E9s were developed in the mid-1960s and sold between 1968 and 1974. The wheel and tyre options then are very different from those prevailing now. Tyres for 14” wheels are becoming more difficult to find and tyre characteristics have changed considerably, often for the better. This article aims to provide you with an overview of the current options available.

Over the last 50 years, tyre options have drastically changed and are nothing like the ones fitted to the cars when they left the factory. This development, however, leaves us with both pros and cons, as better rubber may be available, but no longer in our size. This presents a problem for most of us, as we now have to look for alternatives. Thankfully, this technical does exactly that, in addition to providing you with the required knowledge to fully understand what you’re looking at and for.

Before we start, please understand that the scope of this article is limited to explaining the theoretical basics in order for you to make an educated choice when looking for your next tyre.

INTRODUCTION
The E9 was initially designed to use the Italian-designed and manufactured 6”J x 14” Cromodora (1) magnesium-alloy wheels. These wheels took advantage of the latest alloy casting techniques. They contributed significantly to reducing suspension “unsprung weight”, which, in those days, was one of a car designer’s principal ways of improving road holding. As development continued and BMW gained vital experience on the race track, the CSL was introduced in 1972, boasting 7”J x 14” Alpina-designed (2) wheels. As these new “Alpina-style” wheels were hugely popular, BMW quickly introduced a similar design for the CS and CSi models from 1974 onwards, known as the turbine-style wheels (3), although measuring slightly smaller at 6”J x 14”. Finally, as an austerity measure, the limited run of 844 “2.5 CS” built in 1974/75 (not sold in the UK) were fitted with 6”J x14” steel wheels. All models were fitted with German Continental tyres except for the CSL, which was supplied with Michelins.

TYRE CODING
The table on the left (4) lists the original wheel and tyre specifications per the BMW workshop manual. Most of these options are no longer available and have been replaced by modern equivalents. However, their coding hasn’t changed since, and we’ll explain that first.

When we look at the “wheel” coding, we start with the width in inches, followed by the rim design, and lastly, the diameter in inches. The “J”, in this case, is a wheel that has a hump just inside each rim to form a shallow indentation into which the bead (edge) of the tyre seats, which stops the tyre coming off the rim under very high cornering forces.

The first number in the “tyre” coding (5) starts refers to the width in mm. The second number is the depth of the tyre as a % of the width, and the two letters indicate the speed rating and tyre type. The final number is the diameter of the wheel in inches to which the tyre can be fitted. Note that the load index is not stated in the table; however, we will discuss this below.

SPEED RATING
In the 1960s and 70s, tyre treads were thicker and heavier than they are today and could rip off if subjected to prolonged high-speed running due to the heat generated by flexing.  Manufacturers (as well as insurers and police) were strict on cars being fitted with the correct speed-rating tyres. Even today, police officers can pull you over and fine you for using incorrect tyres. The table on the left (6) lists the current ratings against their maximum speeds.

When looking at the speed rating, it is wise to stick to the figures specified by BMW. The problem is that our E9 can achieve 138mph (220kmh), which means many tyres are unsuitable for us. However, unless you plan on taking your car onto the German Autobahn, you shouldn’t be driving anywhere near these speeds, as most countries have speed restrictions in place that are well below our maximum speed. Having said that, it is best not to take any risks as most insurance companies will look at this when you file a claim, and ratings less than H should only be considered if nothing else is available.

The second letter in the code, “R”, stands for Radial, which refers to the tyre’s construction type and is considered the industry standard. Other types include “B” for bias-ply, “D” for diagonal, and “XL” for extra load (or reinforced) construction. Most, if not all, standard road tyres are of the radial design.

LOAD INDEX
The load index is a designation of the weight a tyre can carry, and it tends to be more important for commercial vehicles. The relationship between load index and a given load is set out in a standardised motor industry table (7), which shows all index figures vs their respective maximum weight. In short, the load a tyre is expected to carry is estimated as the combined load that all four tyres can carry plus 25% of the maximum total load. The 25% allows for uneven loading plus additional cornering & breaking loads.

If we take the CSi as the heaviest E9, its specified maximum total weight is 1770 kg. Adding 25% on top, we now have a load of 2212.5 kg. Since we have four wheels fitted to the car, we divide this by 4 and end up with a load of 553 kg per wheel. Comparing this to the Load Index table, we find our car between 87 and 88. However, always make sure you round up! Thus, for an E9, the minimum load index would be 88. Fitting anything less than this could lead to overheating and failure of the tyres!

I hear you think that’s all good and well, but what about weight distribution and its effects? As mentioned, this would be included in the 25% on top of the 1770kg max load. However, a standard E9 is distributed roughly 54% at the front and 46% at the rear. If we apply the math to the max total weight of 1770kg, we will end up with 496kg (Load Index 84) on each front wheel and 390kg (Load Index 76) on the rears. Both are well below 553kg; therefore, the load index of 88 still applies, leaving you with a healthy safety margin before abnormal loading occurs.

For those considering fitting 15” or even 16” wheels instead, keep in mind that lower profile sizes tend to have lower load ratings than higher ones. Unless you go for wider tyres to keep your sidewall depth at 70% of the diameter, you will end up with a lower depth and potentially a lower load rating as a result. Always try to fit a tyre with a load rating of 88 or higher.

ROLLING RADIUS
Time for some additional and important math. You could probably argue that once you know the diameter of your wheel and the aspect ratio (depth to width as a percentage) of the tyre, it would be straightforward to calculate the radius. Combine this with the tyre revolutions, and you should be able to work out your speed. In practice, however, it turns out it’s not that simple.

All tyres have a flat spot where they are in contact with the surface, even when correctly inflated. This flat, or compressed, section of the tyre affects the radius because it becomes less than the “overall” radius of the unloaded section. So, in reality, your rolling radius will be slightly less than you would’ve calculated using the theory above. (8) But why does this matter? Well, changing your tyres to a different size will directly affect the rolling radius and, therefore, also the speed displayed on your speedometer, which could now be incorrect because it has been calibrated for a different radius.

Thankfully, tyre manufacturers have already done the complicated math for you and have stated the actual rolling radius for each tyre they produce. In fact, the true radius is measured empirically, which means it’s done by observations rather than theory alone while the wheel is loaded under a “normal” load and inflated to a specific pressure. Inflation is key here, as it directly influences the size of the flat portion in contact with the road and, therefore, the actual radius. Have you ever compared an inflated tyre with a flat one? This is also why your car manual will tell you to inflate tyres to a higher pressure when carrying extra passengers and luggage, as it aims to keep the flat spot constant, resulting in a true speed reading on your gauges.

Some manufacturers publish a “distance run per revolution” figure instead of the rolling radius. Did you know the two are directly related by Pythagoras? Neither did we! Anyway, let’s put the theory to the test. If we take the “Michelin XWX” 195×70 VR14 for the CSL, it gives us a theoretical radius of 314.3mm (9). However, the true rolling radius is 306.2mm (10), which gives us a distance run per revolution of 192.2cm. If we had used the true radius, we would have ended up with a distance run per revolution of 197.8cm. Although 5.6cm is only a small difference per revolution, you can see how this would affect your actual speed compared to what’s indicated on your gauge.

To save you the hassle of calculating the radius for each tyre and then comparing them against each other, you can use tyre calculators online that will do the work for you. They’ll even let you enter the dimensions of two (or more) tyres and present you with a table showing the differences between them. One such website is called “TireSize”, which we used to compare the original 14” tyre against a 15” and 16”, with the results presented on the left (11). You’ll notice that we’ve compared tyres of 195mm width for each of them, which is why we’ve ended up with quite a big difference between them. There is a rough rule that “For every two increases in section width (2x5mm) requires a 10% reduction in profile”. A common and popular alternative to the standard tyre is the 205/70R14, which increases the size of the tyre contact area with the road and only very slightly affects the dynamic suspension geometry, the latter which will be unnoticeable with the power steering.

However, as we have learned above, it will directly affect the rolling radius, and result in your speedometer (and odometer) to underread as the rolling distance is actually higher than “anticipated” by your gauges. (12) Keeping to the “10% reduction” rule, you would have to reduce the profile from 70% to 63%. A 205/70 will underread by 2.4%, whereas a 205/60 will overread by 4%; the 205/65R14 would be the closest option (underreading by 0.8%).

Unfortunately, hardly any tyres of this size are available, while there are plenty of options when shopping for a 205/70R14. There is the option of going up in wheel size, like a 15” or 16”, but this comes with the requirement of needing new wheels in addition to your tyres—a costly endeavour to say the least. Alternatively, you can change the diff ratio to keep the speedometer correct; not a simple nor cheap solution either.

TYRE AGING
New Tyres
Tyres are made of a blend of either natural or synthetic rubber mixed with several chemicals, fillers, and other ingredients. Through an elaborate process and multiple steps, this liquid mix is heated and pressure-pressed into the final shape before cooling and curing into a solid product. Due to sulphur being one of the chemicals used in the production process, tyres start ageing from the moment they are cured. As the tyres are exposed to ozone and sunlight, the sulphur continues to cure, resulting in the tyres hardening at a slow rate post-manufacture. At the same time, rubber oxidises, changing its elastic properties and strength. Both of which have a negative effect on the overall quality of the tyre. For this reason, most manufacturers will not sell new tyres older than 3 to 5 years post-production.

That doesn’t mean that these tyres are removed from the distribution chain, which can result in tyres being sold at your local dealership that are (much) older. So, always check the date stamp (13) to ensure you’re not buying a tyre that should have been retired years ago.

In Use Tyres
You probably gathered that tyres will continue to age while fitted to your car, let alone the negative effects of daily use. Once in use, they’ll be affected in a range of ways, whether standing still and exposed to sunlight and ozone or cruising down the road and exposed to friction, heat, and flexing.

The general opinion when it comes to slowing the degradation process as best as possible is that tyres which are rarely used degrade quicker than those that do an average of 10,000 miles per year”. Surely, this would be contradictory as tyres in use are exposed to additional effects compared to those standing still. Come to think of it, this principle probably applies to the entire car: Standing engines will age quicker than those that run and circulate oil through them, let alone all the other components fitted to your vehicle. Anyway, I digress.

The most likely cause of increased tyre wear when not in use is because they’re pressure-checked less often, which brings us to the real killer: under-inflation. Closely followed by poor wheel balance and incorrect wheel alignment in third place. At the top of the list should really be tyre abuse, but since very few of us (if any at all) are into burnouts and drifting, I figured it wouldn’t be much of a player in our case. Unless most of you aren’t willing to admit to being guilty of these activities…

General Rule
The constant use and ageing of the tyres mean they won’t last forever. Almost all tyre manufacturers will agree that tyres should be changed after roughly ten years since production. Don’t use this as a fixed rule, as local effects might shorten the lifespan of your particular tyres. The real indication of the condition of your rubber is sidewall cracking (14), which is a direct effect of the continuous hardening process. You can also find these hairline cracks in the throughs between the raised part of your tyre tread.

Whatever age your tyres are, always make sure you have the minimum legal amount of tread depth remaining and replace them before you get anywhere near these limits! (15)

TUBES AND TUBELESS
Nowadays, tyres are mainly designed to be tubeless, a significant change from the late 60s and early 70s when many tyres still required inner tubes. If you still use tyres with inner tubes, it’s safe to assume that any new tyres you purchase will be tubeless. Although extremely rare, a few modern options are available that still use an inner tube. If yours are included in this list, be sure to continue using them accordingly.

However, for tubeless tyres, it’s essential to understand that if a tyre isn’t holding air for reasons other than the valve, it likely has damage to the wheel or tyre structure, making it unsafe for use. This damage could stem from improper seating on the rim, sidewall issues/damage, or a hidden puncture through the tread. Additionally, using a tube in a tyre not designed for one can cause friction against the sidewall, generating heat and increasing the risk of a blowout (16).

In the end, buying a new tyre is the best and safest option instead of attempting to repair a leaking tubeless tyre by fitting a tube, especially if a tyre specialist advises against it.

PRESSURES
Both the original driver’s handbook and workshop manual recommend a tyre pressure of 28 psi (1.93 bar) for the standard 195/70VR14 tyres when cold (17). It also lists a note to add an extra 4 psi (0.28 bar) for warm tyres, and 7 psi (0.48 bar) when driving over 120 mph. All tyres are set to the same pressure, with no difference between the front and rear. These pressures should remain the same if you use a different brand tyre, as they’re based on the car’s weight to maintain that flat spot as explained in the rolling radius section. That being said, the original XWX tyres ran at a slightly higher pressure.

It is important to ensure your tyres are inflated to the correct pressure for the reasons explained previously. In addition to this, under-inflated tyres heat up quicker, which causes them to lose grip and increase wear. On the other hand, over-inflated tyres may result in a small improvement in fuel consumption, but this comes at the cost of losing grip, which is far more important from a safety aspect. Therefore, ensuring correct pressure will allow the tyres to operate at the correct temperatures and grip levels, providing you with the highest possible safety levels.

SUSPENSION GEOMETRY
Although not a player when it comes to finding a suitable tyre, it does play a large part in the lifespan and safety of any tyre you fit onto the car. All of the information and statements above assume that the geometry of your car’s suspension is correct. A simple way to check this is by driving on a straight road and carefully taking your hands off the wheel. If the car continues in a straight line, perfect. If it starts pulling to either side, you potentially have a problem with the car’s geometry.

This can be caused by a multitude of things, including uneven tyre wear, worn bushes, and suspension damage. To rule out the majority of these things, paying a little extra to have the wheel alignment checked when replacing your tyres is highly recommended. Also, always replace the left and right side pairs simultaneously, minimising the difference between the sides. Swapping the rear with the front wheels at regular intervals will also help with the even wear across all four wheels. Lastly, try to fit the tyres with the best tread to the front, as it will provide better grip and steering response when it matters.

CURRENT ORIGINAL-SIZE TYRE OPTIONS
When it comes to selecting and buying your next tyre, things such as availability, budget, appearance, and quality come into play. However, contrary to the common belief, there is no direct correlation between price, performance, and durability. For instance, there are cheap tyres which provide fantastic grip but wear out after a few thousand miles, and expensive tyres with poor grip that last a lifetime.

The overview published on the left (18)  is based on an extensive internet search for tyres which are predominantly available in the UK, and availability may differ in other countries. Also note that the list is not exhaustive, as several “own brands” without any online footprint have been omitted.

As there is quite a bit of competition in the marketplace for tyre sales, a collection of available tyres has been listed on a price basis of “Low” (less than £50), “Medium” (£50 – £100), and “High” (£100+). Note that a lot of the 14” tyres on offer are “H” rated. You’ll also notice that there is a significant difference in options when moving away from the 14” to a larger 15” and 16” tyre, some of which are listed below. Another option is to go wider, like a 205 width, keeping in mind the previous caveats mentioned.

There is a great website (www.tyrereviews.co.uk) (19) that posts independent comments from tyre users, sometimes including one of the tyres listed. We recommend reviewing these and similar sites before deciding on your tyre model before buying. Additionally, and possibly the best advice, is to speak to other owners about their experiences. After all, they’re fitted to the same car as you intend to and will have the same handling characteristics.

Some of you may have already noticed that Pirelli recently launched a new 14” tyre that’s aimed at providing us with a quality classic tyre, called the Cinturato CN36 (20). So far, reviews have been positive, and it is definitely a worthy consideration when looking for your next tyre.

Note that Michelin’s “XWX” tyres aren’t listed, as they are only available in 205/70R14.

REAL-LIFE EXPERIENCES
John W.
When we first started researching this topic, we kindly received a short piece on tyre experiences from John, a sage regarding anything CSL. He prefers performance tyres, and it is important to note he has 15” wheels with 7” rims fitted. That being said, his approach and experiences are still valid and worth considering.

Tyres that I have tried on rear-wheel-drive cars and mainly my CSL over the years have been many and varied. One set was put on before a Brands Hatch track test day and removed the day afterwards!

Rear wheel drive cars seem to need something slightly different to front wheel drive. I would be very happy to be on Michelins in a front-wheel drive car but not on a rear-wheel drive, like our E9s. The qualifying sentence in years gone by regarding Michelin’s was ‘spit on the road, and you’re off it”, – as E9s tend to be tail-happy in the wet, and the XWX is not the route I would choose. When you go into that too smooth roundabout in the wet or have to brake very hard because someone pulls out on you, you are hoping or expecting your tyres to cope, and sometimes they don’t.

To find out which tyres suit my car best, I like to treat them to a track day. There you have the time and the space to try out all the limits, and different manufacturers have different qualities. From my experience, I have usually found that Pirelli tyres ‘talk’ to you more than others and go on trying no matter what you ask of them – I have used P-6000, P-Zero Asymetrico, P-Zero Rosso, and I felt happy and in control on all of them. Next, I would put Yokohama and Bridgestone, S02’s being particularly good.

For tyre types which tell you least about what’s happening on an E9 are Michelin, Continental, and Toyos, and in wet conditions, I would be most unhappy to use any of their tyres in anger. (I had one set consigned to the scrap bin the day after a very frightening track day.) A surprisingly good budget tyre has been Nanking and I have not yet tried Falken. Dunlop usually has a reasonable grip but feels too spongy or soft for my liking.

Having had a very quick look at the internet, I find that there is only a limited selection of 14” tyres available, and out of those available, I would probably only be happy with Bridgestone, Hankook, and possibly Nexen. 15” tyres are better when it comes to choice. Personally, I don’t like the look of 16” wheels on the E9 much, and 17” is a no, no! Pity……terrific selection.

Chris G.
Chris kindly pointed out that the Alpina wheels can develop leaks because the rims become porous, something he has experienced with modern Mini and BMW rims. A simple and frequently effective remedy is to remove the tyre, paint the inside of the rim with Hammerite (or something similar), refit the tyre, and hope it works.

William J.
William, a member of the GB club who competes in his CSL, said “I race on 15” wheels because the best tyres are available in the 205/50R15 size. That means changing the diff ratio if one needs to keep the speedometer & odometer accurate. Good tyres such as Continental Premium Contact 2, Dunlop, Kumho, and Vredestein are available in the 205/60R15 size. The overall diameter of the wheel & tyre is only about 1-2 mm less than a 195/70 tyre on a 14” wheel. If I were using my CSL on the road, I would use 205/60 tyres on 15” wheels which is the same as John Wilkinson recommended.”  There is no doubt that upsizing to 15” wheels is a good move. The size still allows the use of a reasonable profile tyre with far more manufacturing options and is not as extreme as 16” wheels, which some people feel are out of character.

William V.
I recently acquired new Pirelli 205/70R14s with great results. They are noticeably better – smoother, more confident, and more precise cornering. She looks a bit more ‘beefed up’ but not out of proportion. They do look different, but I quite like them, and my wife noticed how much more comfortable and precise the handling was, as well as the braking.

CONCLUSION
Armed with the newly gained knowledge, it’s time to head out there and shop for your next tyre set.  When sticking to the classic 195 wheels, you can splash your cash on a set of the new Pirelli Cinturato tyres, or choose from a reasonable availability from brands such as Dunlop, Falken, Kumho, Vredestein, and Toyo – most of them at a fair price. However, these tyres differ significantly in fuel consumption, grip, and durability when comparing them against each other. For that reason alone it is recommended looking for available test results and reviews before committing to buying. As mentioned, although the list only presents you with a small portion of the available tyres out there, it should hopefully provide you with a good starting point.

Enough tyre talk. Time to get rollin’!