Bob,
I can see the difference in indicated speed in driving it, because the front tires driving the
speedometer remain the same.  Only the rears run the bigger tires so they are effectively increasing
the final drive.  The top speed is definitely increased by about 3 or 4 mph.
Don't know where you got 1.13%.  I get 7.54% difference.
Here's my work:

BFG measured diameter:           26.5"  (they are worn)
Goodyear measured diameter:    28.5" (also worn)

BFG circumference:                 26.5 * pi = 83.25"
Goodyear circumference:         28.5 * pi = 89.54"

Difference in circumference = 6.28"
Percentage difference from BFG = 6.28 / 83.25 = 7.54%
At 60 mph that's a 4.5 mph difference in speed for the same engine rpm.
Let me know if I made a mistake.
-Hans

----- Original Message -----
From: "Bob Stevens" <mtbiker62@HOTMAIL.COM>
To: <vanagon@GERRY.VANAGON.COM>
Sent: Tuesday, December 06, 2005 3:38 PM
Subject: Re: 27" BFG all terrain tires on 82 diesel

> "Changed the top speed by about 4 mph I am guessing."
>
> I think you'll find that's more like 1.13% difference. If you've traveled an
> indicated 250 miles, multiply that by 1.13 and you get 282.5. And at 60 mph,
> it's more like 67.8.
> bob
>