I've got 295/75's on my 02 350, and I'm wanting to calculate fuel mileage. Is there a chart or formula to use that factors in the larger diameter of taller tires?
Rule one is you must see liquid diesel at the cap when you fill up - before and after the tank of fuel you are checking to determine MPG. That takes several minutes of trickling diesel through the foam until you see liquid diesel and no foam at the cap.
If you have calibrated your speedo to 626 tire revs/mile, then it's simply dividing miles driven per the tripmeter by the gallons of fuel used.
If you have not calibrated the speedo, then you need to determine tripmeter error. Some folks say you can use a GPS to do that, but I prefer the old fashioned way:
On an interstate highway, find a mile marker that is not near an entry, exit, overpass, underpass, or any other reason the highway crew couldn't put the marker exactly where it belongs. Set the tripmeter to zero, then drive at least 10 miles down the interstate to another mile marker that is also not near anything but open highway. Then determine the percentage of tripmeter error.
If you go exactly 10 miles per the markers, then every tenth of a mile error is one percent error. For example, if you went 9.4 miles per the tripmeter in an actual 10 miles per the markers, then you have a 6 percent tripmeter error. Your tripmeter is "slow", so you need to add miles to the tripmeter number when you compute fuel mileage.
For example, you used 21.4 gallons in 342.4 miles per the tripmeter. Without correcting for tripmeter error, you got 16.00 MPG. So you have to add 6 percent to the miles per the tripmeter, = 362.9 actual miles. 362.9 divided by 21.4 = 16.96 MPG. And we won't tell anyone if you round it off to brag about 17 MPG.
Using 10 miles for your tripmeter check will get you within one percent of the actual tripmeter error. If you're persnickety like me, that's not good enough. So I use 100 miles, then each tenth of a mile tripmeter error is only one tenth of one percent.
The above procedure is the only sure-fire way to determine tripmeter error.
You can use simple math to approximate the difference between your tires and stock-size tires, but that method assumes
your tripmeter was accurate with the stock-size tires. The formula is simply the percentage difference is tire revs/mile.
For example, stock-size tires on your 2002 have 655 revs/mile. Your 295s have 626 revs/mile. 655 minus 626 = 29, divided by 655 = 4.43 percent. So assuming your tripmeter was accurate with stock-size tires, your correcting factor would be 4.43 percent. Add 4.43 percent to your miles per the tripmeter, divide by the gallons used, and you'll get a corrected MPG number that will be in the ballpark, but probably more than one percent off.