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Old 04-07-2009, 02:59 PM   #7 (permalink)
ConnClark
DieselMiser
 
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Join Date: Jun 2008
Location: Richland,WA
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Das Schlepper Frog - '85 Mercedes Benz 300SD
90 day: 23.23 mpg (US)

Gentoo320 - '04 Mercedes C320 4Matic
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Welcome to the forum

Quote:
Originally Posted by Ernie Rogers View Post
As for tires, I am still working on ideas here. Some rules I believe to be true--

1) Soft rubber makes more efficient tires
2) Big wheels with low-profile tires are really bad for fuel economy
3) Narrow, large-diameter tires are best
4) The second number on the tire (profile) should be 60 or higher
5) High tire pressure improves fuel economy AND tire life
6) Pavement matters. Smooth (or lightly grooved) concrete can raise mileage by as much as 10% over smooth asphalt, 15% for rough asphalt.

By the way, larger-diameter tires allow your engine to run at lower RPM where it's more efficient, but I think most of you already knew that. Be careful to correct all data for the change in tire diameter. Watch your speed.
Some things to note about tires.

A harder rubber compound makes a more efficient tire. It improves mileage in the same way as higher tire pressure does. Typically tires rated to last the longest have a harder compound. A harder rubber compound won't give you as good of grip as a softer one so you loose traction.

Also an over inflated tire tends to reduce the life of the tire because it causes more pressure and wear on the center of the tread.

Generally the low profile tires have less side wall flex which reduces rolling resistance. Unfortunately going to a larger rim to accommodate a lower profile tire may increase the rotational inertia of the wheel. Which way to go depends on if you do a lot of stop and go driving or if you do a lot of driving at constant speeds for extended periods of time.

Also its important to note if you go to a larger diameter wheel with a larger profile this will make your car handle worse and make it more prone to rolling over.

Changing tire diameter doesn't always help. Some cars have their gear ratios set to optimize the engines maximum efficiency rpm range to the typical speed limits.

In your situation with a diesel, peak efficiency typically occurs at the rpm where peak engine torque is generated.
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