On 14/02/2019 13:38, NY wrote:
"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.

They are frequently smaller all around.
The limit to how small they can make them is the clearance on the brakes.