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Old 05-18-2011, 11:24 PM   #1 (permalink)
SentraSE-R
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Join Date: Jun 2009
Location: Bay Area, California
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Mean Green Toaster Machine - '06 Scion xB
Team Toyota
90 day: 48.92 mpg (US)
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Tall tire test (taller gearing advantage offset by wider, heavier wheels/tires?)

I did some testing of my new 205/50-17 wheels/tires v. the stock 185/60-15 wheels/tires. There are too many variables for this to be a realistic comparison, but this testing is better than just guessing what happens if you swap to 6% taller tires and wheels that weigh 12 lbs more, each.

Car with stock tires/wheels

Car with new tires/wheels

Internet tire specifications:
======= ..... new ...... old
diameter ......... 25.07"...23.74"
revs per mile.... 829.6 ....876.1
speedo diff...... -5.605%...0%
diameter diff..... 5.31% ....0%

In their favor, the new shoes are taller geared, look better, and have 51 psi sidewall maximums (v. stock 44 psi). Working against them is their extra weight, worse aerodynamics, and wider tread. On balance the newer, taller tires should get me worse mpg, as it's likely the minuses outweigh the pluses.

New weight 47.3 lbs Old weight 35.3 lbs (tire + wheel weight)

distance calibration (tested against GPS after 40 miles, minimum distance)
new 1.099 old 1.033 (odometer shows 10.0 miles, I've traveled 10.99 miles with the new tires, or 10.33 miles with the old tires)

The drive is a 10 mile loop (cruise control, 32 mph) with minimal traffic, and no stop signs. I do have to turn around at each end to go the other direction. It's at sea level with a few hills - the tallest one perhaps 100' high. Temperature was 65-70 degrees, partly cloudy, winds 0-5 mph from the west. The route runs north and south. Roads were dry. Mpg calculated with Ultragauge.
new1..49.2 mpg.....old1..50.1 mpg
new2..50.4 mpg.....old2..50.0 mpg

Essentially identical performance. It's roughly confirmed by my 900 mile trip last weekend with the new tires. I got 53.7 mpg my first tank and 47.7 mpg topping off the tank in preparation for the drive home. The 53.7 is my best highway tank, and 47.7 is about average for me when I had the old tires. The new tires get similar mpg to the old ones.

Considering the 17" tires/wheels weigh 12 lbs apiece more than the stockers, are less aerodynamic, and are 20 mm. wider, I'm pleased that I didn't lose mpg with the trade.

*Edited 5/19/11 to correct incorrect original tire diameter

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Darrell

Boycotting Exxon since 1989, BP since 2010
Have you ever noticed that anybody driving slower than you is an idiot, and anyone going faster than you is a maniac? George Carlin
Mean Green Toaster Machine
49.5 mpg avg over 53,000 miles. 176% of '08 EPA
Best flat drive 94.5 mpg for 10.1 mi
Longest tank 1033 km (642 mi) on 10.56 gal = 60.8 mpg

Last edited by SentraSE-R; 05-20-2011 at 02:11 AM..
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