Re: tire sizes

[DeWitt Harrison <de@aztek.com>]

On Fri, 11 Apr 1997 16:07:40 -0400, Mark Trank wrote:

>A recent post re: differing tire sizes caught my attention, since I 
>recently acquired an 87 5kcstq that runs 205/50/15s on the front 
>(firestone firehawks) and 215/50/15s on the rear (same tire).  A friend 
>who runs a tire store in town says it won't make any difference in 
>driving or traction, though the aesthetics of the front v. rear are 
>noticeable on close inspection.  Am I in trouble?

Maybe very little. Your 215/50s are roughly 1 cm taller than the
205/50s. I calculate a nominal 59.6 cm vs. 58.6 cm. In terms
of rate of rotation, the shorter tires will make about 17 extra
revolutions beyond each 1000 revs made by the tall ones. 
This represents some extra work for the center differential.
I wouldn't want to venture an opinion as to whether this has
any wear and tear implications. I am sure you wouldn't want
to lock your center diff. except in very low traction situations.

The shorter tires would be a little better at applying torque
to the pavement since their reduced radius would result in a
slightly higher overall gear ratio engine-to-front vs. engine-
to-rear; about 1.7% higher. There could be some subtle
drivability differences compared to same all round, but I
suspect you would have to make some sort of direct A-B
comparison to notice.

My suspicion is that "trouble" would be overstating the
consequences but I would immediately yield to first hand
experience to the contrary.

DeWitt Harrison   de@aztek.com
Boulder, CO
88 5kcstq