MPG & Tire Size

[CaptainRay]

I have a 2002 Jeep Wrangler 6 cyl, and I'm getting 18 City, 21 Hwy, running stock 27 inch tall tires. Anyone had any luck with taller tires, say 30 inch to 33 inch to up the MPG a bit and lower the RPM at hwy speed (65). I've heard going to tall with the tire will reduce mpg and power a lot. So I was looking for that perfect size and rpm match.

[GreenJeep10]

I have a 97 stock 6 cylinder with 30's and I get around 16 to 17 mpg, if I keep my foot out of it. On a trip a couple years ago I got around 19 on highway trip to Tenn.

[fredjacksonsan]

Yeah, generally upping your tire size will hurt mileage, unless you make a change to the gear ratio as well.

[Saxonwolf]

I think I read somewhere in this forum that when you go to a larger tire that it throws your odometer and speedometer readings off because youre actually traveling farther and faster than they read because of the larger tire. I have 33" tires on my TJ and as close as I can tell I get about 18 to 19 combination mpg.

[fredjacksonsan]

It does; check out this site:

http://www.1010tires.com/TireSizeCalculator.asp

It'll give you the % change, from which you can figure your new mileage and speedo indication.

[CanaMark]

I have bad gas mileage. But my o2 sensor is busted off.

[Saxonwolf]

Originally Posted by CaptainRay

I have a 2002 Jeep Wrangler 6 cyl, and I'm getting 18 City, 21 Hwy, running stock 27 inch tall tires. Anyone had any luck with taller tires, say 30 inch to 33 inch to up the MPG a bit and lower the RPM at hwy speed (65). I've heard going to tall with the tire will reduce mpg and power a lot. So I was looking for that perfect size and rpm match.

As far as power goes when you go to larger tires. Its been my experience that larger tires actually give you MORE power, at least in the lower gears. Where you loose it is in your top end speed. It seems at least with my TJ that my 5th gear just isnt able to turn the larger tires and maintain any kind of speed. But like Fredjacksonsan said, if you get it regeared it solves the problem.

[fredjacksonsan]

Actually, you have more [i]traction[/] with larger tires, due to the contact patch being much larger. The engine has to work harder, as the larger tires require more torque to turn them. That's why you can't keep speed in 5th, the motor isn't developing enough power to maintain the speed uphill.

[Saxonwolf]

Originally Posted by fredjacksonsan

Actually, you have more [i]traction[/] with larger tires, due to the contact patch being much larger. The engine has to work harder, as the larger tires require more torque to turn them. That's why you can't keep speed in 5th, the motor isn't developing enough power to maintain the speed uphill.

OK, that makes more sense. All I know is in 1st through 3rd I'm zippin' around like a sports car in 4th and 5th ..... not so much , but who needs 4th and 5th when youre goin up a mud hill?

[fredjacksonsan]

That's when you put it in 4 LO, and have all the power you need.