Mileage/RPM/MPH Question

[Jeepin05]

I'm curious as to what RPM or MPH the 3.7/42RLE drivetrain is most efficient with gas. I think this was roughly discussed a long time back, but I'm not sure anything came out of it. I have a ~60 mile drive to work each day round trip. Back when gas was $3.50 - $4.00 a gallon, I drove to work with the speed limit locked at 65, given that the RPM was lower. I'm just not sure that was better or possibly worse for gas mileage...

So basically just looking for others opinions or facts regarding the KJ. Again this is with the 42RLE since I think it has different gear ratios. The KJ is mechanically stock.

And yeah, I know not to expect good gas mileage out of the KJ, just looking for suggestions for my daily drive!

Thanks in advance.

 

[speedracerbubba]

hmm... never tested it myself but I think if you knew the drag coefficient and the power curve along with the rpm/mph relationship you could get close.
It is also going to be pretty subjective due to different ride heights, tire sizes (diameter, weight, even tread pattern have an effect (albeit, very small)), etc.. But, for most 3.7 4x4 auto (or whatever you have) it should be ridiculously close as long as tire diameters are similar.

I know with the cruise control between 70-75 and about 15 miles of city driving in fresno, ca I got 17mpg.

 

[JeepJeepster]

The slower the better. Mine does pretty good around 60-65. Drops quickly above 70mph. Usually set cruise at 75 or so and I get 17-18.

 

[Atrus]

Agree with Blake. 60-65 seems best. 70 is OK. Anything above that and it exponentially drops. I guess we can't complain since we drive bricks with wheels

 

[Jeepin05]

Thanks for the feedback...I guess RPM does seem to be the key factor (lower RPM = better mileage) .

 

[JeepJeepster]

Thanks for the feedback...I guess RPM does seem to be the key factor (lower RPM = better mileage) .

Not necessarily. Once it gets so low it will lug the engine and/or cause the torque to start to unlock and lock. Down around 45-50 mine isnt very happy on rolling back country roads. She purrrs right along at 60mph turning about 1800 rpms.

 

[yellocoyote]

My kj is happy between 60-70mph, typically.

 

[Dave]

Not necessarily. Once it gets so low it will lug the engine and/or cause the torque to start to unlock and lock. Down around 45-50 mine isnt very happy on rolling back country roads. She purrrs right along at 60mph turning about 1800 rpms.

Same here. Once I get past 45-50 where the 42RLE shifts into o/d and can hold it at 60 (or between 60-65 max) it seems best. 1800-2000rpm's on mine.

There are so many variables depending on which auto ****** you got (different gear ratios of each), tires, etc, etc.

Dave

 

[jnaut]

The slower the better. Mine does pretty good around 60-65. Drops quickly above 70mph. Usually set cruise at 75 or so and I get 17-18.

I have found around 70 - 75mph for me personally. You may inspect my gas milage trend in my signature. While you can't make out the numbers, the really high bars were roadtrips where most of the driving was done between 70 and 75mph. Usually get between 20 and 21 mpg.

That last bar on the very end (for now-- my sig changes as I update my mileage graph) represents a road trip I just took to Mt. Hood in Oregon.

 

[tan's2002kjlimited]

My kj is happy between 60-70mph, typically.

C'mon, yours isn't happy unless you're doing 5-10 mph on the trails.

[-X (libsilver)

Usually get between 20 and 21 mpg.

Lift it 3 inches, get full skids, Moabs, 245/75/16 A/t tires and watch
that drop more than just a little bit.

 

[tjkj2002]

Actually the closer to the peak torque the better it will run,performance and mpg's.

Though I do not have the 42RLE my KJ with the 45RFE got much better mpg's at 80mph when I still had the 3.73's(engine at about 2800rpms),after the 4.10 gear swap It was very happy at 75mph(2800 rpms) and could almost get 22mpg's with all 5500lbs.With either gear ratio between 60-70mph I could barely get 15mpg's due to the trans having to keep shifting in/out of OD where as I could hold OD at 78-80mph and get almost 22mpg's.

 

[LibertyFever]

I suppose if you had an OTIS console you could see your instant gas mileage.
I should wire one up and see where I get the best mileage.

Once upon a time the government thought everybody would get the best gas mileage at 55 mph. A pretty generalized statement Eh?

 

[flair1111]

according to my evic 65-70 is best for mine depending on the road.

 

[ptsb5a]

If speeding tickets cost less than gas, I might try the cruising at 80mph, but driving at 130kph is like 30 over the limit and would cost me my licence in no time flat. Note to self.... petition legislators to increase speed limit on highways in order to reduce green house gas emmisions and our dependancy on oil. Those green tree hugging b*****ds would love that.

 

[ThunderbirdJunkie]

ThunderbirdJunkie's KJ seems happiest at 75 with the big tires and stock gears.

 

[Boiler]

Actually the closer to the peak torque the better it will run,performance and mpg's.

Though I do not have the 42RLE my KJ with the 45RFE got much better mpg's at 80mph when I still had the 3.73's(engine at about 2800rpms),after the 4.10 gear swap It was very happy at 75mph(2800 rpms) and could almost get 22mpg's with all 5500lbs.With either gear ratio between 60-70mph I could barely get 15mpg's due to the trans having to keep shifting in/out of OD where as I could hold OD at 78-80mph and get almost 22mpg's.

I think this whole shifting thing is my problem with gas milage too. Unfortunately I rarely get the opportunity to drive 80 without a weekly ticket. All 55 and 60 mph on my daily drives.

Will a reflash affect this speed requirement, and if so, in what way?

 

[Andynator]

What sort of gearing would I have with the off-road package and LSD? Even on the stock tires, I was pulling 2200 rpm at 55mph. I might get 16mpg with a tailwind.

kopkrab.gif

 

[ThunderbirdJunkie]

3.73 unless you have a 4 ******

 

[AlexKJ]

Even on the stock tires, I was pulling 2200 rpm at 55mph.

O/D OFF?

 

[JeepJeepster]

Actually the closer to the peak torque the better it will run,performance and mpg's.

Thats a confusing statement. The engine may be more efficient near the peak torque but it by all means wont be using less gas. If that was the case we would all be running around at 3600 rpms.

I suppose if you had an OTIS console you could see your instant gas mileage.
I should wire one up and see where I get the best mileage.

Once upon a time the government thought everybody would get the best gas mileage at 55 mph. A pretty generalized statement Eh?

If you want instant mpg you can get a scan gauge. It will tell you that and more.