"Jim K.." wrote in message
o.uk...
Still hopefully the same external diameter, otherwise the car would (try
to)
go round in circles.

We call them differentials...

Would a differential iron-out the tendency for two wheels on the same axle
with different diameters to want to steer in one direction, needing constant
counter-steer to correct it?

Maybe it would. Obviously it allows them to rotate at different speeds when
they are turning on different radii of curvature, but would a car that is
going in a straight line tend to keep that line if the tyres were different
sizes?

I *thought* that a spacesaver spare tyre was always (nominally) the same OD
as the wheel that it is replacing, allowing for different amounts of tread
wear, and that it was only the tyre *width* that was narrower. I agree that
one wheel may be a different size to the other, but that is immaterial as
long as the OD of the tyre is the same.