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Black 08 Tundra crewmax 4X4 5L
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Hey guys, i have a 2008, 5.7l, crewmax, sr5 which gets 11.5 to 12 miles / gallon, i drive it mainly in los angeles, a lot of freeway driving at 75mph and city driving, stop and go at 40,
I am buying 20" wheels, up from tyhe stock 18"s, and was thinking of going from a 275/65/r18
that is 32 inches diameter to a 275/65/20 which is 34 inch diameter, reducing the revs per mile from aroung 650 to 620.
Any one have experience on whether that will help with milage? obviously ill loose a little towing capacity, but i rarely tow anyway.
thanks
David
 

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Mark it eight, Dude...
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Not sure how that works, but if the new set of wheels and tires are heavier than what you have now that will decrease mpg.
 

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Under the right conditions it might improve things a little, but overall you will probably not see any improvement.
As MaylonTRD says, any increase in the rotating weight will reduce your mileage.
 

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Under the right conditions it might improve things a little, but overall you will probably not see any improvement.
As MaylonTRD says, any increase in the rotating weight will reduce your mileage.
No increase in tire size with today standard gearing will ever give you an increase in mileage. Larger, heavier rotating mass, plus to some degree(with the size you mentioned) reduces the effectiveness of your gearing, which means your trans will downshift more(especially with the pitiful 6th gear in the Tundra).
 

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When you use a larger wheel and larger tire, more mass is distributed from the axle. This means 'moment of inertia' is increased, making it more difficult to accelerate and decelerate. This means the engine and the brakes have to work extra hard. It seems, from reading mpg reports from various members, that the 'distance advantage' of larger tire is smaller than the harder working engine disadvantage. Has anyone observed an improved mpg after installing a bigger tires and wheels?
 

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Driving style will probably afford you greater mileage. If you slow down a little on the freeway and limit your stop and go driving, you can probably get a couple more miles per gallon. When you can't avoid stop and go driving, limit racing up to the next stop light and slamming on your brakes. If you know you are gonna have to stop at the next light anyways, gently accelerate to a reasonable pace and gently slow to stop at the light. Even when I drive around town I get 14 or 15 mpgs in my 5.7.
 

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think of if this way.... if your odometer is calibrated for a certain outer diameter tire size, it knows that if it tuns X revolutions you have gone Y miles. When you increase your tire diameter size and do not adjust your odometer, that calculation is not no longer accurate and will underreport how many miles driven and still report how much fuel was burnt so your fuel mileage goes down.
 

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I have 275/45X20s on my '04 DC. No complaints. I don't know that they affect gas mileage one way or another. They were "stock" on my truck ... part of a package the Gulf States Toyota Distributor cobbled together and called the TSS--Toyota Sports Series.
 

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Wheel weight and the HP needed to drive heavier tires is a small part of why bigger tires give worse MPG. The primary factor is because you increase the frontal area of the vehicle, and increase the coefficient of drag (multiple ways).

Expect a .01% reduction in COD every inch the vehicle is raised. (*.01% doesnt seem like much but its murder to MPG)
Expect an increase in frontal area directly related to a 1" section out of the middle added to the overall frontal size for every inch the vehicle is raised.

There is also a hefty loss in wider tires because of frictional losses and increased FA related to the wider tires vs OEM sizes.

~JH

AND...

auto manufactures spend a great deal of effort getting the air to go over the body and very little going under. The more air going under will also hurt the overall CdA numbers.
 

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Wheel weight and the HP needed to drive heavier tires is a small part of why bigger tires give worse MPG. The primary factor is because you increase the frontal area of the vehicle, and increase the coefficient of drag (multiple ways).

Expect a .01% reduction in COD every inch the vehicle is raised. (*.01% doesnt seem like much but its murder to MPG)
Expect an increase in frontal area directly related to a 1" section out of the middle added to the overall frontal size for every inch the vehicle is raised.

There is also a hefty loss in wider tires because of frictional losses and increased FA related to the wider tires vs OEM sizes.

~JH

AND...

auto manufactures spend a great deal of effort getting the air to go over the body and very little going under. The more air going under will also hurt the overall CdA numbers.
That's an interesting factor. I suppose the faster one drives (e.g., a lot of highway driving) more significant will the drag.

Let me try to understand this. How does raising the truck increase the frontal area of the vehicle? It seems that the area increase due to a larger tire size will be very small. I assume that the coefficient of drag is proportional to the frontal area.

Or rather, is the coefficient of drag related also to the volume of the air flowing under the truck? I could see how, since the upper side of a truck is more aerodynamic than the underside, more air flow under the body translates to higher air resistance. Does the 'effective frontal area' takes this into account? Perhaps then the effective area includes the area between the road and the top of the truck. Then I could see how the frontal area of the truck will increase with a larger tire size.
 

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Let me try to understand this. How does raising the truck increase the frontal area of the vehicle? It seems that the area increase due to a larger tire size will be very small. I assume that the coefficient of drag is proportional to the frontal area.

Or rather, is the coefficient of drag related also to the volume of the air flowing under the truck? I could see how, since the upper side of a truck is more aerodynamic than the underside, more air flow under the body translates to higher air resistance. Does the 'effective frontal area' takes this into account? Perhaps then the effective area includes the area between the road and the top of the truck. Then I could see how the frontal area of the truck will increase with a larger tire size.
The 'true' aero number is called CdA. CdA is coefficient of drag X Fa (frontal area). With the CdA number you can calculate a number of factors including HP per MPH or MPG, HP needed to achieve (X) speed and other factors. A Boeing 747 has way better aeros then the tundra but requires 1000's of HP needed to move it while a motorcycle has bad aeros and can go fast (relatively) with a small amount of HP. This is why Cd and Fa are needed to get a true profile of a vehicles aeros.

I could get real technical about the physics behind the reasons the vehicle will gain in frontal area when its raised but it has to do with the turbulent flow under the vehicle, ground affect and other factors that basically mean every inch the vehicle is raised, the air sees it as a wall 1" taller. You would need to have a vehicle raised a good 5-6' off the deck before air could effectively go under the vehicle without being disturbed and negatively affecting the Cd-CdA.

There is some good info here:
Car Aerodynamics 101

but....its riddled with generalizations and some misinformation (overall its pretty good)


~JH
 

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The 'true' aero number is called CdA. CdA is coefficient of drag X Fa (frontal area). With the CdA number you can calculate a number of factors including HP per MPH or MPG, HP needed to achieve (X) speed and other factors. A Boeing 747 has way better aeros then the tundra but requires 1000's of HP needed to move it while a motorcycle has bad aeros and can go fast (relatively) with a small amount of HP. This is why Cd and Fa are needed to get a true profile of a vehicles aeros.

I could get real technical about the physics behind the reasons the vehicle will gain in frontal area when its raised but it has to do with the turbulent flow under the vehicle, ground affect and other factors that basically mean every inch the vehicle is raised, the air sees it as a wall 1" taller. You would need to have a vehicle raised a good 5-6' off the deck before air could effectively go under the vehicle without being disturbed and negatively affecting the Cd-CdA.

Car Aerodynamics 101

but....its riddled with generalizations and some misinformation (overall its pretty good)


~JH
Thanks for the information and the link. That's helpful. The physics behind it is simple. When air flow is streamline flow, the friction is proportional to the speed whereas when air flow is turbulent, the friction is proportional to the square of the speed. So since typical lifting still causes turbulent flow under the truck, the 'effective frontal area' increases when the vehicle is raised thereby increasing the drag. In order to find out how high you have to raise the vehicle before getting non-turbulent flow has to be determined empirically.

I assume you mean that when the vehicle is raised 5-6 of the ground, the flow under the vehicle it is no longer turbulent. Do you know if the mpg is improved when the vehicle is raised more than 5-6? (discounting other factors, of course). If that's the case, there is a very good argument for raised trucks.
 
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