Thread: Different Tyre Diameters

That's a big difference, 28mm in diameter, personally I wouldn't.

Baz.

Originally Posted by Redback

That's a big difference, 28mm in diameter, personally I wouldn't.

Baz.

756mm vs 728mm is under 4% difference. That is also the OD, not the diameter of the casing which determines your rolling radius. I would expect the casing diameters to be even closer than that as the 45's have a lot more tread.

Has anyone done it?

It's the circumference difference that will take it's toll on your driveline. The diff is 86.39mm per revolution or 36.398m per kilometer, that is a hell of a lot of difference.


Tyre Size Difference Comparison

Select Wheel/Tyre 1 / 275 / 40 R20
Select Wheel/Tyre 2 / 275 / 45 R20

Results for your selected Wheel/Tyre 1 Rolling Radius 364mm Circumference 2287.08mm Diameter 728mm

Results for your selected Wheel/Tyre 2 Rolling Radius 377.75mm Circumference 2373.47mm Diameter 755.5mm

Differences between Wheel/Tyre 1 & 2 Rolling Radius 13.75mm Circumference 86.39mm Diameter 27.5mm When your speedo reads 100km/hr you're actually at 103.64 km/hr

I have 255/60 R 18s on mine (3 worn, 1 only a quarter worn) - no issues.

The spare is a new 255/55 R18 and one of the worn tyres on the car got a flat and I had to put the spare on - opposite side to the good 255/60. Again no issues but I did keep it on for the shortest possible time.

Difference in rolling diameter is 26mm.

Ok for emergencies and short periods only I think - until you can get some plugs into the puncture.

Garry

It will log a fault eventually & you'll get various suspension/transfer case/ABS /TC fauls logged due to the different wheel speeds throwing the car out of whack. The suspension will drop to the ground eventually too.

We chased these faults for days on one car before finding a different size tyre on one corner. It wasn't much difference, it could have been like Garry's tyres from memory.

Originally Posted by Dougal

756mm vs 728mm is under 4% difference. That is also the OD, not the diameter of the casing which determines your rolling radius. I would expect the casing diameters to be even closer than that as the 45's have a lot more tread.

Has anyone done it?

Why ask if you don't see a problem

Like I said I wouldn't do it.

Baz.

Originally Posted by Redback

Why ask if you don't see a problem

Like I said I wouldn't do it.

Baz.

I ask to see if anyone has done it and either experienced a problem or not.
Garrycol's experience suggests there won't be a problem for the use it will get. The smaller tyre would be a spare wheel which may never be used (punctures aren't that common). If it is used it would only be until the usual tyre can be repaired or replaced.

Rich, 3% is 3%. It doesn't matter if you want to call it 30km in 1000km or 30,000km in 1,000,000km.
The speedo won't read 3% out if only one wheel is 3% different in size, the speed difference would be halfed by the differential on that axle and halfed again by the centre differential. Total difference in measured speed would be 1/4 of 3% which is 0.75%.

Tyres don't run on the outer circumference for turns per km, they run on the radius of the steel belts in the outer casing. The tread blocks flex when entering and leaving the contact patch to make up this small difference in rolling diameter.
Just like V belts and timing belts.
If you were to pump a tyre to around 200psi to make deflection minimal then the difference in rolling circumference to actual circumference also becomes minimal.

I travelled 400km with the smaller diameter on the back left BUT as said it was a new tyre and the one on the same side was a little worn larger tyre so while in theory there was a big difference in reality it may have been marginal. The only reason I have the difference is because I need new tyres soon an will do the lot at the same time.