Calculating Fuel Mileage?

[slowneasy]

Question for all you math geniouses.

My truck has a 4. 10 rearend and came with 245/75 R16's. I upgraded to 255/85 R16's and now would like to know how to figure out the mileage. How do I go about doing this?

Thanks a ton for the assistances.

 

[High Voltage]

Drink more beer and line up the cans #ad

 

[Alan Reagan]

In your case I'll round it off. Multiply your speedometer reading by 1. 09 and that will be close to your actual mileage.

Here's the deal. You had 245/75 tires. 245 is the width and 75 is the aspect ratio (H/W x 100) all in millimeters.
(Side Height X 2) + rim diameter = Tdia
245/75 = 183. 75 mm side = 7. 23" = 30. 46 Tdia
255/85 = 216. 75 mm side = 8. 53" = 33. 06 Tdia

33. 06 divided by 30. 46 = 1. 085. That's the ratio of the new to the old. For every mile your speedometer has turned the truck has actually traveled 1. 085 miles. That's an 8. 5 % error in your speedometer now. When you read 55 you are doing almost 60. At 70, you are doing about 76. Just multiply your speedometer by 1. 085 and you got it.

Hope this helps. Just remember, when doing the speedo, you can't just say its "X" number of mph off. It is off a different number at each speed because it is a ratio. To be safe just add 10% to your displayed speed and you'll be allright in a speed zone.

I really prefer the beer can method but I was bored this morning.

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2001 HO 6 spd 2500, 4. 10 LSD, Towing Package, Electric Brakes, SPA EGT/Boost Gauges, Fog Lights, Real Backup Lights, Molded Running Boards

[This message has been edited by Alan Reagan (edited 05-08-2001). ]

 

[slowneasy]

Thanks Alan for all the infor. That's exactly what I was looking for. Haven't tried the can method, is it any easier? #ad


Thanks again.

 

[Alan Reagan]

No, but after about three beers it seems to be easier #ad
. Plus no one will argue with you cause you are always right . BTW... . This also assumes that you are running the same manufacturer. What you really need is the exact diameter of the old tire and the new tire, multiply both by 3. 14 (pi) and then calculate the ratio. Different manufactures tire sizes can vary up to 1/2 inch in overall diameter causing a little more difference. 1. 09 or 1. 1 is pretty safe in this case.