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Some believe that gas mileage and tire size seem to be related in terms of how you measure the mileage factor. Your odometer may reflect better mileage with bigger tires. Some experts refute this by pointing out that larger tires will make the odometer read a greater distance traveled-unless the odometer is recalibrated for the larger tires. Your actual gas mileage and tire size issues may have more to do with the fact that your odometer isn't calibrated for the bigger size than an actual savings. Your best bet to increase gas mileage is to use fuel efficient tires with better rolling resistance, and keep those tires properly balanced and inflated.
This theory is flawed. A larger tire has a larger circumference, therefore lowering the revolutions per mile and thus your speedometer and odometer are going to read lower than with stock.
The statement "Some experts refute this by pointing out that larger tires will make the odometer read a greater distance traveled" is backwards. A larger tire will make the the odometer read les miles, so you might think you mileage is less. Fact is it might take more fuel to get a larger tire rolling, also it will be heavier.
In over drive with a small motor it also might make the transmission need to down shift up a incline.
Food for thought.
Larger tires will not make the odometer read a greater distance traveled. By increasing the circumfrance of the tire, the car goes further for each revolution of the wheel. The odometer uses revolutions of the wheel to calculate distance. A larger wheel will cause the odometer to read lower than the actual distance traveled.
Unlike this tip claims, larger tires will NOT make the odometer reflect a greater distance traveled. The odometer will read LESS distance than what is actually traveled, and any fuel economy calculation will appear reduced by the same percentage difference that the distance traveled is recorded. In other words, if you replace a car's original tires with tires that have a 10% greater circumference (quite a large increase for most sedans), distance the car travels will be 10% greater than the mileage recorded by the odometer. So if the car indicates 100 miles on a gallon of fuel, but the larger tires have meant the car ACTUALLY traveled 110 miles on that gallon, the ACTUAL economy has improved 10%, even though there will appear to be no change in economy. Other factors are at play here, of course, and larger tires may or may not help with efficiency, but larger tires WILL NOT create the APPEARANCE of efficiency based on odometer readings.
And another thing, based on the specs from major manufacturers, (I didn't check all tire brands and models available, but I did check in excess of 20 different tires, various styles of tires, and more than 9 manufacturers) a tire of size 205/60-15 was listed as requiring between 834 and 849 revolutions to travel 1 mile. the vast majority of the tires fell between 840 and 844 rev/mile. Only about a 1.7% difference between small and large, if one stays with the same size tire as original equipment.
Are you sure you have the facts straight about tire size? A larger tire will go further per rotation then the smaller tire, but if the car is calibrated for the smaller tire your odometer will actually read a lower millage then the true distance traveled. Thus it should give the appearance of being less fuel efficient.
I think you got it backwards. A bigger tire will turn the wheels fewer times per mile, and make the odometer register fewer miles for the same distance traveled.