Larry, you might be experiencing a loss of power due to friction. More tread
= more friction and the rubber may be significantly different enough to
cause even more friction.

Just my basic physics that I remember from static's.

Sean Garrett

-----Original Message-----
From: Larry Chase [mailto:lchase@ATTGLOBAL.NET]
Sent: Saturday, December 14, 2002 4:52 PM
To: vanagon@GERRY.VANAGON.COM
Subject: Engine Performance vs Tire Size

A while back my stock engine was rebuilt (stock 2.1L to a Eurocars 2.2L)

After the rebuild my "Seat of the Pants" conclusion was that I had
gained 10-15% in Engine performance (Acceleration & Top End MPH).

Now to the topic ..... Engine Performance vs Tire Size

Since then I've had the opportunity to try two different tire sizes on
the same vehicle. Since I was planning on going to a large than stock
tire, I anticipated sacrificing a little power / performance.

But it seems to be more than a little.

Tire Details:

Size / Width / Diameter / Rev per Mile

Original Owner Tires

27X8.5R14 / 8.6" / 26.5"  / 795 ... Dunlop Rover - 10-15% Engine Power
Increase (as noted above)

First Replacement Tire Set

215/65R15 / 8.7" / 26.1" / 818 ... Michelin  - No Change Engine Power
Increase

Second Replacement Tire Set ... Current Tires

215/75R15  / 8.5" / 27.7" / 755 ... Michelin LTX 0% Engine Power
Increase

Does it make sense that I'd loose 10-15%  in performance by increasing
the tire diameter 1.2"?

If that's the case ..... I might consider changing to a tire closer to
the OE Tire diameter.

Larry