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Old 06-12-2007, 07:33 PM   #1
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Need Math Help!

Heres the deal. My speedometer reads 65mph, when im doing 70. (this verified by a Ford Focus, U-Haul, and 3 Random truckers ) Now, multiply 5 times 100 and divide by 70 and you get roughly 7. So, at 65mph the speedo is 7% off. Assuming the speedo is 7% off throughout the entire speed range, my MPG results are off by 7%. Correct?

So... since the speedo is "slow" I increase my MPG results by 7%. Correct? So, since is usually get 13-16 MPG, I can (roughly) add 1mpg to my results?






Is the above math, and theories correct?
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Old 06-12-2007, 07:43 PM   #2
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Your math is correct, but it is your odometer that you need to check the accuracy of. The speedo being off by 7% does not mean the odometer is off by 7%.
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Old 06-12-2007, 07:46 PM   #3
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Your math is correct, but it is your odometer that you need to check the accuracy of. The speedo being off by 7% does not mean the odometer is off by 7%.
You have a point there. Ive just always ASSUMED one off equaled the other being off as well. Damn, I hope my odo is off. THe 1mpg boost is nice
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Old 06-12-2007, 08:08 PM   #4
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what car/truck is it off in?
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Old 06-12-2007, 10:10 PM   #5
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Funny, 7.1% is what my car is off by, in the same direction. I simply multiply my odometer reading of how many miles I went by 1.071 to get my "real" miles driven. Sure does help the gas-mileage . That should be a new tip in the sticky...Or not.
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Old 06-12-2007, 10:50 PM   #6
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what car/truck is it off in?
Ford F150


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That should be a new tip in the sticky...Or not.
LOL!
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Old 06-13-2007, 02:55 PM   #7
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what year? im gona assume cable driven speedo. its not the best way to measure your accuracy with another cars speeds cuz who knows how much it could be off. but since you had multiple drivers tell you that, i would say that it is a safe bet...
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Old 06-13-2007, 03:10 PM   #8
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what year? im gona assume cable driven speedo. its not the best way to measure your accuracy with another cars speeds cuz who knows how much it could be off. but since you had multiple drivers tell you that, i would say that it is a safe bet...
85'

Yes cable driven.


I marked out a mile with a newer vehcile (2000 F150) and drove it with my truck. The two odo's where right on with each other. I guess I dont get to tack on an extra MPG
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Old 06-13-2007, 03:18 PM   #9
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You are assuming the 2000 is accurate.

Since I have 14" rims on my CRX, I was assuming the odometer was reading low. I used Google Earth to measure the distance between two locations on the freeway about 28 miles apart (did this at a resolution of about 1000 feet to be as accurate as possible around corners, etc). I then compared this distance to my odometer and found it was within 1/10th mile with the larger rims. With the stock 13's, it must be a few % high.
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Old 06-13-2007, 05:01 PM   #10
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Don't you have mile markers where you live? most highways around here do.
Thats the best way drive at least 10 miles and compare.
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