Why Are Wheels Different Sizes?

Different wheels suit different cars. Older BMWs, such as the 2002, came with 13" steel or alloy wheels to suit their small wheel diameters and the higher profile tyres of the 1960s and 70s. The E30 three series was supplied with 14" rims which became 15" on the E36 and may well be 16" on the new E46 when it comes out soon.

So why the increase in wheel size? Tyre technology is the driving force. In the 1970s even supercars like the Lamborghini Countach only had 14" wheels, most Ferarris had 15". As tyre design has advanced lower profile tyres with stronger sidewalls have become available. Overall wheel diameter is still broadly the same but modern wheels with low profile tyres and larger metal rims offer a lower "slip angle" due to the lower flex in the wheel as a whole, this improves steering response.

One problem with low profile tyres is that because they offer less sidewall flex the driver gets less warning when the tyre is about to lose grip and start sliding. Higher profile tyres give more feedback before breaking traction. So a 17" wheel and 40 section tyres may offer more ultimate performance but just be sure you know what happens when you lose traction with little or no warning.

So the difference between 15,16 and 17" inch wheels is basically due the profile of the tyres you want to fit. Bigger wheels also let you fit larger brakes, you must use 17" wheels on an M3 for example.

As a general rule 15" wheels are fine for most BMWs. 16" are a worthwhile upgrade for enthusiastic drivers who want faster steering response and better handling at the expense of a slightly bumpier ride.

With 17" or larger wheels you'll feel every bump and learn a whole new respect for potholes! It's very common where I live for M3 drivers to bend wheels on bad roads, not a cheap experience. The tyres for these larger wheels are progressively more expensive too.

Some BMW owners have separate set of wheels for road and track use. This give you practicality for everyday use and saves the real performance for the place you can use it. Others, like me, run alloys in the summer and cheap steel wheels with snow tyres in the winter.

Tyre Types

All tyres have a rating printed on the sidewall such as 225/50VR16. This is decoded below.

The total diameter of the wheel above is (16 x 25.4 ) + ( 225 * 0.50) x 2 = 631.4mm.

You can use the JavaScript calculator below to compare the sizes of two wheels, very handy if you're thinking of an upgrade.

Don't forget that as tyres wear the wheel diameter will decrease by up to 14mm!

If you decrease the overall wheel diameter by fitting lower profile tyres you are effectively increasing the final drive ratio of the car and improving low end acceleration a little. This has the side effect of making the speedometer read high though and decreases cruising fuel economy.

Running very wide tyres isn't a good idea. The weight of the car is spread over a larger area resulting in less pressure for a given area of tyre. This causes the car to aquaplane at lower speeds in the rain and have problems cutting through ice and snow in winter.

Z rated tyres tend to have stiffer sidewalls than V rated and handle better. In my limited experience H rated tyres are cheaper than V but don't last as long. Avoid remould tyres as they don't last. The previous owner of my 635CSi fitted remoulds to the back end, they handled tollerably but only lasted 9,000 miles! The front TRXs hadn't worn nearly as much in the same period.

Always ensure your tyres are at the correct pressure. Incorrect pressures result in poor fuel economy, longer stopping distances and increased or uneven tyre wear. For enthusiastic drivers a few extra psi won't hurt, just don't over do it. Don't forget to check the spare either.

Snow Tyres

Rear wheel drive cars need a little help in the winter, especially if they run tyres wider than 200mm. I use 195 section snow tyres and steel wheels on my 318iS in the winter instead of the normal 205 section alloys.

So what makes a snow tyre different? Normally they are narrower to increase contact pressure on the ground. They use a softer rubber compound which is more pliable at low temperature and can dig into the road surface, this also makes them wear terribly in warm weather. Lastly snow tyres have more open tread patterns and deeper tread to disperse water and grip through snow.

Snow tyres have a lower speed rating than normal tyres and tend to have softer sidewalls which spoil the cars handling. I can't wait to get mine off every spring but the difference in stopping distance and traction is worth it.

Always fit four snow tyres to a rear wheel drive car, never two. Getting used steel wheels is a good idea because it saves your alloys from the salt and muck. You also don't have to pay someone to swap your tyres twice a year.

TRX Tyres

In the 1980s BMW fitted tyres known as TRX to some higher performance models. TRX were advanced for their time and used a special design of wheel and tyre lip to hook the tyre to the rim for improved high stress cornering performance. To avoid the dangers of fitting normal tyres to a TRX rim they were all made in metric sizes which only a TRX tyre would fit. Only Jaguar and a few other manufacturers used TRX and it became the Betamax and 8-track of tyre design.

TR means "Tension Repartie" (spread tension). Michelin's design called for a shallower rim that allowed freer movement of the tire sidewalls under compression but provided a progressively increasing lateral stiffness during cornering. This was thought to be a boon for the high-end luxury-sporty car market as it in theory would allow for a more comfortable ride without sacrificing cornering ability through controlling the slip-angle with sidewall stiffness. In essence BMW thought they were providing the ride quality of a 70-series tire with the handling benefits of a 60-series profile.

Modern tyres have better tread patterns and rubber compounds than TRX. TRX are also quite expensive and harder to get hold off than modern tyres. When the TRXs on my 635CSi wore out I bought and refurbished some used 15" cross spoke alloys and bought modern tyres.

TRX tyres may be spotted by a designation of 220/55VR390 or 415 on the tyre sidewall.

Buying Wheels

You tend to get what you pay for where wheels are concerned. Cheap wheels will have a cheap finish and poor quality alloy as a rule. With 17" rim this results in a wheel that buckles and chips easily. If you must get new wheels either pay the money for good ones or get a proper BMW used set. BMW owners magazines like the Roundel or BMW Car have good classifieds sections with used wheels.

I bought used 15" BBS cross spokes for my 635CSi for 35 GBP per wheel. After a good clean I decided to respray a few of them. I bought the correct silver paint from the dealer and applied a few light coats after 1000 grit wet and dry paper. I then used a few coats of clear lacquer and voila! BMW dealers will sells you stick on centre roundels cheaply to finish things off.

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