Tyre Myths - Wheels Magazine November 2015

[carl888]

Carl,

I would believe that if all roads were perfectly flat and straight but I would suggest that the differentials can cope just as well with slightly different wheel diameters as it can with curves.  I would suggest the note in the driver's manual is for other reasons.

That's actually not the case, transient lateral rotational differences are accounted for as in any car by a differential.  For example, turning a corner or lifting a wheel as you advertise the fact you don't have an LSD.  Longitudinal differences are also accounted for by a differential however in both cases, a differential is NOT designed for constant discrepancies in RPM, particularly under load.  Porsche mention this in their manuals for 2WD and 4WD vehicles.  I am unsure of what they quote in their current models however in the 1980s they stated a 30% tread difference per axle assuming an identical tyre was fitted to the same axle.  

The issue with the differing tyres in terms of manufacture is that the assumption is that all manufactures have the same circumference for a given size. It is interesting to line up a Michelin and a Pirelli to see that this is not the case because there is no standard rim size for measuring section width, it is up to the manufacturer to determine that, and there are differences because of this.

[Redracn]

I had an experience one with a prestige tyre brand. Got an unrepairable puncture on the front. Put a new tyre on and swapped the other front for the unused spare. So two new tyres on the front same brand same model same size same tred pattern no visual difference whatsoever.

But now it pulled to the left really bad making it very tiring to drive any distance. Checked alignment no issue. Looked into the tyre more closely and one was made in the USA and sold in Australia. The other made in Japan and came with the car. Contacted the manufacture and explained the problem. They then replaced the Japanese tyre free of charge. Problem solved no more pulling to the left.

They told me the tyres internal construction was substantially different to cater for differnt market tastes and expectations and even though every aspect was identical to look at (except the country of manufacture) the tyres were not identical and should never be used on the same axle.

When it comes to differing diameters it is not black and white. The manufacture will have a tolerance that is acceptable in respect to wear and life as well as the electronics ability to adapt etc. The bigger the diameter difference the more wear etc on diffs, clutch pacs, viscus couplings etc. so the question really is how much faster do you want to wear it out assuming you are not beyond the electronics (ABS, diff controllers etc) ability to adapt to the deviation from standard specification.