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Discussion Starter · #1 ·
After going from 31" tires to 32",I had to lower the PSI to 20 lb to put all the tread on the road (8" rims).Now I notice my mileage is down about 25%. Could the air pressure or the increase in tire size be hurting me that much. I was already over geared with the 31" and crappy 2.73 R&Ps.

 
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It's more than likely a combination of lower air pressure, wider tire (read bigger contact patdh on road), and the larger tire diameter.
You are correct that your diff. gears are to high. The taller tires just aggravated the problem. The only real solution is lower gears.

Gary
 
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Your gas milage probably has gone down SOME, but not as much as indicated. Larger tires move the vehicle further for each revolution of the wheel, but your speedometer is geared to revolutions NOT how far the vehicle moves per revolution, so you have to get the proper gear for your speedometer. If you still have the stock gear, your speedometer is probably way off.

Doug '97 TJ
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/wwwthreads_images/icons/tongue.gif Doug's got it figured out. You are actually penalizing yourself just in the numbers dept, not in reality. ALTHOUGH that gear ratio is nowheresville with that size rubber/wwwthreads_images/icons/frown.gif.

CJDave
 
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That's an awfully high ratio. Recon it's really a 3.73 instead of a 2.73??? Nothing like being able to run 70 mph in second gear. /wwwthreads_images/icons/tongue.gif

Doug '97 TJ
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You might be right. Nothing like trying to run 70 in high gear at 118 rpms, or being able to finally shift into high at 161 mph. /wwwthreads_images/icons/smile.gif/wwwthreads_images/icons/tongue.gif/wwwthreads_images/icons/smile.gif

Doug '97 TJ
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Discussion Starter · #8 ·
Ha ha ha. Fun at my poor Jeeps expense. But you are right. I can only use 4th effectively after surpassing 55mph. At 70mph I turn 2000rpms, but they're hard to hold. I have included the change in tire size for all calculations, using the "Gear ratio calculator" in the "Ask Grant" section on "Dirtroad.com". My speedometer if off about 10%. My hi gear ,4th, is equal to a 3.73 diff in .75 overdrive,according to that nifty calculator(check it out). I guess I'll air up those tires and check the mileage again. Wife says, "No more Jeep parts this X-mas". :(

 
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There's another problem we have not addressed. If the axle ratio - or overall gear ratio might be more accurate - is too high, you could easily hurt your gas mileage simply because the engine isn't strong enough to pull with the high gears. You keep your foot in it trying to keep the speed up, and the engine isn't strong enough to do it. No reflection on the engine either. With the overall ratio you describe here - 2.73s, large tires etc., your problem may very well be as described above as opposed to low tire pressure. Tire pressure will have an effect, but I think your overall gear ratio is the main problem.

Doug '97 TJ
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and i thought my 3.55's with 33's was bad.


79/CJ-7/AMC360/TH400/Q-TRAC/d30/d44/33's/Herculiner

 
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Hey, the 3.55s are a lot better. You can shift into overdrive at about 103 mph!!??!! /wwwthreads_images/icons/tongue.gif I'll tell you a story. In '83 I bought a new Mack tractor. It was my second rig, and I thought I had learned everything there was to know with my first tractor, an '80 GMC Astro single axle with a 290 Cummins and a 9 speed Fuller. That rig was WEAK. Anyway I got the Mack with a 440 hp Mack V-8 and a 9 speed. Don't remember whether it was a Fuller or a Mack tranny, but it was a 9 speed OD tranny. Don't remember what the axle gears were either, but I did some calculations, compared the numbers to the 290/9 spd combo that I had been driving and decided that I could change the rear axle ratio on the new truck and have a real strong hwy rig. Cars and Jeeps have a useful engine rpm range from roughly 1,100 to 1,5000 up to 4,000 or 5,000 rpms. Lots of usable rpm range. Big ole desel trucks, and the Mack particularly have a very narror rpm range. The 440 I bought worked from 1,200 to 1,900 rpms. Another thing to remember - back in '83 the speed limit was 55 all over the US. Something else, going from a 290 (hp) Cummins to a 440 Mack (also hp) is about the same as going from a 4 cyl Jeep to the same Jeep with a Dodge V-10 in it. Anyway, part of the deal when I bought the new Mack was changing the ring and pinions. (2 sets, a "twin screw," as opposed to a "single asle" - one driv axle)
When I took delivery on that new tractor, I thought I had the biggest, baddest thing on the road. At that time the Mack 440 V-8 was (900 and some odd cubic inches - 990, I think) without equal. It was a brute. It was so much stronger than anything else on the market, there just wasn't any comparison. Well, let me back up on that. There was a Cummins that had been designed for freight trains or tug boats or battle ships that some people had put in trucks that was stronger. Of course with the added weight of that engine a trucker could only haul about 3 or 4 bushels of produce, but he could haul it at better than 100 mph up hill and down hill. It really wasn't a practical truck engine. The Mack V-8 was almost up to the 600 hp Cummins specs, but with a size and weight that made it practical for over the road trucks.
Back to my "modified" Mack. I got the axle ratio changed. Took off for California on my very first trip. Got on the interstate under a load and it felt like the "4 wheeler" when he says, "I never even felt that trailer back there (boat trailer or a U-Haul) - until I shifted into overdrive, that is. To shorten this long story, I could not drive the speed limit - 55 mph. High gear, direct, one to one, worked real well in the high 40s mph and low 50s mph., but overdrive which was in the .7 something or other range, didn't get into the rpm band of the engine until the truck was running about 61 or 62 mph, well above the 55 mph speed limit. To get into overdrive I had to be on fairly level ground and let the truck gradually pick up speed until the engine was in its pulling zone. It would run 85 or 90 without any problem at all. Just would not run 55 mph.
All this is to say that gears are very important. Even with a very strong engine you can gear it improperly and lose everything the engine has to offer. A gasoline engined 4 wheeler generally has a much greater rpm range to work in, but even so, it is still possible to gear it wrong and wind up with a vehicle that cannot do anything very well.
My truck??? The return trip from California was through Dallas. I had the stock gears replaced,. Had to pay for it too. Cost a little over $2,000 if I remember correctly only because they were able to reinstall my original gears.
Jeeps, or any normal gasoline powered family vehicle, have a bigger rpm range to work in than my truck expampe, but any engine can be geared improperly and suffer greatly because of it. My truck expample is easier to understand because the working rpm band of the engine is so small, but any engine can be overwhelmed by the gears attached to it. One must be aware of the useful rpm range of his engine and gear accordingly.

Doug '97 TJ
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Re: gas, gears, and guts

/wwwthreads_images/icons/smile.gif Doug's (aka Papa Thermodyne)tale reminds me of my truckin' daze, when a 220 Cummins was considered really hot stuff. We always tried to gear for direct-direct for loaded cruise speed, and then have the over in the brownie for coming back empty. My '97 ZJ is intentionally geared so high that I cannot run overdrive unless everything is perfect, but oh, the fuel mileage!! When you put HUGE tires on your Jeep and don't change gears, THINK of the stress on the crankshaft! SHEESH!/wwwthreads_images/icons/crazy.gif

CJDave
 
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I have 3.07 with 33x12.50x15, it runs a nice 2200 rpm @ 70 mph. It works a lot better with the 350 than it did with the 258. I was thinking of gear changes too, but decided to swap in a 350./wwwthreads_images/icons/smile.gif I could drive this thing to the ends of the earth, I just wish my wife liked it as much as I do./wwwthreads_images/icons/frown.gif

80 CJ7 350, SM465, and lifted 4"
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Re: Jeep \"retail therapy\"

/wwwthreads_images/icons/smile.gif Here's what you do..Swayse. Every time you get your wife into the Jeep, you take her someplace where she can go in a store. Over time, she will associate the Jeep with "retail therapy", and it will subconciously work on her psyche. EVERY TIME she sits in that Jeep it needs to be associated with some good stuff; HER good stuff, not yours. Like you drive her to the next county to some tourist-trap antique stores. Chicks love that kind of stuff./wwwthreads_images/icons/crazy.gif

CJDave
 
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