Tire size and gas mileage

I've noticed recently that my gas mileage is down a little (2003 Elantra GLS) and also my GPS shows the speed as a couple of MPH lower than the speedometer. This got me to wondering if my recent set of tires might be smaller in diameter than the original Michelin tires? Of course it could just be my speedometer needs calibrating, but I wonder how much variation there is in tire diameter (195/60-15) and if it can be enough to noticeably change MPG?

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On 11/17/2011 3:00 PM, Victek wrote:

If your new tires are the same as your old ones I don't think that there is significant variance between two tires that are marked the same size. What I have noticed is that in two of our families Hyundais there has been a significant difference between real and indicated speed (4-6 mph @ 60mph) and I believe there was a TSB about it because the one they readjusted. The other was around 2 mph which they said was within tolerance.

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Actually there can be more of a difference in you might think-

Here are the Revs per mile for a selection of 195/60-15 tires:

Yokohama AVS ES100 - 858 Bridgestone Potenza RE960AS Pole Position - 861 BFGoodrich Advantage T/A - 859 Bridgestone Ecopia EP422 - 860 Continental ContiProContact - 850 Fuzion Touring (H&V) - 861 General Altimax HP - 850 Goodyear Assurance ComforTred Touring - 863 Hankook Optimo H418 - 858 Kumho Ecsta LX Platinum - 862 Michelin Primacy MXV4 - 862 Pirelli P6 Four Seasons - 859 BFGoodrich g-Force Super Sport A/S H&V - 859 Bridgestone Potenza RE92 - 861 Firestone Precision Sport - 860 Goodyear Eagle GT (V) - 863 Goodyear Eagle RS-A - 864 Michelin Pilot Exalto A/S - 867 Sumitomo HTR A/S P01 (H&V) - 874 Yokohama AVID ENVigor (H&V) - 858 Bridgestone Turanza EL400 - 861 Dunlop Signature - 863 Pirelli P4 Four Seasons - 855 Yokohama AVID TRZ - 859 Goodyear Assurance TripleTred - 863

The range is 850 to 874, or say 862 +/- 1.3%

If you went from the 850 rev per mile tires to the 874 rev per mile tires, and your speedometer was orginally "perfect" with the original tires, the actual speed with a 60 mph speedometer reading would decrease from 60 to 58 mph. On the other hand, assuming all things were equal, your indicated (not actual) fuel mileage based on the odometer reading would appear to increase by something like 2.6% (say go from 30 to 31 mpg). In the real world chagning tires can effect fuel economy in other ways. Different model tires have different rolling resistances. Tires with higher rolling resistance will reduce your fuel economy. For a given tire size, I don't think the small change in the overall gear ratio beacsue of the slight variation in the tire's rolling radius (revs per mile) will have a measurable effect on fuel economy.

Unless you are keeping a gas log book and make comparisons over many tanks of gas, I doubt your gas mileage measurements are sensitive enough to detect the sort of small changes that would be related to changing tire models (but not tire sizes).

Ed

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"C. E. White" wrote in message

Actually there can be more of a difference in you might think-

Here are the Revs per mile for a selection of 195/60-15 tires:

Yokohama AVS ES100 - 858 Bridgestone Potenza RE960AS Pole Position - 861 BFGoodrich Advantage T/A - 859 Bridgestone Ecopia EP422 - 860 Continental ContiProContact - 850 Fuzion Touring (H&V) - 861 General Altimax HP - 850 Goodyear Assurance ComforTred Touring - 863 Hankook Optimo H418 - 858 Kumho Ecsta LX Platinum - 862 Michelin Primacy MXV4 - 862 Pirelli P6 Four Seasons - 859 BFGoodrich g-Force Super Sport A/S H&V - 859 Bridgestone Potenza RE92 - 861 Firestone Precision Sport - 860 Goodyear Eagle GT (V) - 863 Goodyear Eagle RS-A - 864 Michelin Pilot Exalto A/S - 867 Sumitomo HTR A/S P01 (H&V) - 874 Yokohama AVID ENVigor (H&V) - 858 Bridgestone Turanza EL400 - 861 Dunlop Signature - 863 Pirelli P4 Four Seasons - 855 Yokohama AVID TRZ - 859 Goodyear Assurance TripleTred - 863

The range is 850 to 874, or say 862 +/- 1.3%

If you went from the 850 rev per mile tires to the 874 rev per mile tires, and your speedometer was orginally "perfect" with the original tires, the actual speed with a 60 mph speedometer reading would decrease from 60 to 58 mph. On the other hand, assuming all things were equal, your indicated (not actual) fuel mileage based on the odometer reading would appear to increase by something like 2.6% (say go from 30 to 31 mpg). In the real world chagning tires can effect fuel economy in other ways. Different model tires have different rolling resistances. Tires with higher rolling resistance will reduce your fuel economy. For a given tire size, I don't think the small change in the overall gear ratio beacsue of the slight variation in the tire's rolling radius (revs per mile) will have a measurable effect on fuel economy.

Unless you are keeping a gas log book and make comparisons over many tanks of gas, I doubt your gas mileage measurements are sensitive enough to detect the sort of small changes that would be related to changing tire models (but not tire sizes).

Ed

Thanks Ed, you covered the variables pretty well. As a practical matter I wonder if I should trust the speed readout on my GPS Vs the speedometer? The GPS says the car is moving about two or three MPH slower than the speedometer, but I have no idea how to assess it's accuracy. If I knew someone in the CHP I could calibrate against his radar gun, but that ain't gonna happen :-)

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wrote:

Speedometers are not allowed to read under the actual speed but can read under. GPS can be more accurate

Some information is here http://en.wikipedia.org/wiki/Speedometer GPS devices are positional speedometers, based on how far the receiver has moved since the last measurement. Its speed calculations are not subject to the same sources of error as the vehicle's speedometer (wheel size, transmission/drive ratios). Instead, the GPS's positional accuracy, and therefore the accuracy of its calculated speed, is dependent on the satellite signal quality at the time. Speed calculations will be more accurate at higher speeds, when the ratio of positional error to positional change is lower. The GPS software may also use a moving average calculation to reduce error. Some GPS devices do not take into account the vertical position of the car so will under report the speed by the road's gradient.

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On my 2002 SantaFe, i replaced the factory 225x70R16's with four 245x70R16's wider and taller tires....without any loss of economy . Then i disconnected the air filter box inlet hose so it sucks in warm engine air, and picked up 2.5 MPG increase at each fillup . Ill return the inlet hose for all outside air come Springtime. No peformance loss thats noticable for this mod. And no driveability issues whatsoever. I also reduced the cold idle speed of the motor from an excessive 1900 rpms to 1000 rpms and picked up another 1 MPG at each fillup.

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