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1999-2007 General Questions General questions related to 1999-2007 Super Duty trucks. If it doesn't fit the other categories, post it here. Gas engine discussion that pertains to all models is allowed. Specific gas engine questions should use the Gas Engines forum.

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Old 04-06-2008, 11:35 PM   #1 (permalink)
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Formula for figuring MPG with oversized tires?

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?

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Old 04-07-2008, 10:16 AM   #2 (permalink)
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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.
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Old 04-07-2008, 08:04 PM   #3 (permalink)
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Assuming your speedo was correct with the original tires. Measure the circumference of the original tires. Measure the circumference of the new tires. Subtract the old size from the new size. Then divide that number by the old size. This is the percentage difference. Say your old tires were 104" and the new ones are 135". 135 minus 104 = 31. 31 divided by 104 = .29808 or rounded off .30 or 30%. So your new tires are going to go 30% further per revolution. So, when you fill up with fuel and check how far you have driven you will multiply that number by 100%+30% or 130%. For example you drove 280 miles and used 18 ga. of fuel. 280 X 1.30 = 364. This is how many miles you actually drove. Now divide that by the gallons of fuel used, 364 divided by 18 = 20.22 MPG. This is also how far off your speedometer is. Say you are traveling 60 mph on the speedo. 60 x 1.30 = 78 MPH.
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Old 04-08-2008, 10:19 AM   #4 (permalink)
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Fuel mileage

Fill up your truck.

Drive it until nearly empty.

Fill it up again.

Divide the total miles between fillups by the gallons in the last fillup.

That's your MPG.

Did I miss somethiing? Did you want to know your MPH, not MPG?

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Old 04-08-2008, 10:27 AM   #5 (permalink)
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Did I miss somethiing?
Apparently.

He has oversize tires, and has not calibrated the speedo to match the new tirees, so his tripmeter is not accurate. Several percent tripmeter error. So how does he determine miles driven?
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Old 04-08-2008, 10:45 AM   #6 (permalink)
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Measure the circumference of the original tires. Measure the circumference of the new tires. Subtract the old size from the new size. Then divide that number by the old size. This is the percentage difference.
Close, but no cigar.

When you mount the tire on the truck and put weight on the tire, the rolling radius is a lot less than half the circumferance. And the rolling radius is what you must use to accurately calculate rolling circumferance.

Simply measuring the radius of the tire from the ground to the center of the axle is also close, but no cigar. The rolling radius is different than the static radius.

So you need to use tire revolutions per mile. That's accurate, based on the rolling radius at a certain speed.

Example:
Circumference of LT265/75R16E is 31.8", or a radius of 15.9". Static loaded radius is 14.6, so the static loaded circumference is 29.2. There's a heck of a difference between 31.8" and 29.2". But even that is not accurate when computing revs/mile, because the rolling radius is slightly different than the static radius of the tire.

Yes, it gets complicated. But the easy way is to use the tire revs/mile, which Ford and BFGoodrich tells us is 655 for that tire.

If you like math problems, then tell us the rolling circumference of the tire when the revs/mile = 655.

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