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Discussion Starter #1
You own a 6 speed manual transmission Jeep with 30" tires. You drive 100 miles to get to your favorite wheeling spot. When arriving, you check your mpg on the jeep readout and are getting 20 mpg.

Of course you need bigger tires. You install tires that are the same width and weigh the same, but are 33" tires. The gears stayed the same. You drive the same 100 miles again, but this time it shows the jeep is getting only 18 mpg.

For the sake of discussion, we won't count wind resistance. We will also assume that you are a good driver and operated the jeep as efficiently as possible. So what is causing the drop in mpg? :confused:
 

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The fact that the Jeep readout isn't taking into account the larger tires because you didn't mention recalibrating the computer. Your odometer will also not register 100 miles. In all actuality you traveled more miles than what the computer is figuring into your mpg readout.
 

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Discussion Starter #3
That was fast - We have a winner. :winner:

The reason I brought this up is I seem many threads about recalibrating and getting better mileage. Sometimes up to 5mpg on a automatic and up to 2 on a manual. I am sure optimizing shift points on a auto tranny helps a lot. The reality is that some of it is the odometer getting bad data.

If going from a 30" to a 33" tire, it is a 10% change in diameter and circumference. So the jeep travels 10% further per rotation. Being as the jeep traveled the same 100 miles, the odometer measured 10% less or only 90 miles. But it still took basically the same amount of energy (gas) to move the jeep. The computer figures the same gas amount but used it in 10 less miles thus making the mpg appear worse.

So when looking at your mpg drop with bigger tires, take into account the odometer error can be contributing.
 

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Taller tires will also reduce MPG because it alters the aerodynamics, increasing frontal area.
 

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Longer moment arm working on the rotational weight (mass) of the larger tire requires more energy to accelerate requiring more fuel. Just because it is the same weight doesn't mean there is no weight effect from a larger tire.

Just because one tire says 30" and another says 33" don't assume that the larger tire will read 10% less miles traveled, you need to measure each tire to get a true set of numbers for the calculation. If they were true sizes then yes, r2/r1 would give you the % change in circumference, but they never measure as advertised.

And yes, you need to use actual distance travelled, most people program their new tire size so they are closer to comparing happy apples to crabby apples when they report lower mpg numbers.
 

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Discussion Starter #6
Hey this was a high school question. Yes there are many other factors that play into true mpg reduction. The main point here was that poor mpg readings are significantly effected by bad data in the calculation and are not true mpg readings. Hence the need to recalibrate the speedo.
 

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In my high school, these questions always pertained to trains. If they woulda been about Jeeps, I think I woulda paid more attention!!
 
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