fastboat

I have own a Cherokee in past and am thinking of buying another one just as a daily driver to and from work. Is there any rear end gear ratio change or computer change that will enhance the gas mileage for street only? This will used on highway only, no off road. I would like to increase the mileage to help out on the commute........Id appreciate any advice or ideas on this idea.......
Thanks

Andy

ol"blue

Welcome to CF Andy!

BuckB91XJ

With the 3.55 OEM gears, move up to a 235/70R/15 or a 30x9.50x15 tire and that gives you about the best commuting final gear ratio you can get out of an XJ. Keep it at 70mph/2500RPM. You should get 20-22MPG

5-90

The 242ci and AW4 automatic will come with 3.55 gearing, and responds nicely to 30" or so tyres.

The 242ci and five-speed manual (BA-10/5 or AX-15, or NV3550 in the 2000-2001,) will come with 3.07 gearing - which is a mistake. Gear to cruise around 2500-2800 with whatever tyres you're using - I'm thinking, offhand, around 3.73:1 with 30" or so tyres.

You should be able to fit 30's neatly, and 31's while losing just a bit of steering and articulation, without modification to the chassis.

fastboat

Thanks guys for the info.......Ill remember this when I get the Jeep.....

Bigbike

If you buy an auto you can swap axles with 3.07 gears in and get better mpgs in theory but you will lose power.

BuckB91XJ

The manual 5-speed tranny that you get from the factory with 3.07 axle gears has a lower first gear and bigger variety of gears 5 vs. 4, then the automatic. Which is why the automatic XJs come from the factory with 3.55 axle gears. I believe you would actually hurt your MPG by changing to a 3.07 axle gear with an automatic. You would bog the motor a lot, thus causing yourself bad MPG. Common sense does tell you that the higher 3.07 axle ratio would give you better mileage but, other factors play in like tranny gear ratios and engine RPM to get you the best MPG.

5-90

^ Essentially correct.

The AW4 is well-matched by the 3.55 gearing from the factory, and it works well with OEM-sized tyres and with 30's. 31's are a bit raggy, and if you go larger you should consider 3.73 gearing.

The 3.07 gearing used with the manuals is a mistake, and you'd be better served with about 3.73 instead (I'd have to do some maths to make sure.) The manual likes to cruise at a slightly higher RPM than the auto, and I've picked up about 3mpg by not using fifth gear on the freeway (cruise around 2500-2800rpm or so.) "Best cruise" is usually the crankshaft speed at or slightly below "peak torque output," (learned that in Ground School years ago...) and peak torque happens in the 242ci somewhere around 3000-3200rpm. That's a little high for cruising, and pulling that down to 2500-2800rpm does make a small difference (I spent the best part of a year working on this.)

Gearing should be selected driven by transmission ratios and tyre selection, and that overall should be driven by the engine's torque output curve. You wo'nt get exactly what you want, but the 242ci I6 is fairly forgiving, and you can get close. It's handy having the broad, flat torque output curve that we have...

88XJLaredo

I am probably a bit late in response, but Top Gear had done testing on MPG in correlation to RPM. Every time, with every motor tested, they came up with the same results; lower rpm equals better MPG. Seems odd, but a lot of people don't change out the speedo gear, and that's what makes it seem like worse mileage.

djb383

Install one of these, keep the needle above 15 and your mpg will go up.

http://www.jegs.com/i/JEGS/555/41008/10002/-1

Much cheaper than gears and a 15-20 minute install.