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Calculating MPG with larger tires

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Old 07-12-2013, 04:14 PM   #1
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Calculating MPG with larger tires

Is there a chart here somewhere that will help me to get a ratio or divider to figure my milage with tires larger than stock?

Thanks
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Old 07-12-2013, 04:29 PM   #2
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It's hard to calculate just on size, since the weight is also a factor. And even the same brand and size can have a 15 lb difference from a P to E rated tire.
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Old 07-12-2013, 04:37 PM   #3
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try to find the sizes approx. by height. So lets say one is 31", and you went to 33".

So 33"/31"=1.0645 these are always approximate. If you had a good distance each rolled per turn it could be more accurate.
The 1.06 is going to be what you need to multiply your tripmeter by to get actual miles closer.

Better yet, just keep close track with a GPS, fill up at the same pump and same setting.

Use the tire size calculator uner the "mods and tutorial" near the top of the screen too.
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Old 07-12-2013, 04:45 PM   #4
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Quote:
Originally Posted by bolio View Post
It's hard to calculate just on size, since the weight is also a factor. And even the same brand and size can have a 15 lb difference from a P to E rated tire.
Nothing to do with it. All e needs is his distanced traveled and volume of fuel used.
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Old 07-12-2013, 06:01 PM   #5
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Did you recalibrate the speedo/odo when you upsized (e.g.--Hypertech calibrator)? If not you will be off a little on miles to calculate MPG...
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Old 07-12-2013, 07:34 PM   #6
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Quote:
Originally Posted by Erik65 View Post
Did you recalibrate the speedo/odo when you upsized (e.g.--Hypertech calibrator)? If not you will be off a little on miles to calculate MPG...
Thanks, that would make sense. How does one go about that?
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Old 07-12-2013, 11:18 PM   #7
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Interested about this recalibration as well.
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Old 07-15-2013, 06:10 AM   #8
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If a tire is 10% bigger in diameter then the stock tire, it's 10% bigger in almost all respects, rolling distance, revolutions per mile etc. Add 10% to the miles traveled, and divide by the gallons consumed. Close enough for me.
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Old 07-15-2013, 03:20 PM   #9
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Quote:
Originally Posted by Goosedog View Post
Thanks, that would make sense. How does one go about that?
Here's what I bought:
http://www.autoanything.com/performa...A3757A0A0.aspx
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Old 07-15-2013, 07:04 PM   #10
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Quote:
Originally Posted by jdmcq View Post
Nothing to do with it. All e needs is his distanced traveled and volume of fuel used.
Really? Then how come when I went from P245/75/16 Dunflops to LT265/75/16 BFG All Terrains, I lost two mpgs? Doing this made my speedo correct to my GPS? Going to this tire was a 16 lb. per tire increase, so yes weight is a factor. No different if you replaced your front bumper with a heavy duty full replacement bumper with grill guard. More weight = less mpgs.
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Old 07-15-2013, 07:09 PM   #11
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^There are two ways of interpreting OP's question.

1. Predicting MPG loss with larger tires.
2. Already installed larger tires, need to correct milage with larger rolling dia.

For #2, you need the "rev per mile" info from the tire mfr's web site. For example, the Dunlop AT20 P245/75R-16 that's OEM on Tacoma SR5 is 686 revs/mile.

The BFG All-Terrain in LT265/75R-16 is 654 revs/mile.

So if you calculated 17.0 mpg from the gas pump reading and odometer reading, your actual mpg is 17.0 x 686 / 654 = 17.8 mpg

It's better to use mfr's data, as two tire models of the same P-metric size can vary by 6%.
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Old 07-15-2013, 07:13 PM   #12
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Quote:
Originally Posted by KenLyns View Post
^There are two ways of interpreting OP's question.

1. Predicting MPG loss with larger tires.
2. Already installed larger tires, need to correct milage with larger rolling dia.
Not referring to Op's question, referring that extra weight does not affect mpgs. Go ahead and throw an extra hundred lbs in the bed of your truck and see if your mileage doesn't go down.
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Old 07-15-2013, 07:30 PM   #13
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Quote:
Originally Posted by Boilerman View Post
Not referring to Op's question, referring that extra weight does not affect mpgs. Go ahead and throw an extra hundred lbs in the bed of your truck and see if your mileage doesn't go down.
No one said extra weight didn't drop mpg.
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Old 07-15-2013, 08:17 PM   #14
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Quote:
Originally Posted by worthywads View Post
No one said extra weight didn't drop mpg.
Go back to jdmcq's response. He is referring to weight.
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Old 07-15-2013, 08:34 PM   #15
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Quote:
Originally Posted by Boilerman View Post
Go back to jdmcq's response. He is referring to weight.
But the context of jdmcq's response was about the op's question, which wasn't about weight. jdmcq did not say weight doesn't matter.

Your speedometer might now be correct but if your odometer was accurate before you put on bigger tires it isn't now.
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Old 07-15-2013, 08:42 PM   #16
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Quote:
Originally Posted by worthywads View Post
But the context of jdmcq's response was about the op's question, which wasn't about weight. jdmcq did not say weight doesn't matter.

Your speedometer might now be correct but if your odometer was accurate before you put on bigger tires it isn't now.
Quote:
Originally Posted by bolio View Post
It's hard to calculate just on size, since the weight is also a factor. And even the same brand and size can have a 15 lb difference from a P to E rated tire.
Nothing to do with it. All e needs is his distanced traveled and volume of fuel used.
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So if weight doesn't matter, then according to jdmcq size of tire doesn't matter either???
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Old 07-15-2013, 09:12 PM   #17
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Quote:
Originally Posted by Boilerman View Post
So if weight doesn't matter, then according to jdmcq size of tire doesn't matter either???
Sure it does, he referred to distance traveled which changes, that's what op wants to know how to calculate.
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Old 07-15-2013, 09:29 PM   #18
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Quote:
Originally Posted by worthywads View Post
Sure it does, he referred to distance traveled which changes, that's what op wants to know how to calculate.
Modern cars are designed to work as a complete package. A car's fuel economy is a factor of engine efficiency, transmission and differential gear ratios, aerodynamics, wheel size and weight and tire size and construction. Car manufacturers use all of these factor to maximize the efficiency. Changing one of the factors---tires, for instance---without taking the others into consideration will usually reduce the fuel efficiency of the car.

Changing the tires on a car so they're taller or shorter will provide a false change in perceived fuel economy. The speedometer of a car is calibrated to the revolutions per mile of the original tires, and changing tire size will result in an inaccurate odometer reading. Shorter tires will take more revolutions per mile, resulting in a higher calculated gas mileage using the uncorrected odometer reading. In fact, shorter tires will reduce fuel efficiency because the engine must run at a higher rpm to achieve the same speed and travel the same distance.

If shorter tires reduce fuel mileage, do taller tires increase fuel economy? If you could get a taller tire without changing any other factors, the theoretical answer is yes. A taller tire will increase the weight and rolling resistance. However, the taller tire is also not designed for the gearing of the engine and drive train. If the car is a newer model, the computer will not know there is a taller tire and may cause the vehicle to run poorly and use more gas. Changing to taller tires will result in fewer tire revolutions per mile, but the offsetting factors will probably negate any theoretical fuel economy .
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