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  1. #1
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    Default Tire size to MPG random thougts

    While at the pick'n pull I found an xj that just rolled in to the donor yard.
    It had 215 75r15 and I still have the stock 215 70r15 (I know...) and they were in good shape vs. mine are still bald (remember I've only had the xj for 3 weeks and have other things that are in great or need...). & they wanted 100.

    The math works out to 1" taller (.87 +/-) and that comes from the tread width ~ is 1" narrower.
    That equates to 3% further per rotation...
    ... yes, my odometer will be off by 3%. & it will require # more torque off the line.
    But the thinner tread is less resistance and the energy to maintain the highway speeds should remain the same.

    So everything else being equal, I should net a 3% gain in MPG.
    Right?

    ....... granted my previous best average was 26 and a 3% gain on that is only gonna be like 26.6 or something. But over a tank of gas, that's like an extra 5 in gas.

  2. #2
    Join Date
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    Default Re: Tire size to MPG random thougts

    No, it will work the other way. 3% loss
    230 hp 4.3L Chevy
    Built 4L60E with manual lockout
    Atlas 4 speed
    Griffin radiator
    On board air
    Warn 9.5ti on custom bumper
    7.5" RE front and bastard pack rear on custom long arms
    30 gal gas tank
    lockers
    And a bunch of other stuff

  3. #3
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    Default Re: Tire size to MPG random thougts

    Quote Originally Posted by 4.3LXJ View Post
    No, it will work the other way. 3% loss
    The diameter is an inch and some longer
    So it would be further per rotation. My spedo will be under - so 60 mph = 61.3 Actual
    ...according to the tire calculator

  4. #4
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    Default Re: Tire size to MPG random thougts

    Here's the screen print of the cal ulator
    rrent Tire Size: / R
    New Tire Size: / R


    Side by side Overlay

    Current Tire New Tire Difference
    Diameter inches (mm) 26.85 (682) 27.7 (703.5) 0.85 (21.5) 3.2%
    Width inches (mm) 8.46 (215) 8.46 (215) 0 (0) 0%
    Circum. inches (mm) 84.35 (2142.57) 87.01 (2210.11) 2.66 (67.54) 3.2%
    Sidewall Height inches (mm) 5.93 (150.5) 6.35 (161.25) 0.42 (10.75) 7.1%
    Revolutions per mile (km) 751.13 (466.73) 728.17 (452.47) -22.96 (-14.26) -3.1%
    Speedometer Difference
    Speedo Reading Actual Speed
    20 mph (32.19 km/h) 20.63 mph (33.2 km/h)
    25 mph (40.23 km/h) 25.79 mph (41.5 km/h)
    30 mph (48.28 km/h) 30.95 mph (49.8 km/h)
    35 mph (56.33 km/h) 36.1 mph (58.1 km/h)
    40 mph (64.37 km/h) 41.26 mph (66.4 km/h)
    45 mph (72.42 km/h) 46.42 mph (74.7 km/h)
    50 mph (80.47 km/h) 51.58 mph (83 km/h)
    55 mph (88.51 km/h) 56.73 mph (91.3 km/h)
    60 mph (96.56 km/h) 61.89 mph (99.6 km/h)
    65 mph (104.61 km/h) 67.05 mph (107.91 km/h)

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