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Old 08-27-2006, 01:02 AM   #1
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Mileage curiosity question...thinking caps on?

You change tires and you are now running 2 mph slower (taller tires?) than what you show on your speedometer...(speedometer says 60, but you are actually going 58 mph...checked by radar)...and you want to check your mileage after every tank full...how would you figure the mileage? Besides the miles traveled divided by the gallons to fill up, would you add or subtract miles to the total mileage traveled on that tank full due to the change in the speedometer?

Am I figuring this right? At 60 mph, you are actually traveling 58 mph (about 3.5%? difference?)...on a 300 mile fill up, that would add up to 10 1/2 miles on top of the 300 mile fill up using 9 gallons. Would I add this or subtract it to the total miles? I know it wouldn't make a huge difference (about 33.33 versus 34.5 mpg), but was just curious...and I haven't been in school in 40 years, 2 beers, early morning for me and tired...


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Old 08-28-2006, 01:41 AM   #2
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You are thinking correctly in that the mileage/speed has changed, but you've gone the wrong way with it. If you've gone from a stock rim/tire, to a larger rim/tire, the speeds you read on the speedometer are reading LOW. If the speedometer read 60mph, you'd actually be going faster, depending on how much bigger the new rims/tires are. This is because you're readout on the speedometer, is going by the rotation of the wheel. If the circumference of the wheel is 3ft, then every rotation, you are traveling 3 feet. The speedometer is calibrated for a 3ft wheel, and gives you the speed using 3ft in the formula. However, if you change to a larger wheel with a diameter of 4ft (just for example), then every rotation, youa re TRAVELING 4ft, but the speedometer is calibrated for 3, so it believes every rotation you are only traveling 3ft, when in fact you are traveling 4. The only thing that's ACTUALLY less than it was before, is the # of times the wheel has to turn in a certain amount of time, to travel at 60mph.

As for figuring out MPG's and such, i'm not going to get into all of that, but it sounds like you're onto it there. So recalculate with the info i've posted above, and you're all set.
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Old 08-28-2006, 06:27 AM   #3
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Thanks for the info...

Didn't know if I had taller tires or not...but I do know that I am now traveling at 58 mph actual when the speedo says 60...so my tires must be a shade smaller?

Then the percentage is about right at what I was thinking? and I would add the difference in miles to my total miles at fillup?
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Old 08-28-2006, 06:54 AM   #4
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In the 2000-2004 Focus at least, there is a built in 2mph, how should I say it, over-read. Its not a percentage, at least not according to the ECU readout, I'm doing 23 at 25mph indicated, 63 at 65mph. It came up once in discussion and apparently Ford has a history of doing this, for reasons they'd be able to better explain.

So, that difference shouldnt affect your total mileage, since the ECU outputs the corrected speed. However, it was pointed out once before that whatever overall height the ECU uses for the tire, once the tire worn down to 3/32 or less it should throw off the distance traveled per rotation significantly. I'm at the end of my OEM tire's life tho and my mileage results are still coming back good, I'd guess the overall result isnt worth much consideration
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Old 08-28-2006, 07:18 AM   #5
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Thanks, I didn't know that either. Learn a lot on this forum.

Yeah, it would have only made a difference of about a mile a gallon. I appreciate it.
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