If you use a GPS unit to see how much your speedometer is off, then apply
the Miata tire size calculator, you can get your speed right on, based on
the tire size.  However, as Ken indicated, your odometer will now be off.

http://www.miata.net/garage/tirecalc.html

Jack

-----Original Message-----
From: Vanagon Mailing List [mailto:vanagon@gerry.vanagon.com] On Behalf Of
Kim Brennan
Sent: Friday, March 23, 2007 5:39 PM
To: vanagon@GERRY.VANAGON.COM
Subject: Re: What size of profile?

By law, US suppliers of vehicles had to make their speedometers read
at or above the true vehicle speed. Unforunately I don't think there
was any attempt to calibrate things, so a 2wd Vanagon's offset might
be completely different than a Syncro Vanagons offset.

Not to even get into the discussion of differing wheel sizes and the
tires that can fit.

And the fun part is that the speedometer and the odometer aren't
necessarily in sync. So if you get your speedometer working
accurately, it is almost certain that your odometer now won't be
accurate.

I find that a 205/65R15 Tire gives pretty accurate speed readings on
my Syncros speedometer. However it makes my odometer read about 4%
off. But that is with a specific brand and model of tire. Different
brands and different models can give a different result, even at the
same tire size.

On Mar 23, 2007, at 5:20 PM, Zoltan wrote:

> It is a "tire question", I know, it sucks.  But I am not certain
> what is the right size if profie that has to be used to make the
> speedometer show correctly the speed.  Right now, I think, I have a
> smaller one on that has a smaller circumference, that makes me go
> 'faster' to be at the right speed.