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Johnny Bling Bling
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Discussion Starter #1 (Edited)
i had the original 265/70/17's with oe aluminum rims on my expy when i first dyno'd it. the result was 339 rwhp, and 440 rwtq. a year later i went back to get the speedo corrected with the 275/55/20'sand oe aluminum rims, and redyno'd at 345rwhp, but 430 rwtq. he said that the tq loss was because the bigger wheel/tire combo was heavier. he didn't know why the hp went up! the time of year was almost exactly a year apart, and the weather was similar(cold, winter). makes sense to me.

does this have an effect on mpg as well? i have a 1200 mile 1 way trip in 6 weeks, and if i can pick up some mpg's easily by switching back to the oe smaller setup, it may be worth it. if not, i'll leave the 20's on....they look a lot better!

original rims 27 lbs ea x 5; 265/70/17 wrangler rt/s 39lbs ea x 5

20" oe rims 44lbs ea x 5; 275/55/20 scorpion atr 48lbs ea x 5
 

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i'm no professional, but it seems like you'd get better MPG with a bigger wheel/tire. more distance traveled per axle rotation.

that sounds right, but the weight may be the factor
 

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A larger tire has the same effect of higher rear gears. That's why you lost torque and gained a bit of horsepower.

You might see a bit of improvement in mpg, but I doubt it would be more than 1 point.
 

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original rims 27 lbs ea x 5; 265/70/17 wrangler rt/s 39lbs ea x 5

20" oe rims 44lbs ea x 5; 275/55/20 scorpion atr 48lbs ea x 5
[(265mm * .70) *2] *0.0393700787 + 17 = 31.6062992 inch diameter
[(275mm *.55) *2] *0.0393700787 + 20 = 31.9094488 inch diameter

Change of 1%

So not only does the vehicle have to move the wheel more for a full rotation but it has to push heavier stuff to do it. Yes that's making the truck do more work than the OEM. The larger gripping width will also hurt MPG, that's why the high-mpg cars have such thin tires. Will the 20's cause a noticeable decline in MPG? Doubtful.

And if he did an SAE corrected graph like any reputable place will, then any difference in temperature is already taken out of the equation.
 

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Johnny Bling Bling
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Discussion Starter #5
so....it's not going to make enough of a difference in regards to mpg, and i should just leave the 20's on?
 

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Gauge master
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The fuel economy will suffer due to the heavier tires. It is a matter of inertia. The larger tire & wheel combination (which is heavier than the stock ones) have a higher amount of inertia. Basically, the main effect of inertia is that it resists a change in speed. The higher the inertia, the harder it is to either speed up or slow down the flywheel, which is basically what the wheel and tire combination becomes. As the weight moves outward from the center of rotation, the inertia increases. So it is not just a matter of the weight, but where that "radii of gyration" is in relationship to the rotational axis.

We manufacture some large hammermills that have flywheels on them to help the machine "power" through a surge of material that is to be shredded. We have to calculate the amount of inertia (in units of lb-ft^2, if anyone cared) so that the motor manufacturer can specify a motor that has a load capacity greater than that number.

The heavier tires and wheels will most likely move the weight farther away from the rotational axis and require more energy to speed up and greater braking forces to slow down. This is why some companies strive to reduce the weight and the inertia of their wheels. It also affects the unsprung weight of the suspension. This is also a measure of the amount of inertia, but this time it affects the response time of the suspension.

I hope that makes sense.
 
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