Tire size and gas mileage

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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?



Re: Tire size and gas mileage
On 11/17/2011 3:00 PM, Victek wrote:
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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.


Re: Tire size and gas mileage

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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



Re: Tire size and gas mileage




Quoted text here. Click to load it


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 :-)



Re: Tire size and gas mileage
wrote:



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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.

Re: Tire size and gas mileage
Quoted text here. Click to load it

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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