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Going from 3:07's to 4:10's

RichP

RichP

NAXJA Forum User
Location
Effort, Pa
What can I expect, will my milage take a big hit ? Currently at 75mph my tach runs at ~2200. I'm assuming that because I see it as a 25% change in gearing that my rpms will go up, will the engine run better and more efficiently. I know that currently I don't use 5th till I get above 65 or it lugs on the interstate. Just curious specially with gas prices the way they are..
 
RichP said:
What can I expect, will my milage take a big hit ? Currently at 75mph my tach runs at ~2200. I'm assuming that because I see it as a 25% change in gearing that my rpms will go up, will the engine run better and more efficiently. I know that currently I don't use 5th till I get above 65 or it lugs on the interstate. Just curious specially with gas prices the way they are..
Click to expand...


What size tire? Also you have a 5 speed, you can adjust your RPM's just about any way you want to. I know except to go lower at freeway speeds.

FWIW, I have 4.88's and 33's now and before I had 4.10 and 33's. I do more city driving, but I have been able to get more miles out of my dented 18 gallon tank. I had to gas up once a week and now it's about every 8-9 days.

BTW, mileage and XJ's don't mix. :D
 
Currently I'm running 3:07's and LT235/75R15 BFG AT/KO's, no lift and average about 18-20mpg mixed. At 195,000mi I also think it's time to do the O2 sensors so I'm hoping that will help some and get it back up to the 24+ it used to get when new. I just fear that the mpg will drop to the low teens and I can't afford that right now...
 
If your 2200 RPM at 75 is correct, your new RPM at 75 will be a shade over 2900 at 75 MPH.

Back in the 70s before the days of gasoline "shortages" and overdrive transmissions, AMC I-6s were typically geared to run 24 MPH/1000 RPM, so 72 MPH was 3000 RPM. My 1966 Rambler American routinely delivered 28 MPG geared like that, and my high-output 343 Javelin V-8 was good for 22 to 24 MPG highway with similar gearing.

I think the 3.07 gearing for the 5-speed XJs is undergeared and does not allow the engine to operate at the most efficient speed. 4.10 might be slightly beyond optimum on the other end of the scale (I think 3.54 or 3.73 would be about right for best highway fuel economy), but I don't think you're going to see a big hit. It certainly should not drop you from the 20s all the way down to the low teens.
 
ran that combo (4.10s and 235s) w/ an automatic and loved it. got 21mpg pushin 85mph at approx 3200rpm for four straight hours in the MJ.

LOVED it. drove much better, fast as hell and fun to drive. If you can keep your foot off the skinny pedal, you should do better w/ mileage.
 
DO IT! you'll love the power, and i suspect you will actually get better MPG

I used to think, like many people, that the lower RPMs the better MPG you're going to get. This is not true in most cases. The best indicator of MPG is how far you have the throttle down. The more the throttle is down, the more air is going into the engine, the more air that's going in, the more gas the injection system will mix in.

I found this out driving cross country last fall in my MJ with 4.10s, manual, and 33s. If I put it in 5th at 75, I got 17mpg. If I put it in 4th at the same speed, I got 18mpg. I'm sure if I would have had a gear in between I would have been up to 20.
 
I did the 3.07 to 4.10 route, but along the way, I went from 235's to 32's, no-lift to 4.5". So, it's hard to judge what combo had the most effect. I like where it's at right now (2500-2600 or so at 75mph), but my mileage has also dropped considerably. From around 18-20, to 13-15. Then again, that might not be a tire/gear issue at 210K miles.

I will say this though. As much as I like the combo now for off road stuff, it felt better (power, etc.) for regular use (and held to mileage) w/ 31's.

You'll probably be fine.

Mike in NJ :patriot:
 
Thats refreshing, no doomsayers, only positives. Guess as soon as the 8.25's gears get here I'll setup the appointment for the install.
 
Am I the only one who is HAPPY to get anything over 17MPH highway in their XJ?? Stock AW4 gears w/ no lift and 29 inch tires.

Getting about 14mph city, which is up from 8 - 11 MPG i was getting in the winter.
 
90Pioneer said:
Am I the only one who is HAPPY to get anything over 17MPH highway in their XJ?? Stock AW4 gears w/ no lift and 29 inch tires.

Getting about 14mph city, which is up from 8 - 11 MPG i was getting in the winter.
Click to expand...
<chuckle> My 94 2dr 4wd 5spd in stock trim got almost 25mpg on 1 tank of gas. 60 mph, no stops. Normally, on freeway trips, it was more like 21-23 and 18-20 in city driving.

With 6" of lift, 33x12.50s and 4.56 gears, it's more like 15 mpg. Heck, I even got better than 12 mpg driving from Chicago to Moab towing a 1 ton pop-up camper (4.10 gears, though.)

Rich, if you're lugging in 5th, you'll just love those 4.10 gears. 5th will be usable, and you'll get a usable 1st gear.
 
RichP said:
Currently I'm running 3:07's and LT235/75R15 BFG AT/KO's, no lift and average about 18-20mpg mixed. At 195,000mi I also think it's time to do the O2 sensors so I'm hoping that will help some and get it back up to the 24+ it used to get when new. I just fear that the mpg will drop to the low teens and I can't afford that right now...
Click to expand...

24+ WHEN IT WAS NEW???? You suck. I get 20-21 almost bone stock. Damn. Let me know if you do, I'd like to pick up 2-3 agallon
 
red91inWA a.k.a. scott f. said:
24+ WHEN IT WAS NEW???? You suck. I get 20-21 almost bone stock. Damn. Let me know if you do, I'd like to pick up 2-3 agallon
Click to expand...

The O2 sensors have a major impact on milage, I gotta wonder how mine are functioning after using the varied additives and cleaners I have used, Techron, seafoam, valvoline, stp, gumout, 3M. While all say 'sensor safe' ya gotta wonder.
The 25mpg was rock solid the first 2 years I had it, consistant on the hiway and that was loaded to the gills for 2 weeks vacation up at Cape Cod with bikes luggage on the roof and 4 people inside. That could also be because I have the 3:07 gears and was running the p225's. Milage did drop 1-2mpg when I put the Pirelli Scorpions 235 AT's on it which dropped my rpm's down. Hoping the 4:10's will help by putting me in back in the correct powerband.
 
you'll love 4.1's with 235's !! I had that originally when I regeared and I was in heaven!!

Btw on the gas thing... different states have different gas variations of additives and such so yeah some might get 24mpg and some might get 19mpg but if you flip them (as in swap gas for same vehicles at same altitudes) the mpg will follow.
 
RichP said:
The O2 sensors have a major impact on milage, I gotta wonder how mine are functioning after using the varied additives and cleaners I have used, Techron, seafoam, valvoline, stp, gumout, 3M. While all say 'sensor safe' ya gotta wonder.
The 25mpg was rock solid the first 2 years I had it, consistant on the hiway and that was loaded to the gills for 2 weeks vacation up at Cape Cod with bikes luggage on the roof and 4 people inside. That could also be because I have the 3:07 gears and was running the p225's. Milage did drop 1-2mpg when I put the Pirelli Scorpions 235 AT's on it which dropped my rpm's down. Hoping the 4:10's will help by putting me in back in the correct powerband.
Click to expand...

Sounds like its the tires in my case. went to 235s when i got the rig, and I've been pushing 'round 20 in the hiway. Guess when I got to 31's I'll have to get 3.54's and see what happens.

And here I thought it was me (which it usually is) :confused1
 
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