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Old 06-01-2008, 07:53 PM
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Lets discuss Tire size and MPG

I have been reading the latest Diesel World while my wife has been in the hospital and it brought me to wonder some things. They are doing some testing with a 6.0 and mileage. Im sure some of you have read it. Anyways they upgraded the tire size from a 255/75r16 to a 305/70r16. That resulted in a 1.7 inch increase in tire size which caused a speedometer and odometer error of 5.6 percent which meant that the speedometer and odometer were reading low. So my question is if tire size is causing the odometer to read less than the actual mileage that means that my calculations of 13mpg is off by alot. This also means that when my speedometer reads 70 and Im sitting at 2k rpms I am actually running 65mph or so correct. The article doesn't say what formula they used to figure this out so does anybody know? They are also claiming 24mpg with a bed cover, exhaust, intake and xzillaraider chip. Sounds pretty good to me. What do yall think.
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Old 06-01-2008, 07:55 PM
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im out 6 k/hr so for every 100km I drive its really 106km .. thats going from 265-295 tire
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Old 06-01-2008, 08:05 PM
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I read that article also. When my speedo reads 60 I am realy going about 65. I have to multiply my mileage by 1.0857 then I can divide that by the # of gallons I buy to figure my mpg. I truly get about 15 mpg mixed driving as opposed to 13.5 uncorrected. Made me feel better about my truck.
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Old 06-01-2008, 08:27 PM
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See thats what doesn't make sence to me. If they went to a 305 from a 255 and I went from a 265 to a 315 wouldn't my speedometer and odometer be reading even slower than theirs. Not faster? The smaller the tire the faster it is having to spin thus more rotations per mile which would mean higher speed right? a larger tire means less rotations per mile thus allowing for a slower speed to be read at the wheels.
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Old 06-01-2008, 08:29 PM
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Another thing that people aren't aware of....it is cumulative.

If your going 30mph and it's 3mph off,....that doesn't mean that when your driving 60mph,..that your actually going 63.

I don't have the exact numbers calculated,...but at 60mph,..your closer to 64.6mph....but even then...it is depending on tire size.
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Old 06-01-2008, 08:30 PM
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your speedo is driven off the rear diff.. tone ring on the ring gear.. so for every axle revolution you make a larger tire will travel a farther distance a smaller tire will go a shorter distance.. this is why gear change in these trucks dont throw out the speedo so you will go faster and farther with a larger tire then the speedo says
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Old 06-01-2008, 08:37 PM
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Exactly what I thought.
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Old 06-01-2008, 08:40 PM
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I'm running 315/75 R16's and cruising at 70 (speedo) I'm actualy doing 75 MPH. I was told there was a 10% error with that tire size.
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Old 06-01-2008, 08:45 PM
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Quote:
Originally Posted by POWERSTROKER1 View Post
I'm running 315/75 R16's and cruising at 70 (speedo) I'm actualy doing 75 MPH. I was told there was a 10% error with that tire size.
True....but at 50mph,..you probably are only about 8.5% error
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Old 06-01-2008, 11:08 PM
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I see what your saying. But I don't completely understand it.

Wouldn't it be 10% across the board?

You said if your 3mph off at 30 you won't be 3mph off at 60, thats true, but its still 10% error, you'll be 3mph off at 30, 5mph off at 50, 7mph off at 70, 10 mph off at 100.....and so on. The only thing changing is the mph you will be off at a given speed.

If this is the wrong way to look at it, please correct me so that I will know.
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