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MPG/RPM


Lenard
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I don't know the exact mpg but when i run a pretty consent speed of 65-70mph i can get around 220-250 miles to a tank of gas, but when i run a consent speed around 75-80 i can hardly get to 200 miles to the tank. I would understand a 1000rpm change making a difference, but it don't sound like it is more than 5-600rpm. So i guess my question is why would 5-10mph change my mpg"s that much? Its a 4.0L ,AW-4, 31" tires, 3.55 gears.

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Also, all other things being equal, fuel usage goes up (against time) as rpms go up. Usable power goes down with rpms when past the torque peak. Take these two together, and after the torque peak, fuel economy takes a nose dive. Add to that the higher air resistance at the higher speed, and it makes sense. For a Renix era engine, the sweet spot seems to be around 2000-2200 rpm.

 

Stock 4.0l engine, AW4 transmission and stock tires with 3.55 gears I could get 24mpg cruising at 60mph. Add to that a supposed 23.5 gallon tank that seems to hold 25+ gave me a radius of 600 miles. I have gone over 500 before filling up on occasion, and 400 miles with gas to spare multiple times.

 

Surprisingly, when I added a 4.5" lift, mileage stayed the same. I guess shoe box aerodynamics are bad enough that the added lift makes no difference.

 

Now with the same set up, safe for ~6.5" lift and 33x12.50 tires I get the best economy around 70-75mph. IIRC it's only about 18 mpg, though. I'm guessing that is because of increased air resistance at the higher speed, higher rolling resistance of the wider tires, and possibly stock gearing not being ideal for that size tire.

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As noted, it's the "brick on wheels" factor. Aerodynamic drag increases exponentially with speed.

 

During my brief sojourn with the '99 Grand Cherokee, I took a 500 miles (each way) trip and used the opportunity to conduct an experiment. Going from CT to Pittsburgh (western PA, for those who don't know) I ran with the cruise control set at 70 (speed limit on I-84 is 65). For grins & giggles, I had plenty of time on the return so I set the cruise control at 55. IIRC my gas mileage on the return was either 3 or 4 MPG better than it was on the first leg of the trip.

 

And a '99 Grand Cherokee is a slipperier brick than an MJ, by a substantial bit.

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Fuel consumption can easily take a nose dive as speeds increase. It's not the rpms, it's the increased air resistance.

 

Resistance plays it's part but increased rpms will do more than the resistance. An engine idling at 500 rpms will burn less gas than one idling at 900 and there is no resistance to factor in. Faster an engine goes, it requires more fuel.

 

My TJ (same brick like design) with 33 inch tires had my engine rpm running around 2300 at 65 mph and was giving me 17-18 mpg. Changed the gearing which put me closer to stock performance but increased my rpms. So now I'm running 3000 rpms at 65 mph and only pulling 13-14 mpg. Roughly a 75 miles per tank difference. Same speed, higher rpm, lower mpg. Once you pass the "sweet spot" that the engine runs at, the rpms start eating away at your mpg's a lot faster. I'm sure there is some sort of rpm to fuel consumption curve that would explain this alot better.

I've been wanting to test a theory by putting 35's on my rig which would lower rpm's, to see if I pulled better mpg even with the larger tires adding higher weight and resistance. In theory it should work.

 

 

:cheers:

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did you change your speedometer gear to compensate for the bigger tires?

no, but i know it reads 5-6mph under.

 

:smart: Unless your speedometer is broken it will not read 5-6mph or whatever under, it will read a certain percentage under(or over) your real speed! Just to keep it simple, lets say you measure with GPS your speed to be 50mph and your speedo shows 45mph, then accelerate until your GPS shows 100, at that speed your speedometer should show 90mph. This means your real speed are 10% faster than what your speedometer shows! The same comes to your odometer, quite important when you measure youre gas milage.

 

And I have heard that you get the best gas milage at peak torque rpm...theoreticaly! It makes sense but peak torque are, from what i know, only measured at full throttle and who measure they mpg at full throttle?? There must be something more to it, I don't hestitate that my MJ would get great mpg around 2400rpm (peak torque rpm with a renix), but no way that i would run my volvo at 4800rpm (peak torque rpm with my swedish brick DD) for more than a few seconds!!

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Fuel economy goes up with available torque, down with rpm, and down with air resistance. Those are the three main things to consider. Torque peak if good, lower rpm is good, lower air resistance (speed) is good. Add all three together, best economy is a bit under the torque peak in the highest gear available. From that point up to the torque peak will reduce it some; once past the torque peak it will take a nose dive because all three factors are now working against you.

 

My personal experience, when I was still bone stock 4.0l, AW4 automatic, 3.55 gears, I got best mileage 60-65mph. Same drive train but 33" tires I now get my best mileage at 70-75mph.

 

My 94 Cherokee (Chrysler) 4.0HO, AX15 stick, 3.07 gears stock seems to get it's best mileage around 75mph.

 

On my wish list for both vehicles is cruise control.

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get the proper gear and have no doubt. :thumbsup:

 

http://gojeep.willyshotrod.com/HowtoSpeedoGears.htm#chart

ok thanks pete

did you change your speedometer gear to compensate for the bigger tires?

no, but i know it reads 5-6mph under.

 

:smart: Unless your speedometer is broken it will not read 5-6mph or whatever under, it will read a certain percentage under(or over) your real speed! Just to keep it simple, lets say you measure with GPS your speed to be 50mph and your speedo shows 45mph, then accelerate until your GPS shows 100, at that speed your speedometer should show 90mph. This means your real speed are 10% faster than what your speedometer shows! The same comes to your odometer, quite important when you measure youre gas milage.

 

And I have heard that you get the best gas milage at peak torque rpm...theoreticaly! It makes sense but peak torque are, from what i know, only measured at full throttle and who measure they mpg at full throttle?? There must be something more to it, I don't hestitate that my MJ would get great mpg around 2400rpm (peak torque rpm with a renix), but no way that i would run my volvo at 4800rpm (peak torque rpm with my swedish brick DD) for more than a few seconds!!

ok makes sense, the speed boxs on the hwy say 70 when my spedo says 65, but if i run like 20 according to my spedo the box say 15mph
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