Date:         Tue, 6 Dec 1994 16:16:20 -0500
Sender:       Vanagon Mailing List <vanagon@vanagon.com>
From:         tauxejd@ornl.gov (John Tauxe)
Subject:      

>  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? 

Just work it out from your geometry knowledge.  A larger tire will cover
more ground in one rotation (remember - circumference = pi * diameter) (pi
=~ 3.141592654).  
So covering the same ground, it will rotate at a lower speed, and will give
a lower value on the speedo.  You will travel farther than your odometer,
calibrated to the smaller tire size, says you did.  the amount you went is
proportional to the diameters of the two tires.  This is diameter, not the
185/195 stuff.  Take a tape out to your car for the best estimate.  

If your old tires were 20" diameter and your new ones are 21", then when you
thought you went a mile, your really went 21/20 miles.  So you are getting
not just 17 mpg, but rather (21/20)*17 mpg.  Feel better?

Finally - some engineering I can do! :)

   John Tauxe     tauxejd@ornl.gov     Oak Ridge National Laboratory