Date:         Mon, 05 Dec 94 20:30:57 CST
Sender:       Vanagon Mailing List <vanagon@vanagon.com>
From:         Joel Walker <JWALKER@ua1vm.ua.edu>
Subject:      Re: Tire size question

On Mon, 5 Dec 94 20:10:26 CST David Carment said:
>  A simple question for any one with time on their hands...what effect
>does running a larger tire have on the odometer, speedometer and trip
>meter? If for example there are 195s instead of 185s on the front wheels would
>it be necessary
>to increase the distance travelled and speed by say 18% (the difference
>betwen 185s and 195s)? I am curious and also looking for an excuse why my
>mpg is lower than it should be. TIA - DC

well, this is what my dippy little program puts out.
assuming we have a 185/70R14 ... it has
circumference of 76.02 inches, makes 833.50 rev/mile, indicated 60 mph = 60mph
  (assuming NO speedometer error. ;)
now, changing only the width of the tire to 195/70R14, we have
circ. 77.75, 814.94 rev/mile, indicated 60mph = 61.37 mph actual, 2.28% off
which is not a whole lot off. and, given the normally erroneous low reading
of most car speedos, it might actually be more correctly indicating the true
speed.
if you change the tire again, to 205/70R14, we get
circ. 79.48, 797.18 rev/mile, or 60mph indicated = 62.73, 4.5% off.

this is what my calculations indicate. :)
joel