Jamie,

You can use this site to calculate speed differences based on tire size.
http://www.miata.net/garage/tirecalc.html

The recommended tire size for our vans is 185/80-14

That size tire shouldn't result in a 20 mph error.  The larger tire you are
running should only impact your speed by 2.1 mph, so when you are going 60
mph, it would actually be 57.9 mph... a 3.5% too fast reading with non-stock
tires.  So, at 70 MPH, you would really be going 67.5.

Back in the 80's when 55 was the legal limit, many of the vehicle
manufacturers inflated the speed slightly.  I believe the Vanagon speed is
off about 3 mph at 60... so if you apply the engineered speed error with
your tires, at 70 MPH, you would be about 64 MPH or so.

Jack R.
84 Westy Wolfie.

-----Original Message-----
From: Vanagon Mailing List [mailto:vanagon@gerry.vanagon.com] On Behalf Of
Jamie Fitterer
Sent: Monday, May 19, 2008 1:31 PM
To: vanagon@GERRY.VANAGON.COM
Subject: Tire Size for 84 Westy

I am currently driving on LT 195 75 R14 tires.  My speedometer is off by A
long shot.  I am cruising along thinking I am doing around 70 and really I
am traveling 50 MPH.  ANy suggestions on a good size 14 inch to run or any
suggestions on how to adjust speedometer?  I like the LT tires because they
last longer with the extra weight on them.

Not sure deciding to go with LT tires was a mistake.