Searching for proof of better MPG after calibrating for larger tires

I recently upgraded to 35s and I'm planning to get a programmer soon. But all this MPG stuff has got me thinking...

So everyone claims to loose MPG when they get bigger tires, pretty common knowledge. I think everyone also agrees that MPG will go back up a bit after you program the new tire size. I'm not trying to debate these things, I'm just trying to understand the facts on how people get this data.

With larger tires and no computer correction people agree the speedo will be slower than your actual speed. This is because with larger tires you travel more distance for the same amount of wheel revolutions. But if this is true for the speedo it will also be true for the odometer. Therefore, if you calculated your "bad MPG" using the milage shown on the odometer, it is inaccurate. Your odometer may show 200 miles traveled, when in reality you may have traveled 225 (these are made up numbers used to illustrate my point).

Using these made up numbers for math:
If it takes 15 gallons to go 200 miles.
200/15= 13.33mpg

If it takes 15 gallons to go 225 miles.
225/15= 15mpg

So you correct the computer for the new tire size, now your odometer shows 225 miles instead of 200 for that same tank of gas. And there you have "better" MPG.

The only way to get accurate data is if you track the miles driven using something other than the jeep odometer (maybe there's an app for that). Then calculate MPG that way, both before and after the computer correction.

Maybe you guys already know this and I'm just late to the party, but I haven't seen it discussed. Hopefully this is some good food for thought.

I'd love to hear if anyone has this type of accurate data. But I will find a way to accurately track my mileage now, then update this post once I get a programmer.


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Welcome to Wayalife. :crazyeyes:


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When you program for the bigger tires the trans doesn't hunt for gears as much, which makes it downshift less therefore getting the better mileage


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I've read about better shift points a lot, and I actually do believe it's true. I just think the numbers are inflated. Calculating MPG with the wrong numbers as illustrated above will show gains without taking into account the shift points. So people claim it goes up like 3-4mpg. I'm trying to get enough data to figure out how much of that is just the math, and how much is actual gain.

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You drive a brick in the wind. Adding bigger tires will only make things worse. If MPG is really that much of a concern, a Jeep may not have been the right vehicle to buy. If you really enjoy what you have, the sooner you get over MPG the happier you'll be. But hey, that's just me.

You're thinking about MPG's too much LOL. Driving a jeep, that should be the last thing on your mind. I do say that living in TX where gas is dirt cheap right now though :cheesy:

I think most people buy the programmer to correct the shift points and speedo. This is for drivability, not MPG.

If you're worried about gas mileage, buy a Prius. Then you can park behind Jeeps and complain about paint chips.

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An LS will get a built up jeep around 20mpg at 70mph. That's right about what a stock sport gets. The wrangler isn't appealing because of fuel economy, it's about smiles per gallon and adventures.

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Sell your Jeep, buy a Prius and you may even get a handjob from a hot little eco-nazi that follows AL Gore.

I love my Jeep, wouldn't trade it for 100mpg. I knew the MPG was going to suck, and even more with bigger tires. But I'd still like to save a few bucks at the pump... those bucks can be put toward more parts... so there's that. Plus I've just been thinking about it a lot, and I figured there might be someone out there that has already thought about it as well.

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DWiggles said:

An LS will get a built up jeep around 20mpg at 70mph. That's right about what a stock sport gets. The wrangler isn't appealing because of fuel economy, it's about smiles per gallon and adventures.

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Click to expand...

Sorry I'm drawing a blank... LS?

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JeepinDingo said:

Sorry I'm drawing a blank... LS?

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Click to expand...

It's a motor. You'd have to do an engine swap.

Stay thirsty my friends!


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JeepinDingo said:

I love my Jeep, wouldn't trade it for 100mpg. I knew the MPG was going to suck, and even more with bigger tires. But I'd still like to save a few bucks at the pump... those bucks can be put toward more parts... so there's that. Plus I've just been thinking about it a lot, and I figured there might be someone out there that has already thought about it as well.

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Click to expand...

Stop thinking about it. No one in their right mind should own a jeep if they're worried about MPGs.

You use the word "claim" in your original post saying that owners claim to lose MPGs when modding. This is not a "claim", this is FACT. Correcting your speedo does not change what you're actually getting MPGs anywhere except for the stupid MPG readout you like to stare at. Adding a tune that will change shift points CAN improve your MPGs but it will still be less than stock numbers. So when owners say that a tuner improved their MPGs that's what they are talking about. Actual physical numbers, not the stupid lying ass trip computer. Stop looking at that and stop thinking about fuel economy. You are not going to save money at the pump with a Jeep. Not even a stock jeep.