Do Bigger Tires Calulate to Less MPG?

[Bridge]

Before my modifications (listed below) I was getting around 12 mpg. Now, after TFI plus, plus, I cannot get over 9 mpg. It was 8.5 before tfi and after. I did not do much measuring right after putting on the 31 inchers. but the bigger circumference should mean less rotations per mile. If they were going to throw the odometer off, wouldn't it be for the better?

[Crazy_Jeepman]

It will throw the odometer off, you will travel more miles than what is registering. Bigger tires will help with increasing MPG in lower geared rigs, by cutting the engine RPMS down. However if you have high gears it will make it worse, because the engine is always under load or more often than not pulling to keep up to speed, working it harder on the hills, ect. ect.

[Bridge]

Based on that Jeepman, I will continue reading about lowering my gear ratio? I am pretty sure that my 1990 GW has 2.something, but I could swear. I see a "ratio plate" that has a number starting with a "2" so I guess I have room to change to 3.31.

[Millerluck]

With larger tires you will travel farther than a real actual mile....drive on the highway and use the mile markers to find what a true 10 mile trip = on your odomater. Then you will need to find the % it's off.

So if it's off and you find that you drove a true 10 and you only rolled on 9.8. Take that into acount the next time you go through a tank of gas and then add up 98 mile for every 100 you get.

Or....

9.8/10=x/miles or 9.8 times miles div by 10

[Bridge]

Makes sense. Thanks.

I hope the ratio is such that I can finally get into double digit mpg. Once this milestone is achieved, I might be able to relax on all the work I am doing on the GW....naaaaa.

[SpruceMoose]

you should be getting 12-13 mpg. i am, and your rig is similar to mine. if you are getting substantially lower, sumtins rong! check the timing, mixture, etc.

sm.

[Mikel]

Don't forget that bigger tires make your tire taller and less aerodynamic (in a FSJ, hahaha!!!), and the inertia when starting to move and when stopping increases too.

[Bridge]

Now we are onto something Sprucemoose. I have never adjusted the timing. (Just didnot want to spring $70 for the gun). The "Juicebox" guy said timing would not need adjusting after the TFI. I'll get it checked out though.

[PhilSine]

You can do a simple percentage calculation based on the OEM tire size and your new tire size.

My OEM tire size was a 31" tire. My new tire size is 33".

By calculating the circumference of both tires and dividing the bigger by the smaller I get a difference of 6.4%. Now when I go through a tank of gas and it says I went 200 miles I multiply that 200 by 1.064 and get an actual distance of 212.8 miles. If I went through 19 gallons of gas then that would be a difference of .67 calculated MPG.

As it is, my MPG is around 13. By using my example, you can see that larger tires should not affect your MPG by that much. .67 MPG is not a lot cansidering 2" more of verticle rubber from OEM. Chances are there is something else wrong with your rig that is causing the added fuel consumption. As others stated, a gear ratio that is too low coupled with larger tires would have a negative effect on MPG. I'm running 3.54 gears and if my tires were any larger than a 33 then I would have to change my gearing.

[derf]

I'm running 3.73's with my 31" tires. Great for off-the-line performance and towing but I have a hard time breaking into the double digit gas mileage range.

[Bob Barry]

Also realize that not all factory FSJ speedos are 100% accurate from the factory. With my nominal 31" tires (actual about 29"), I checked the speed indicated on my speedometer against the speed that is shown on those unmanned radar guilt billboards (you know the ones: "Speed limit is 55mph; your speed is 87mph"), and my actual speed was consistently about 10% higher than my indicated speed (66mph, when the speedo said 60mph). So though I calculated about 12.7mpg for part of my trip, the actual figure was just over 14mpg. I have no idea how much the tires I'm running contribute to this inaccuracy.

[DirkR]

Speeding. Don't forget that with bigger tires
you'll be goin' faster than yer speedo says if
you don't have the speedo gears swapped out.

It's best to follow someone instructed to keep
at a sharp 60mph, and calculate what percentage
yer speedo is off as well.

Ya don't want to get stopped by Ol' Smokey.

-Dirk R.

[Manhattan]

I used my GPS unit to calculate my speed - I was off by about 6% with 31"s. I'll check again after swapping on the 32"s this week.

[Tim728]

Manhattan- Thats a great Idea, I am always looking for new ways to use my GPS. By the way, great pics!! do yo plan on posting new ones after the tire change?

[Wart Hawg]

Originally posted by DirkR:
Speeding. Don't forget that with bigger tires
you'll be goin' faster than yer speedo says if
you don't have the speedo gears swapped out.

It's best to follow someone instructed to keep
at a sharp 60mph, and calculate what percentage
yer speedo is off as well.

Ya don't want to get stopped by Ol' Smokey.

-Dirk R.Well that 'splains why I seem to pass everyone when I'm going 65mph. I guess I need to quit yelling "THE ACCELERATOR IS THE SKINNY ONE ON THE RIGHT!!!"

[Manhattan]

Originally posted by Tim728:
Manhattan- Thats a great Idea, I am always looking for new ways to use my GPS. By the way, great pics!! do yo plan on posting new ones after the tire change?Thanks. Yeah - I'll post some pics once I do the new BJ's Offroad 4" system.

The GPS unit is the only way I can accurately judge speed. The unit has a "dashboard" setting with speed, heading, etc. in a large display.