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Tires & Wheels/Size and Pressure Test:

Date sent: Fri, 16 May 1997 16:41:34 MST
Subject: HML: Tire Size/Pressure Test

OK people, I did the test and here are the numbers...

Test Description:

Measure the Height between the center of the wheel and the ground. Measure the distance traveled in 10 revolutions of the tire Perform above measurements at two different inflation pressures.

Test Data:

Tire PressureHeightDistance
35#17"92' 1"
10# *15-3/8"91' 4"
(% difference)9.6%0.8%
* Tires were bulging noticably (about as expected for 10#).

Test Result Analysis:

Reducing the vehicle axle height by about 10% did not reduce the distance traveled by anywhere near 10%. Actual reduction was less than 1%.

Since the distance traveled was changed by only about 1%, the effective torque was also increased by only about 1%. It should be noted that the rolling resistance of a low-pressure tire is also higher, so the net difference on torque is probably near zero, if not negative.

Speedo difference should also be about 1%.

Effective Tire Size (Calculated from distance traveled):


Bottom Line:

Do not lower your tires for increased torque.

As mentioned in a previous posting, reducing tire pressure does greatly increase traction and floatation. These, in addition to smoother ride, are excellent reasons for reducing tire pressure.

Notes for Nit-Pickers:

The test was performed only once at each tire pressure. The truck speed during the 10-tire-revolution test was very slow - about 1 mph. The axle-height to ground was measured at the end of the test. The tire was turned about 1 revolution at low pressure before the test was started to allow for tire deformation while deflating. Test was performed on the rear tires of a '95 Diesel wagon with very little cargo inside. Front tires were not deflated, transfer case in H (not HL). Tires are Goodyear MT tires with about 50% of tread remaining. Altitude: about 5500'. Ambient Temp: 82 deg. F. Surface used for testing: graveled asphalt. Grade: between 1% and 2% down.

Dave Breggin
'95 Diesel Wagon

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