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Recently went from 30x9.5 tires to 31x10.5 BFG AT KOs and my MPG has gone to the crapper. going the same places I used to I fill up more often. It feels like if I went any bigger I would HAVE to get gears. I have an '89 YJ with the 4.2 I6. It also has a Flowmaster 2 chamber muffler, 5 spd, and 3.55 gears. Would 3.73s or so give me back my lost MPG? My dad want's me to go back to 30s, but the 31s look so damn cool. Would upgrading the ignition system with GM HEI, MSD 6AL igniton box, Bosch 4 plugs and MSD wires help? how would the carb work with he HEI? my carb is tuned good so I would only replace it with a whole V8 setup.
Tas




 

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Gears will get you back to were your milage was.
If you are thinking about EVER getting even bigger tires than what you put on it now forget the3.73's and think 4.11 or more likly 4.56(gears are$$$)
Milage died because it takes more pedal to maintain the same speed.....the 258 is'nt the biggest powerhouse around. If you can swing it , get the gears and you'll be happy/wwwthreads_images/icons/smile.gif
DO THE HEI!!!!!/wwwthreads_images/icons/cool.gifyou won't believe the difference.
Forget the bosch plugs/wwwthreads_images/icons/frown.gifI think they are the worst plugs on the market!
I'm kinda partial to a/c delco rapid fires but$5 a plug/wwwthreads_images/icons/frown.gif

85'CJ7 258 4" runnin33's
 

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gears will improve your MPG ( if it puts your normal cruise speed back into the powerband RPM range), but I doubt get it back where it was before the tires, since the new tires are heavier, and have more rolling resistance blah blah..

Ive heard GREAT things about the HEI swap..



ozarkjeep
1977 CJ5 looking for a Hard top near NW Arkansas!
 
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If you want to run 31 or larger tires, go with 4.10 gears...they will keep you in the proper range up to 33" tires.

John......southern CA
84CJ7, 3"lift, 32"BFG, 4.10's, ARB Locker, Solid Axle's, Durabak
 

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I ain't buying it! You only went 3% larger with the tire size, maybe 4% if the old tires were worn slick
unless the 30s were really overstated 28s or 29s. I think your problem is the wider tire with a more
aggressive tread pattern and the mileage won't come back. Check the inflation and make sure the front
hubs aren't locked. The price of gears will buy a lot of gasoline and I don't see anywhere in your profile
or on your webpage that you off-road. When you own a Jeep there are two things you can't worry about,
getting wet and gas mileage.


 
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If it's not already, change the diff fluid to synthetic... I changed all my fluids to Mobil 1 and I could actually feel less rolling resistance... probably only gives you about .5 to 1 mpg though....

Steve

1994 YJ Sahara
 
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Hey Taz,

Your speedo and trip may also be altered, stating falsy that your mileage went to hell. I had this happen to me on my Toyota. I thought my mileage sucked until someone brought this to my attention

My .02

Kevin Speicher
New England 4x4 Club
http://146.189.121.83/
79 Jeep CJ-7
 

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/wwwthreads_images/icons/tongue.gif There have been several(really all of 'em) good posts here, and lots of items that need to be looked at, like the accuracy of the speedo and so forth. Gas mileage usually isn't ONE thing, it's a lot of odds and ends. You need to sweep out EVERY corner on this, and check into every possibility from ignition timing to tire rolling diameter. MATCH those tires by adjusting air pressure till the top of the rim is EXACTLY the same height off the ground (hot). CHECK that timing and experiment a little. Now that the pinion speed is lower, and tooth pressure is higher, the special gear lube MIGHT make a measureable difference. The same goes for the main box and the t-case. Also, what about alignment. If that Jeep is "pushing", it would really suck up the HP./wwwthreads_images/icons/crazy.gif

CJDave
I never believe any statistics unless I've made 'em up myself.
 

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I like the comment about gas mileage and getting wet - oh so true.

Lets look at some math:

Let's assume you've gone from 17 mpg to 15 mpg, a loss of 2 mpg. Thus, 8 gallons used ot take you 136 miles, now only takes you 120. So, you now need 9 gallons to go where 8 gallons used to take you. You've spent an additonal $1.50, approx., to go the 136 miles. To save up, or make up for, the $500 for gears or that whole ignition system and whatever else (just an estimate) you would need to travel 500/1.5 units of 136 miles, or 333 units of 136 miles, or 333 x 136 = 45,333 miles.

Go buy a pizza and relax. /wwwthreads_images/icons/smile.gif Buy the time you put on a winch, tire carrier, hilift jack, etc., etc., your gas mileage is out the window anyway...

Pete

88YJ, 4"susp, 33"BFGMT, 9k#winch, rear homemade swingout, reb.258, 999, 4.10, weber32/36, GMHEI.
 

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/wwwthreads_images/icons/tongue.gif Excellent point Pete. Back in the 70's when they had the man-made fuel crunch, people sold perfectly good Buick LeSabres for peanuts and bought a Volkswagen Rabbit for more than the sticker price. Had they done the math, it would have shown that they could have kept the Buick and BANKED the savings each month that they would have realized from having no car payment. It takes a TREMENDOUS DIFFERENCE in fuel to offset big expenditures, because you are only working with the difference, not the total fuel used./wwwthreads_images/icons/crazy.gif

CJDave
I never believe any statistics unless I've made 'em up myself.
 
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Re: Will gears bring back lost MPG? Hey, DAVE

Well Dave, you have caused me to use my old trucking experiences to make examples, now you jump on my VW Rabbit. I bought one of the first Rabbit Diesels delivered in Dallas. Forget the year. Ordered my second one when I picked up the first one. Took over a year for my name to get to the top of the list so I could get the second one. Great little car. Got close to 50 mpg a time or two, but overall, for all the miles I owned it, the average was over 40 mpg. Don't remember for sure cause it's been so long ago, but I did have a Coupe DeVille back in those days too. May have swapped it for the first Rabbit. Guess you can now give me another name that somehow combines "Papa Thermodyne" and the Rabbit. /wwwthreads_images/icons/tongue.gif That really was a neat little car though.

Doug '97 TJ
My Web Site
 

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Pete 88YJ 's rig is running 4.10's not the 3.50 range OR the 3.31's I have.
It does make a difference(small) BUTit's a good excuse to get a kick in the pants under a stomp on the pedal(remember his "dad" wants him to take off his oversized tires)/wwwthreads_images/icons/crazy.gif
BTW try(even on the street} running 35's with3.50's or lower/wwwthreads_images/icons/frown.gif

85'CJ7 258 4" runnin33's
 
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I recommend going w/ 4.10 gears. I went to 3.73 after I got 31" tires. This improved mileage and power over the stock 3.08 gears. Then went to 4.10 for a little more power. The mieage didn't change a bit from when it was 3.73.
And with 4.10, you'll be better set up if you ever want to get 33" or bigger tires.

-Dorian
88 YJ w/ multi-speed wipers
 

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Dirt Doggie is right, I am indeed running 4.10s with my 33s, 258, and the auto tranny. However, I only did that to go up to the 33s, and I ran 31 ATs on the original 3.07 gears, and the auto tranny for a long time, and it was fine. A little doggish on takeoff, but that's it. In fact, I go back to the 31s in the winter, and take the BFG MT 33s off, and it sorta sucks with the 4.10s and the auto tranny and the 31s, as it tops out at like 60-65 mph on the highway, and its kinda high on the rpms. I would love some 33" winter tires, but just can't swing the $ now.

I guess it's a compromise. I wouldn't go bigger than the 31s with the 3.07s and the auto tranny, but you should be all set with the 31s. I just wanted to say that the mileage gain or lost is really not a big deal, money wise, and it would take a long time to make up for the money spent on the gears. I did it for performance's sake. Now, if you can convince Dad to let you keep the 31s, AND get some new gears, by all means... /wwwthreads_images/icons/smile.gif

Pete

88YJ, 4"susp, 33"BFGMT, 9k#winch, rear homemade swingout, reb.258, 999, 4.10, weber32/36, GMHEI.
 
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