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Old 08-28-2008, 12:42 PM   #1 (permalink)
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89 Metro XFI - MPG Decrease w/ Larger Tires

So I'm pretty confused. I replaced my 12" rims with 13" ones and my MPG has dropped by 3-4 mpgs.

I got 155/80 R13's and am now pulling 49mpg's (last two tanks). Back on the R12's I was getting 52-53mpgs.

Driving hasn't changed (actually driving a bit slower to compensate for the 2mph increase the larger wheels/tires produce). No other mods besides the wheels and tires.

What's going on??

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Old 08-28-2008, 12:46 PM   #2 (permalink)
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what size were your old tires? and how are you tracking your miles? your odometer will be off from the bigger tires as well
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Old 08-28-2008, 12:47 PM   #3 (permalink)
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My guess: your odometer is now off due to larger tire diameter. It's under reporting your distance traveled, as your speedo is under reporting speed. You may only apparently be getting worse fuel economy.
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Old 08-28-2008, 12:47 PM   #4 (permalink)
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Tony beat me
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Old 08-28-2008, 12:57 PM   #5 (permalink)
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155/80 R13 = 22.8" tall tire.

post up your stock tire size and me or someone else can figure up how much your odometer is off.

example, my tires are .07% taller than stock. i multiply my trip odometer reading by 1.07 to get (pretty close to) actual miles. .07 makes a difference of about 15-20 miles over the course of my 18 gallon tank
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Old 08-28-2008, 12:58 PM   #6 (permalink)
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.07% would require multiplication by 1.0007.
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Old 08-28-2008, 01:07 PM   #7 (permalink)
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oops....... well, you should probably have someone else crunch the numbers then, haha
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Old 08-28-2008, 01:29 PM   #8 (permalink)
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i think even my .07 is wrong to begin with.

stock tire size = 28.9
current tire size = 31

yeah, i know, going by what the tire says is not the best way. but i figure its pretty close. i have a stock sized spare, so next time i get a flat i'll measure the static load radius of both tire sizes with vehicle weight on them.

lets say the OD says i went 263 miles. truck thinks i have stock tire size.

so i assume this formula is correct:


28.9 263
------ = ------
31.0 X

***edit--> for some reason the forum is shoving my numbers over. i'll keep trying to get it to read right....

***edit2--> or just put it as 28.9 / 31.0 = 263 / X

Solve for X and you get 282.1. that should be actual miles traveled. am i way off on this?

my prior (failed) math gave me 281.4
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Last edited by Tony Raine; 08-28-2008 at 01:48 PM..
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Old 08-28-2008, 02:33 PM   #9 (permalink)
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This link is really handy for figuring it out....

Tire size calculator

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Old 08-28-2008, 02:35 PM   #10 (permalink)
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sweet!!!!

i totally didn't even thing of the odometer being off. thanks gents!!!

i don't recall the old size off-hand. will look when i get home and post up.

so what i would do is use the calculator above to determine the difference and then use this to calculate the odometer different (i.e. the figure provided will work for both the speed and distance calculations)?

thanks again! much appreciated!

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