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Perhaps the answer to why larger tires appear to get lower gas mileage. This is not a proof but just a theory.

My 2004 Rav4 4x4 tire size: Factory 235/65/16
235mm x .65 ratio =152.75mm ~ 6.01in (sidewall height)
6.01in X 2 + 16in = 28.02in (tire height)
Circumference = 28.02in*Pi = 88.02in. (distance per rotation)

Larger tire: BFG A/T 235/70/16
235mm x .70 = 164.5mm~6.47in (sidewall height)
6.47in X 2 + 16in = 28.95in (tire height)
Circumference = 28.95in*Pi = 90.95in. (distance per rotation)

For the sake of this theory I’m assuming 21 Miles per gallon.
21 miles ~ 1330560 in

To cover 21 miles:
1330560in/88.02inches per rotation =
15116.5644 rotations of my 235/65/16 factory tire

Or

1330560in/90.95in per rotation =
14629.5766 rotations of a Larger 235/70/16 BFG tire.


The odometer is calibrated to factory size tire so regardless of the tire size, the computer recognizes one rotation of the wheel as 88.02 inches traveled.

14629.5766 rotations of larger tire * 88.02in = 1287695.3403in ~ 20.323 mile

So the 21 miles traveled on the larger diameter tire will only show up as 20.323 miles on the odometer, a difference of .677 miles per gallon or 8.124 miles over 12 gallons (roughly the size of the tank). Therefore unless the odometer is recalibrated after switching to larger sized tires the distance traveled will appear less than it really is.

Again, just a theory.
 
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Discussion Starter #2
No offense but funny how you put up all these when everything is but common sense :wink:
 
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haha yeah i kinda realized that after but i was bored so what the heck :)
 

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Isn't the stock tire size 235/60-16, not 235/65-16? That would make even more of a difference when changing to 235/70-16.

Size 235/60-16, with circumference of 85.1", has 744 revs/mile, but 235/70-16 has only 697 revs/mile.
 

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Larger tires will affect the gas mileage in other ways than just throwing off the calculations.

The size of the tire affects the overall gear ratio and as a result affects the load on the engine and gas consumption.

Larger tires often have higher rotational friction (mainly dependent on tread design though) so it takes more force to turn them.

That is why many people who install larger tires also regear their differentials, which compensates for the change in tire size.
 
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bmorton said:
Isn't the stock tire size 235/60-16, not 235/65-16? That would make even more of a difference when changing to 235/70-16.

Size 235/60-16, with circumference of 85.1", has 744 revs/mile, but 235/70-16 has only 697 revs/mile.
DOH! Good catch!
 

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and for those of us who can only use 1% of our brain capacity:

larger tires = more weight
more weight = increased fuel consumption

:lol:
 

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Although heavier large tires will definitely result in higher consumption, weight difference for RAV4 can be ignored. Let's look an example.

Bridgestone "Dueler H/T" tire is a RAV4 OEM tire (not sure which year's RAV4)
215/70R16 - 22 lb
235/60R16 - 28 lb
There is a 6 lb difference for one tire. 5 larger tires will increase 30 lb. Comparing it with a 4X4 RAV4 curb weight plus driver's weight, say 3000 lb, it is only 1% increment.

I don't know weight difference between a steel wheel and alloy wheel. But we can safely assume extra weight of the tires and wheels will not increase significant fuel consumption. Any of other factors like, road condition, temperature, luggage, speed .... will write off fuel saving of small tires.
 
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Don't forget that larger tires also add more frontal area along with the more wieght, lower final drive ratio and increased rolling resistance.

If you were to recalibrate the speedometer you would find that larger tires still consume more fuel.

Also don't forget that RAV4's also can have 215/70/16 tires. I wonder how they compare to the 235/60/16's.

And the discussion of only 1% additional weight...you must calculate the change in rotating mass and it's distance from the centerline of rotation, not the total vehicle weight. This is the place you will see large power consumption differences with just a few pounds added. Since the tires and wheels make up the majority of this mass a small change will result in slower acceleration and greater power consumption. This is why drag cars and formula one cars do not have huge wheels on them, they try to keep the rotating mass down and close to the center of rotation.

Also consider the difference in the weight of the steel wheels compared to the aluminum wheels.
 

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Did you just say that drag cars and stock cars do not have large wheels on them? Not to slag you bro, but you should take a closer look. Indeed stock cars have very large tires on them, and drag cars even larger!

In any case, I'm glad you brought this topic up Sierra. I mentioned it one other time, and no one seemed to get it. I'm glad it seems like common sense now!

nothercarguy said:
Don't forget that larger tires also add more frontal area along with the more wieght, lower final drive ratio and increased rolling resistance.

If you were to recalibrate the speedometer you would find that larger tires still consume more fuel.

Also don't forget that RAV4's also can have 215/70/16 tires. I wonder how they compare to the 235/60/16's.

And the discussion of only 1% additional weight...you must calculate the change in rotating mass and it's distance from the centerline of rotation, not the total vehicle weight. This is the place you will see large power consumption differences with just a few pounds added. Since the tires and wheels make up the majority of this mass a small change will result in slower acceleration and greater power consumption. This is why drag cars and formula one cars do not have huge wheels on them, they try to keep the rotating mass down and close to the center of rotation.

Also consider the difference in the weight of the steel wheels compared to the aluminum wheels.
 

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hey Sierra SnowTurtle
Thank you for description and explanation why, but I was thinking about this and though this can interpreted differently with out many confusing numbers.

-lets say that you get 20 miles per gallon on normal stock tires.

-Now lets say that you change tires for biger ones.

-Bigger equalls heavier.

-Heavier equals more mass to carry.

-More mass to carry equalls with engine working more.

-And if engine works more then it uses more gas.


I hope I'm not far from beig right :idea:
 
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You see, I said "wheels" not tires. F1 cars have small wheels with large diameter and width tires, I did not mention stock cars.?.. They keep the wheel weight down and fill the space with compressed air. Not to slag on you. :)


machman said:
Did you just say that drag cars and stock cars do not have large wheels on them? Not to slag you bro, but you should take a closer look. Indeed stock cars have very large tires on them, and drag cars even larger!

In any case, I'm glad you brought this topic up Sierra. I mentioned it one other time, and no one seemed to get it. I'm glad it seems like common sense now!

nothercarguy said:
Don't forget that larger tires also add more frontal area along with the more wieght, lower final drive ratio and increased rolling resistance.

If you were to recalibrate the speedometer you would find that larger tires still consume more fuel.

Also don't forget that RAV4's also can have 215/70/16 tires. I wonder how they compare to the 235/60/16's.

And the discussion of only 1% additional weight...you must calculate the change in rotating mass and it's distance from the centerline of rotation, not the total vehicle weight. This is the place you will see large power consumption differences with just a few pounds added. Since the tires and wheels make up the majority of this mass a small change will result in slower acceleration and greater power consumption. This is why drag cars and formula one cars do not have huge wheels on them, they try to keep the rotating mass down and close to the center of rotation.

Also consider the difference in the weight of the steel wheels compared to the aluminum wheels.
 
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