Tyre Outer Diameter Question

[Shedding]

If I calculate the nominal diameter of a tyre (eg. for a 315/35-R20, OD = 20 x 25.4 mm + 2 x (35/100) x 315 mm = 728.5mm), is this the actual diameter the tyre manufacturers aim to achieve?

The reason I ask is that I've just had 4 new tyres fitted to my X5 and the X-drive system is not happy. The tyres are staggered with 315/35-R20 on the rear and 275/40-R20 on the front. They are nominally the same outer diameter (to the nearest mm) but I think they must be outwith the tolerance which BMW requires (the tyres are not BMW * rated).

I plan to measure each tyre circumference and am hoping I only need to replace two of the four tyres, hence the question.

[996TT02]

Have you tried disconnecting the battery, maybe the car re-learns the sizes and will only react to a real situation. No idea, just a thought. Those sizes give a pretty much identical rolling circumference. Are they original sizes? If not, maybe they are not supposed to be? I really don't know.

Edited by 996TT02 on Friday 21st July 11:59

[Pica-Pica]

You need the rolling circumference. Calculate here:
http://www.wheelcalc.com/

ETA: they are the same nominal rolling circumference.

My 335d xDrive has staggered sizes, they are not exactly the same RC (within 4mm), but they are *marked and I have never had an issue.

Edited by Pica-Pica on Friday 21st July 13:20

[Panamax]

Forget the "nominal" anything.

Make sure the tyre pressures are 100% correct to manufacturer recommendation.
Get a piece of string and drive a wheel onto it - join the string round the tyre and mark the join.
Remove car.
Measure string.
Then same with the next wheel until you've done all four.
Compare the results.


Don't try to shortcut the process by jacking up one wheel at a time. You won't get the answers you need.

[Shedding]

To answer my own question, and in case it helps someone, I found the ISO standard for passenger car tyres and rims (ISO 4000-1).

The design tyre overall diameter which manufacturers aim for is indeed the formula I quoted previously. Tolerances are also given. For a 315/35R20, the design diameter is 729mm while minimum and maximum diameters are 722mm and 737mm respectively. (BMW * rated tyres have tighter tolerances).

The code also gives the measuring procedure which is to measure the circumference with tyre mounted on rim at a pressure specified in the code and at room temperature. Wheel should not be loaded so jacking the wheel off the ground is appropriate.

I'm going to measure the tyres on my car and hope that either the rear pair or the front pair are close to the design diameter so that I only need to replace the other pair.

[Pica-Pica]

Panamax said:

Forget the "nominal" anything.

Make sure the tyre pressures are 100% correct to manufacturer recommendation.
Get a piece of string and drive a wheel onto it - join the string round the tyre and mark the join.
Remove car.
Measure string.
Then same with the next wheel until you've done all four.
Compare the results.


Don't try to shortcut the process by jacking up one wheel at a time. You won't get the answers you need.

That will not get the rolling circumference at speed. Also string stretches.

[Pica-Pica]

Shedding said:

To answer my own question, and in case it helps someone, I found the ISO standard for passenger car tyres and rims (ISO 4000-1).

The design tyre overall diameter which manufacturers aim for is indeed the formula I quoted previously. Tolerances are also given. For a 315/35R20, the design diameter is 729mm while minimum and maximum diameters are 722mm and 737mm respectively. (BMW * rated tyres have tighter tolerances).

The code also gives the measuring procedure which is to measure the circumference with tyre mounted on rim at a pressure specified in the code and at room temperature. Wheel should not be loaded so jacking the wheel off the ground is appropriate.

I'm going to measure the tyres on my car and hope that either the rear pair or the front pair are close to the design diameter so that I only need to replace the other pair.

Any ISO standard will follow the European Tyre and Rim Technology Organisation (ETRTO) parameters.

[E30M3SE]

Shedding said:

the tyres are not BMW * rated

This is your problem, you need look no further.

[Shedding]

I know what the problem is and obviously one solution is to buy 4 new BMW * rated tyres. It might come to that but hopefully 2 of my tyres are within the acceptable BMW X-drive diameter tolerance and therefore I need only buy 2.