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better gas mileage?

S

Stern

NAXJA Forum User
Location
Chestertown, Md
i have a 92 jeep cherokee with the 4.0, 3 inch rusty's lifts and 31's on her. i get like 12 miles to the gallon. i have put rusty's air tube on and the engine just crossed 200,000...anyway i can get better gas mileage out of the old girl? thanks in advance!
 
What axle ratio? And when you went to the 31" tires did you replace the speedometer gear to correct for the error that will introduce?

If you didn't replace the speedo gear, you are actually getting approximately 10% better mileage than you thought, becase the difference between stock tires and 31s is about 10%.

And if you didn't regear, running 31" tires means that you are running the engine well below its most efficient range. Try an experiment -- run a tank of gas (or run one way of a round trip) the way you normally do. Then follow that my locking out overdrive -- if you have a 5-speed, don't use 5th, and if you have an automatic, use "3" rather than "OD" -- then compare the fuel mileage. You might be surprised by the result.
 
If you still have the stock gears (3.55 with AW4, 3.07 with AX15), do yourself and your Jeep a huge favour by swapping them out for numerically higher gears; 4.10's should do fine but if you plan to install even bigger tires later on (say 33's), you'd better opt for 4.56's instead.
Like Eagle said, your engine's revving too low in each gear with the 31's and you're having to lug the engine to get moving. Numerically higher gears will increase the rpm at any given road speed, restore lost performance, and improve gas mileage.
 
i havent changed the speedo gear and being a kid in college i dont think i could afford to regear the differential, how much does that cost anways? i plan on going bigger in the future but not for a few years. i have the AW4 if that helps any and dana 35c in the rear. i will try the no overdrive deal but is that good for the old girl? i only really use her for long trips. thanks for any help
 
Do you even know how fast you're going or what your real gas mileage is, then? With a stock XJ automatic and 225/75R15 tires, 65 MPH is approximately 2200 RPM in overdrive. Your 31" tires are about 10% larger (maybe 9% but 10% is easier), so for you a true 65 MPH will be about 2000 RPM in overdrive.

The overdrive ratio is 0.75:1, I think, so if yiu run with it locked out you'll be doing about 2650 RPM at 60 MPH. The 4.0L engine is easily capable of cruising all day at 3000 RPM.
 
when i was on the interstate i was watching the milemarkers and comparing it to my tripomoter and they both read the same. i will call jeep and get a new speedo gear and also lock out overdrive and see what happens. what about a throttle body spacer? could that possible help to?
 
Speedo

I just changed my speedo gearing and it was about $12.00. Like Eagle said, do that first and see what your real mpg is. You might find that you're about normal. I like throttle body spacers but I doubt that it will help gas mileage. Better low end torque though.
 
i will get the gear in the next few days..i did notice a drop with 30's as well so we will see. she does have 200,000+ miles on it so that might have somethin to do with it.
 
Stern said:
i will get the gear in the next few days..i did notice a drop with 30's as well so we will see. she does have 200,000+ miles on it so that might have somethin to do with it.
Click to expand...

Have you done a compression check and a cylinder leak-down test? Those are useful for diagnosing the condition of the engine.
 
how much and where to buy gears?

looking to get some 4.56's for my 96 4.0 auto(dana 30 front and chr 8.25 rear)

wheres the best (cheapest) place to get gears?

how hard are they to swap?(and do it well)

im still on stock gears (3.55) and im running 31's

ive done just about every bolt on performance thing
cone air filter with rustys tube
TB spacer
new ign stuff(plugs/wires/cap&rotor)
hi flow stainless cat conv.
Borla stainless exhaust

on the highway i get MAX about 17 MPG

my old 87 XJ (150k miles) used to get about 21MPG!


I would like to go with the 4.56's now, because im sure i will end up on 33's or MAX 35's in the future

The thing of it is--once i tear the diffs apart, i want to throw ARB air lockers in there too...
well...anyway
so-wheres the cheapest place to get em?
 
For gears and driveline stuff, I like Drive Train Direct (www.drivetraindirect.com). Look in the back of any of the 4WD magazines (Jp Magazine is as good as any) and you'll find lots of gear places advertising.

A lot of people seem to like Randy's Ring and Pinion. Must be that photo with the halo, I guess. I found DTD to be cheaper, but if you get a better price from Randy's, be sure to be VERY specific about what you order, and then check when it arrives. I've heard stories about people buying Dana-Spicer gears from Randy and receiving Brand 'X'. When I bought my gears from DTD, they were drop shipped directly from Dana-Spicer and arrived in Dana packaging.
 
i havent done a compression check so i will do that when i get home this weekend and let you know. i am also thinkin about changin my oxygen sensor becasue i have no clue when it was changed last and i have had it for about 50,000 miles now and i dont think my dad ever changed when he had it. when u had your gears changed how much was it just to do the swap. not including the price of gears i guess is what i mean
 
speedo error

You might be surprised at how far off the factory speedometer gear is really off! I went from 225 x 70 x 16 tires (28.25" tall) to 30.25" tall tires and checked my speedo w/GPS to see how far off it was.....It was within 1mph of GPS! (speedo read 1mph slower)I've also tested it on Cop radar...same 1mph off (speedo read 1mph faster).
Find some mile markers on the freeway and drive your speedometer's 60mph....see how long (stopwatch) it takes you to get from one to the other;) .

BLUTO :)
 
clarify

Just to clarify, and answer your question about an air tube, the short answer is no. Nothing you do to increase horsepower will really solve your problem because the engine is not running in the correct RPM range. You can do things that will give it more hp and torque in the range that it is running, but it would still run better, stronger, and be more efficient if it was running in the correct range.

This is why, when you buy larger tires, you really NEED to budget for new gears at the same time.
 
I never really thought about it, but is there another Jeep that has the D30/D35 combo in it? I wwas wondering cause i checked out drivetraindirect and they have packages for all the other recent jeeps. TJ, YJ, CJ, ZJ, but no XJ. So would I still be able to get one of their packages if i said I had a different Jeep, they are all the same axles eh? Just different danas.
 
Speedo gear

I have 2 jeeps , a 99 running 31s and an 88 with 33s. Who sells the 12 dollar speedo gears? Would they know what ratio I need for the both sets? I'd love to know for sure what mileage I am getting..
 
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