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Old 07-17-2008, 10:24 PM   #1 (permalink)
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Correcting for tire size

Despite a bunch of searching, I can't find a clear answer--

How do you correct for non-stock-sized tires? My xB came with 185/60/15's, and I have 195/60/15s. This is mostly because the only tires that I could find in 185s were garbage Walmart tires, which my car came with. The tires I've had since the beginning of June have a circumfrence 2% greater than stock. Is this even worth correcting for?

If so, how do I do it? Multiply my miles as shown on the odo by 1.02?

Needless to say, avoiding math was why I went to school for biology. I think in researching this, I've confused myself further.

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Old 07-17-2008, 10:40 PM   #2 (permalink)
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You just need to DIVIDE your miles by 1.02. Whether it's worth correcting is up to you, but I do it and my correction is only 1.1%.
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Old 07-17-2008, 10:44 PM   #3 (permalink)
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Hi,

I think the most accurate way to do this is to drive a measured mile, or a measured 10 miles at 60mph -- and time it with a stopwatch, and adjust the numbers based on the 1minute per mile.
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Old 07-17-2008, 10:47 PM   #4 (permalink)
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Thanks, guys. I should borrow a GPS to double-check my odometer accuracy anyway. That 2% figure is based on the miata.net tire size calculator.
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Old 07-19-2008, 12:25 PM   #5 (permalink)
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I'm doing the same thing to my car soon...205/60's to 215/60's which is a 2% difference. This does make a difference in your mileage calculations, and since we're all working for every additional mpg, I will definitely compute the change.
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Old 07-19-2008, 01:12 PM   #6 (permalink)
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I just did the corrections on my fuel log, and noted them. It took my actual mpg down slightly, but that just means I need to be more motivated to be efficient. I'm still doing really well the last couple of tanks, though, so I can't complain.

In happier efficiency news, I talked my girlfriend out of driving 50 miles each way to Erie, PA to see a movie. That's just shy of four gallons in her car!
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Old 07-19-2008, 09:24 PM   #7 (permalink)
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Hold the horses!! You went from 185/60s to 195/60s correct?...i.e., taller tires. If so then your odometer is reading 2% too LOW. You need to multiply your odometer readings by 1.02 then divide by the gallons used. You're calculated FE in your log should go up not down. If the tire is taller, you travel further for every "mile" read by your odometer as compared to your old tires. Taller tires give you MORE effective overdrive which in most (not all) cases will increase your mileage. This is one major reason I'm going to taller tires next week.
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Old 07-19-2008, 09:27 PM   #8 (permalink)
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See, that's what I originally thought, then I started second-guessing myself. It's easy enough to change the logs again, though.

The worst part is, I actually had this straight at one point, because I used to have a truck with somewhat oversized tires, so I actually sat and figured it out way back then (2001 or so).

Edit:
Correct corrections added to fuel log and my spreadsheet.

This boosts my actual FE up quite a bit!
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Old 07-19-2008, 09:35 PM   #9 (permalink)
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whokilledthejams -

Doesn't it feel good? It's like a taller-size-tire dividend when I enter my fuel log.

I'm coming up on my GPS correction check. I try to do it every 6 months on the assumption that the tires should be getting smaller over time.

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Old 07-20-2008, 04:52 AM   #10 (permalink)
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Here's another tire size calculator, this one you can compare four tire sizes to the stock size. Looks like I will be getting a better size tire for my Hypermiling.

http://www.1010tires.com/TireSizeCal...bmit&reset=yes

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