differential mileage

[a1b2c3]

Just a simple question. Can the gears in the differentials be switched to improve the mileage on a 99 Cherokee?

[jfxj67]

Yes...

I don't know if it would really be feasible though, as i don't think different gears will change your mileage that much.

I would honestly suggest finding mileage improvements elsewhere first.

[stamp]

well if your running 410s or 456s then a set of 307s would be a great highway improvement but your city milage would suffer 355s which are stock on most autos are a happy medium for stock tires

so i guess the question is what size tires are you running?
and what type of driving do you do

[CT99XJ]

Unless you're running 32's or larger, don't regear. Check your compression, plugs, head gasket, cooling system, and exhaust before you worry about regearing. You'd be surprised how much better your mileage is with an exhaust that has the proper backpressure and new plugs.

[a1b2c3]

Thanks. I'm a rural mail carrier running 235's. Mileage is important since its my job to deliver mail, and the more mileage I can squeeze out, the better for mya bottom line.

[JeeperMatt]

As CT has already mentioned (and yet AGAIN beaten me to the punch, lol) a good tune up and ensuring everything is in good working condition would be a wise move.

Also, as weird as it sounds - throw some power adders on there such as an intake, header, exhaust, etc. Basic principle is the same as how they add the power; factory equipment is restrictive, designed to meet emissions and mpg requirements, and just to get the job done. Aftermarket parts do the same things, however they're designed to be much more efficient at it, which in turn frees up power that was restricted by OEM parts. Less restriction = less power lost to do a job. Less power lost = less gas burned to do said job. On 235's and a 5-spd I was seeing nearly 400mi/tank with mixed city/freeway, and still had enough balls on tap to run with a Mustang GT. Being a rural carrier, I imagine you'd see closer to 400. Plus the extra power is always nice Granted if you use the extra power, your mileage goes away but still - nice to have it on tap.

[a1b2c3]

I appreciate your response. MY hwy mileage before putting on the 235's was 20, and about16 on the route. I haven't had any opportunity to check the mileage since putting on the bigger tires. Time will tell. Tune up wise, the jeep has had everything done to it, just not with a fine tooth comb. After all, output must exceed input.

[jeepgod]

When I built mine before my lift and big tires were added.....I had my throttle body bored out to 62mm and that helped a lot with gas mileage.....and engine response too.

[CT99XJ]

Originally Posted by JeeperMatt

As CT has already mentioned (and yet AGAIN beaten me to the punch, lol) a good tune up and ensuring everything is in good working condition would be a wise move.

Jeepgod's correct, to a point- the bored out throttle body will improve mileage, but only at partial throttle.

Here's a list of the modifications done to mine before the lift, and I saw some pretty decent mileage:

Flowmaster 40 and high-flow aftermarket catalytic converter

Bored-out throttle body

Wrapped header (Keeps exhaust heat from radiating off the header and heating up the engine faster)

"Cold" air intake (cooler, maybe)

Accel plugs/wires/cap/rotor (Surprisingly, work as well as Champions)

Plymouth Neon fuel injectors (24 lbs)

Depending on whether your 99 was an early or a late, you can replace the manifold to gain some mileage and power. I would see 270 miles a tank with all city driving. Also, as dumb as it sounds, check your tire pressure and bearings/alignment. a loose bearing will cause a wheel to wobble, thus making it harder to move in a straight line, thus sucking mileage (however little it may be, it still helps). Alignment should be self-explanatory as to why it could improve/hinder mileage.

Also? run some Marvel Mystery Oil through your gas every once in a while, and substitute a quart of oil with Marvel every other oil change. Running some ATF through your intake Seafoam-style will help clean it out a bit, too.

[jeepgod]

ct99xj, I should of clarified that a little more: With WOT, your engine will be using more gas but at cruising speeds is where you will notice the gains.

Before adding any aftermarket oils and stuff such as lucas or otherwise, be sure to do a thorough research about them because not all of them are what they claimed to be and are a waste of money. Also remember that when adding things such as lucas oil to your engine, if you live in colder climates where winters typical result in freezing temps and snow.......that thicker oil will not circulate properly until the engine is warm which may can accelerated engine wear