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Gas mileage effect: Larger tires


 
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guesswho369
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Joined: 07 Sep 2005

PostPosted: Wed Sep 07, 2005 2:18 pm    Post subject: Gas mileage effect: Larger tires Reply with quote

Hello all

New to the forum and jeeps - and you guys (and girls) soudn like you know your stuff, so here goes...

I just picked up a 97 TJ SE about a month ago and have thus far only done minor work on it (oild change, brakes, SP, dist, etc).

97 2.5 l
50k miles
15x10 in wheels
33 inch tires
Magnaflow catback exhaust
Banks Torque-tube header
High-flow air filter
3-inch (body? Susp?) lift

My question (first of many): I'm not going to be taking this thing off-road as much as I'll be on highway. I'm currently getting about 14-15 mpg average, and have to struggle to keep it at 65 on the highway crossing low-angle overpasses. If I downsize to 31-inchers will I notice any speed or mileage difference?

2nd question: I thought it was a suspension lift, but someone the other day said he thoguht it was a body lift. How do I tell the dif?

Thanks!

Andy
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ehirner
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Joined: 01 Jan 2004
Location: Eastern PA

PostPosted: Wed Sep 07, 2005 3:40 pm    Post subject: Re: Gas mileage effect: Larger tires Reply with quote

guesswho369 wrote:
My question (first of many): I'm not going to be taking this thing off-road as much as I'll be on highway. I'm currently getting about 14-15 mpg average, and have to struggle to keep it at 65 on the highway crossing low-angle overpasses. If I downsize to 31-inchers will I notice any speed or mileage difference?

First, verify that your speedometer gear was changed to compensate for the larger tires. If it was not changed, you're actually getting better mileage than you are now...probably about 10-15% better.

If you downsize to 31s, you'll gain some performance back but possibly at the cost of mileage. The reverse is that slightly larger tires without changing gearing will actually give you better mileage.

Quote:
2nd question: I thought it was a suspension lift, but someone the other day said he thoguht it was a body lift. How do I tell the dif?

Look inside your rear wheelwells. If there's a gap large gap between the top of the frame rail and the plastic fender liner, then you've got a body lift. If it's a 3" BL, I'm sorry. First, because the gaps they create are hideous. But, second, more so, because they really put some strain on your stock body mounts.
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KnightFire
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Joined: 07 Jan 2004
Location: Hanover, MD

PostPosted: Wed Sep 07, 2005 11:35 pm    Post subject: Reply with quote

Just to add to ehirner's post...

I believe that decreasing tire size you will gain both mpg and power, you will have less rotating mass and a smaller foot print to tend with.

Easiest way to tell if your lift is body or suspension, look under the Jeep, and measure the size of the body pucks (the mounts that tie the body/tub of the Jeep to the frame), if you do have a 3" body lift (3" pucks) get rid of em, put a 1" BL and get a 2" suspension lift. Or, if power and mpg is you main concern, simply use a 1" body lift and don't worry about adding a suspension lift, the higher you raise the Jeep, the lower your mpg will be (wind resistance).

KnightFire
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1fox2go
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Joined: 01 Dec 2004
Location: Fishersville, Virginia

PostPosted: Thu Sep 08, 2005 4:36 am    Post subject: Reply with quote

Your powerplant is the 4 cylinder, I think that is where your mileage went. That motor has to work to get those big tires turning

i agree going to a 32 31 would help in take off and mileage because they are both bigger than stock
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Last edited by 1fox2go on Thu Sep 08, 2005 7:31 am; edited 1 time in total
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TJgirl
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Joined: 15 Jan 2004
Location: California

PostPosted: Thu Sep 08, 2005 6:53 am    Post subject: Reply with quote

welcome guesswho, enjoy your jeep!
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guesswho369
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Joined: 07 Sep 2005

PostPosted: Thu Sep 08, 2005 6:57 am    Post subject: Reply with quote

Thanks to all that replied so fast!

It is indeed a body lift...amazing how those gaps are now going to bother me until I get a suspension lift put on. Ignorance is bliss, right? Shocked

I haven't changed the speedo gearing, but have a handheld GPS and figured the ratio out for 40, 50, 60 MPH, so I wind up doing the simple math in my head as I'm driving. It sounds as if I'm going to be downsizing those 33-inchers anyway; I think I'll probably call up0 some tire places and see if they can give me the weights on 30, 31 and 33-inch tires so I can decide whether a reduction to 31 is worth the risk.

P.S. Thanks TJGirl Smile
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'drack
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Joined: 01 Jan 2004
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PostPosted: Thu Sep 08, 2005 7:48 am    Post subject: Reply with quote

Don't forget 32's as well. They'll be an inch shorter and thinner, cutting down on the rolling mass a bit. It's apples and oranges, but on my 6cyl, when I went from 32's to 33's is when I really noticed a decrease in performance. Biggest I ever went with a 4cyl was 31's, still adequate power there, but ultimately decided to go with a 6cyl in the end.
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guesswho369
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Joined: 07 Sep 2005

PostPosted: Thu Sep 08, 2005 7:57 am    Post subject: RE:32 inch Reply with quote

You had one - you know what a 4cyl performs like; while I might notice a difference from 33-32, I'm counting on a bigger one going to 31-inchers. I knew in getting a 4cyl I wasn't buying a highway speed-demon, but I was suprised at what a small headwind did to my ability to keep up, and gas mileage as well. I realize that the "brick factor" has much to do with this, but I'm hoping that by decreasing tire weight and diameter will give me a little more cruising power.
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