Smaller tires... better gas mileage? - JeepForum.com
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post #1 of 16 Old 10-27-2008, 01:11 PM Thread Starter
serg23
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Smaller tires... better gas mileage?

Hello Jeep Lovers I'm a proud owner a 2000 GC Laredo 4x4. I have p245/70 r16 tires on my jeep. I was wondering if I get samller tires would it increase my mpg. I currently get about 23-24 mpg @ 60-65 mph and 20-22 mpg @ 70-75 mph.
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post #2 of 16 Old 10-27-2008, 01:14 PM
jeepdavetj
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From what I understand it would around town but you would loose mileage on the hi-way. Basically it makes it easier for the vehicle to get under power but will spin at a higher RPM out on the open road.


Also probably want to post in Tech forum. Get more and better informed responses most of the time.

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post #3 of 16 Old 10-27-2008, 03:49 PM
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http://www.grimmjeeper.com/gears.html

try this link, it'll give you all of the numbers for different tire sizes vrs rpms, its also helpful if you plan on regearing

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post #4 of 16 Old 10-27-2008, 04:46 PM
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It can be a little misleading because your odometer will not be accurate with smaller tires. It will say that you went further than you actually did which will make it appear that you got better mileage.

Your odometer calculates distance by counting the number of revolutions and multiplying it by the known circumference of your tire. If your tire is smaller, it will rotate more times in a given distance. Since your ECM is unaware of the smaller tires, it thinks you actually went further.

Let's say that you put on a set of 245/60 tires so you kept the same width, but had a smaller diameter. Your mileage would be off by 7%. If you were to simply divide the miles on your odometer by the gallons of gas, you would think you saw an immediate improvement in fuel economy. The truth is, you need to divide the miles traveled by 1.07 (or multiply by .935) before you do your mileage calculations.

For those of you who put on larger tires and assumed that "the gearing" made your mileage go to crap, the same thing applies. You are actually getting better mileage. I actually have to add 10% to my miles traveled when doing my fuel economy calculations.
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post #5 of 16 Old 10-29-2008, 07:12 PM
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I think you have have to look at the type of driving your doing. Smaller tires(to a point) will help MPG around town but hurt you on the highway. Either way too small or large and you could kill MPGs no matter what.

Example I have 35's on 307 gears. I get hurt around town and on the highway if I use 5th gear. But turning my 4th gear into my overdrive I get better MPG. 5th gear drops my RPM way too low and my engine has to work harder to maintain speed. If the tires were made too small I might make 1st gear useless and have to take off in 2nd gear. While in 5th gear my RPM would be too high and cost me MPGs.



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post #6 of 16 Old 10-29-2008, 09:25 PM
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Your mileage is really good for your grand. Most of us WJ guys see the 17 to 18 on the highway if we are lucky.

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post #7 of 16 Old 10-29-2008, 09:28 PM
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Quote:
Originally Posted by bmlawless View Post
It can be a little misleading because your odometer will not be accurate with smaller tires. It will say that you went further than you actually did which will make it appear that you got better mileage.

Your odometer calculates distance by counting the number of revolutions and multiplying it by the known circumference of your tire. If your tire is smaller, it will rotate more times in a given distance. Since your ECM is unaware of the smaller tires, it thinks you actually went further.

Let's say that you put on a set of 245/60 tires so you kept the same width, but had a smaller diameter. Your mileage would be off by 7%. If you were to simply divide the miles on your odometer by the gallons of gas, you would think you saw an immediate improvement in fuel economy. The truth is, you need to divide the miles traveled by 1.07 (or multiply by .935) before you do your mileage calculations.

For those of you who put on larger tires and assumed that "the gearing" made your mileage go to crap, the same thing applies. You are actually getting better mileage. I actually have to add 10% to my miles traveled when doing my fuel economy calculations.
I'm pretty sure the gear that corrects the speedometer corrects this too

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post #8 of 16 Old 10-30-2008, 01:59 AM
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Originally Posted by omarambo101 View Post
I'm pretty sure the gear that corrects the speedometer corrects this too

So does a navigation system. As well as already knowing the exact miles to a place.

I had this fight with a friend that could not believe the MPGs I could pull down with big tires,roof racks or light bars,heavy bumpers and the works. He made the claim my odo, was wrong giving me false numbers. But I knew the miles driven so the odo no longer mattered. To become a believer I drove his truck and scored almost double the MPGs but he said he could never drive like that without having a mental break down.


Chris.

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6.5" Rusty's Lift, Atlas II Rims, 35x12.5x15 Wild Country,Rusty's Tube Bumper, Wilderness Rack
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I-ROK reinforcement kit, ORO U-TURN steering kit,Conversion Headlights/Silverstars And too much $$$
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post #9 of 16 Old 10-30-2008, 02:40 AM
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You have a stock tire size. Leave it the way it is. It will cost you a lot more to get new tires than what you will save on gas. If you want good mileage buy a Prius. 12.3MPG ftw.
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post #10 of 16 Old 10-30-2008, 08:15 AM
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Quote:
Originally Posted by omarambo101 View Post
I'm pretty sure the gear that corrects the speedometer corrects this too
That's absolutely true, but the OP didn't mention that they would be replacing the speedo gear. If anyone swaps tires, I would recommend swapping the gear. It is inexpensive and easy to do.

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post #11 of 16 Old 10-30-2008, 06:43 PM
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Quote:
Originally Posted by kg4kzv View Post
So does a navigation system. As well as already knowing the exact miles to a place.

I had this fight with a friend that could not believe the MPGs I could pull down with big tires,roof racks or light bars,heavy bumpers and the works. He made the claim my odo, was wrong giving me false numbers. But I knew the miles driven so the odo no longer mattered. To become a believer I drove his truck and scored almost double the MPGs but he said he could never drive like that without having a mental break down.


Chris.
ya, except there's a little bit of a price difference between the gear and a GPS, but I know what your talking about, I used a GPS to check my speeds until i got the new gear, and its amazing what going lighter on the gas will do to you milage

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post #12 of 16 Old 10-30-2008, 07:03 PM
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Quote:
Originally Posted by omarambo101 View Post
ya, except there's a little bit of a price difference between the gear and a GPS, but I know what your talking about, I used a GPS to check my speeds until i got the new gear, and its amazing what going lighter on the gas will do to you milage

Even with the gear you could still be off been there done that in the old hot rod days. But yes it is much cheaper but now having the nav system I would have to replace it if it ever broke.

Yes keeping a light foot on the gogo peddle helps but so does a lot of coasting those are near free miles.



Chris.

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post #13 of 16 Old 10-30-2008, 07:23 PM
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The gear is irrelevant, works different in the WJ in question




The increase you'd see from smaller tires would not at all be worth the cost of them. I wouldn't go much smaller than the smallest stock size available (225/75/16) either. Actually, I wouldn't go smaller at all, but then again I lift in the face of high gas prices

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post #14 of 16 Old 10-31-2008, 02:01 AM
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psssttt.......you drive a jeep not a civic or prius.

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post #15 of 16 Old 10-31-2008, 08:58 AM
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Originally Posted by Prot View Post
psssttt.......you drive a jeep not a civic or prius.

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