What rpm are you turning on average on the hwy with the 5 speed? Assuming you are not heavily loaded, a switch to 4.10's will net you nothing but a further loss of mpg...across the board.
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What MPG are you getting?
- Thread starter jsmusic
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What rpm are you turning on average on the hwy with the 5 speed? Assuming you are not heavily loaded, a switch to 4.10's will net you nothing but a further loss of mpg...across the board.
C.J. Roberts
TJ Addict
I honestly don't remember. I haven't done more than a few, (10 or less), miles on the highway for several years now. But it doesn't seem to matter. I can run 60, 65, 70, 75, 4th OR 5th gear. 12 MPG.
I've replaced every sensor or component that my research, (google, as well as here) said might affect fuel mileage to no effect. I've had nobody offer any theory as to why. Literally, the closest thing to helpful anybody has said is, "Well, Jeeps don't get good mileage", and then posting that pic of a cow being more aerodynamic that a Jeep. That, and, "Just stop keeping track".
It's just an honest fact...more rpm is going to burn more fuel. You have something unexplained happening with that vehicle and it's unlikely the final gearing. You have proven that fact yourself by running it in 4th gear on the hwy...lower gears is not the answer for you. Your 5 speed has a greater OD than the 6 speed btw.
C.J. Roberts
TJ Addict
Not without knowing the rpm's you are turning at hwy cruise speeds...this is the main contributor. One can skew results by having a foot in the gas pedal constantly, but I doubt this is the case if you manage 14mpg in city driving. My driving is much the same as yours...seldom get on a hwy with speeds over 50mph.
C.J. Roberts
TJ Addict
Since I get 12 no matter what speed or gear, I somehow don't think RPMs are the answer.
I found out the hard way the mileage I was getting on the highway.
Several years ago, I took a road trip to Ohio from Arizona. I knew I was getting the 14 in the city, so I was anticipating at LEAST 15 or 16 on the highway. Nope. 11 to 12. This was 70 in 5th. I tried running at 65 for a tank full. Then I tried 70. Then 75. Then 60. I tried both 4th and 5th. No change.
While on the trip, I asked on this and a couple other forums what the problem could be. I got varying answers, but the most promising seemed to be O2 Sensors. When I got home, I changed them. Nothing. 12 MPG. Air filter. TPS sensor. And so on. No change. For a while, I was only getting 12 MPG here in town! But it went back up to 14 before too long.
The following year, I again made the same trip, figuring that with all I had done, I should be able to get better mileage. Nope. STILL 12.
I don't know what the answer might be. I have talked to several experienced mechanics and have discussed it on the forums. NOBODY has been able to give me an idea of where to even LOOK for a clue after all I've done to try to correct the problem. Everything that anybody's suggested I've tried. So I just don't road trip The Muddog any more.
The truck is more comfortable for a road trip any way.
Pantera99
TJ Enthusiast
When I put my bikini top on I could only go about 55 on the highway because it seemed like it was gonna blow off. This was also when gas was at its highest so I really started tracking mileage & I was amazed how much better it was. 18-19 mpg when normally it was 15.5-17 mpg running 70 mph. At 2000 rpm I'm going about 58 mph in 5th with 31s, 3.73 gears. I have a long commute & its nearly all highway not that its always actually moving.
That there tells a lot. CJ should be getting comparable mpg...something is sucking fuel/going rich for him to be going backwards. I'm expecting to see those mpg figures if I ever get this one out on the open road. I'll post my rpm's at 60mph when I get that chance.
Pantera99
TJ Enthusiast
The only thing else I can think of is I did change to Bosch 4 hole fuel injectors when one of mine went bad. Didn't notice a huge improvement but it helped. Not so much mileage but acceleration & supposedly atomizes the fuel better. Always keep the fluids fresh also & tire pressure at 33 psi. Shes bumpy but seems to help mileage.
If there is still any doubt that rpm's affect mpg just look to your generator. Switch it to eco mode and it will run all day...turn the eco mode off and let it rip and it will be out of fuel in hours. 
Yeah, 30psi here.
@C.J. Roberts - Find someone with a wideband o2 setup and graph what your AFR's are doing at cruise...positive you will find your answer there. That is the only logical explanation for your situation.
Yeah, 30psi here.@C.J. Roberts - Find someone with a wideband o2 setup and graph what your AFR's are doing at cruise...positive you will find your answer there. That is the only logical explanation for your situation.
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My 99 xj w/ 4.0, aw4, and 31s got 18 as a daily commuter. Sure, it's a tiny bit better aero, and a fair chunk lighter than a tj, but it's still a brick. And the aw4 gets worse mileage than the ax15.
I'd expect to get 16-17 driving your tj.
Fwiw, I average 12 mpg on the highway in my 03 suburban, 5.3, towing a 26 foot sailboat named "Biscuits and Gravy." A tj on 31s should do better than that.
C.J. Roberts
TJ Addict
If there is still any doubt that rpm's affect mpg just look to your generator. Switch it to eco mode and it will run all day...turn the eco mode off and let it rip and it will be out of fuel in hours.
@C.J. Roberts - Find someone with a wideband o2 setup and graph what your AFR's are doing at cruise...positive you will find your answer there. That is the only logical explanation for your situation.
It’s rpm’s alright,I like to keep under 3grand on the highway and with my 3 speed that is 63-64 mph and the Jeep sucks gas at that high rpm,had to climb mountains in Colorado in second,these engines are happy and make power right over 3000 rpm with the right gears,but ya will use lots of gas!
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Not exactly. Climbing a mountain road @ 35mph in 3rd gear with the pedal flat on the floor, and descending the same mountain @ 35 mph in 3rd is gonna deliver the same RPM, not the same fuel consumption. I had 4.88 gears and just regeared to 5.38, My RPM went up 200RPM at any given speed. My mileage improved slightly ( not more than a half of MPG),,
5.38 and 35's gives me the same MPG I got with 4.88 and 33's, and that MPG is a shitty 11.5.
Edit: I think I mentioned earlier, I drive slow, use cruise control over 30mph, I haven't been over 70mph ever, rarely over 60mph.
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You are giving scenario's that are not even in the realm of this thread. Yes, we understand a foot buried in the throttle is a net loss.
A 2002 Jeep Wrangler 4.0 was advertised at 13 City/16 Hwy when they were produced...they didn't just pull that out of a hat. Like I stated earlier, if there is a specific reason one needs lower gears (ie heavy loads, bigger wheels, extreme terrain) that is fine, but it is ridiculous to make a blanket statement that lower gearing is going to equate to more mpg. Not going to happen in the real world as we know it.
A 2002 Jeep Wrangler 4.0 was advertised at 13 City/16 Hwy when they were produced...they didn't just pull that out of a hat. Like I stated earlier, if there is a specific reason one needs lower gears (ie heavy loads, bigger wheels, extreme terrain) that is fine, but it is ridiculous to make a blanket statement that lower gearing is going to equate to more mpg. Not going to happen in the real world as we know it.
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Drove my jk to California with 3:21 gears and 33” tires. The seven speed tranny stayed in 4th gear a lot. Got 13mpg with the headwinds. Regeared to 4:10 and now the same trip I get 19mpg and stays in seventh gear. I love the regear mileage.
Was not aware of any seven speed transmissions. Kind of disingenuous to tout a JK in this TJ thread, especially one that had been obviously handicapped with larger wheels/tires. As stated many times before, a gear change might be utilized to counter self inflicted wounds or some specific need. That 4.10 gear on your otherwise stock JK would have netted you a loss...otherwise the factory would have happily installed it from the beginning. Manufacturers are all about mpg.
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Took the opportunity this afternoon to satisfy my curiosity. Drove 101 miles at 65mph at an indicated 1800rpm for a 16.8mpg figure. I was expecting more, but I suppose I have nothing to complain about seeing where I started from which was 12mpg. The Jeep did throw a CEL # P2096...uncertain if that is going to have any effect at the moment.
Just sharing that I am tracking some attempts to improve mpg on my Jeep. Just did a expensive oil change in every rotating assembly to a high quality synthetic. Tracked the first 4 fill-ups before and will track the next 4. First tank looks encouraging but I will post more details after tank 4.
If your mileage improves after those changes, it is most likely due to subconsciously changing your driving habits while monitoring. In physics we call this “The Observer Effect” - the act of monitoring changes the result. Happens a lot - we spend a bunch of money on mods, and we want the results to be worth the investment.