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I am very surprised to be averaging 19.0 MPG from my 1987 MJ since i got it in April. With a 3 inch lift, 31 inch tires and a 4.0 5 speed with 4WD I was not expecting this. 

 

What kind of mileage is everyone else getting from their MJ's? I am curious, because i had a 4.0 in my 5 speed TJ and it only got about 15 MPG with 31" tires and no lift.

 

My Suburban is another story. 10 MPG empty or towing, running the air or not. good thing it has a 42 gal fuel tank.

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I get about 17.5-18 in town. Haven't checked highway yet. I've got an 88 swb 4.0 5speed 2wd 3.07s and 235/75/15s. when I first got it going I got like 12-14. After working all the bugs out I only expected to get 15 maybe 16 in town. Can't wait to see what I get on the highway

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Wouldn't a larger tire make the speedometer read low, which means you actually traveled more miles than it says?  With a 31" tire you'd cover 6% more distance than the tripmeter would show.  Which would mean 6% better mileage.  What kind of driving were you doing when you got those numbers?

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I used "aCar" for awhile. Available on android only I think. Allows you to track EVERYTHING vehicle related. Tune-ups, services, service reminders, etc.

 

I was getting around 16MPG before I made a lot of changes. I'll report back after I go through a full tank now that I have my truck running right.

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My Chief (4.0, ax15) gets 13 no matter what I do, but I need to sort out many things with it as it seems the o2 sensor (along with other sensors?) are making it dump fuel like a madman. (teacher had renix port adapters for his scan tool)

My 2wd (2.5 ax4) gets 15.7 (only tracked 1 fillup so far) but it has a nasty exhaust leak at the manifold, and I need to check the o2 sensor. (and do a fan delete)

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I am very surprised to be averaging 19.0 MPG from my 1987 MJ since i got it in April. With a 3 inch lift, 31 inch tires and a 4.0 5 speed with 4WD I was not expecting this. 

 

What kind of mileage is everyone else getting from their MJ's? I am curious, because i had a 4.0 in my 5 speed TJ and it only got about 15 MPG with 31" tires and no lift.

 

My Suburban is another story. 10 MPG empty or towing, running the air or not. good thing it has a 42 gal fuel tank.

 

I am in suburban San Diego (El Cajon).  My truck gets used mostly around town for just Home Depot runs and stuff like that.  It is not my daily driver.  The weekend before last, I picked up a long block in Apple Valley.  I had 75 miles of around town driving on the tank before I drove 325 miles round trip to Apple Valley on Saturday afternoon.  Speed was an indicated 75-85 (go with the flow) all the way over the Cajon Pass (4100 ft, for the non So.Cal drivers). When I filled up the tank after 400.2 miles, I used 22.6 gallons of gas.  That works out to 17.75 mpg.   The tank before that one was about 15.8 and that 15.8 is very close to what I get most of the time.  Best ever tank was 23 and a fraction mpg.  That was a back from San Luis Obispo at a 55/60 average speed.  Odometer accuracy was checked against 50 miles of road markers in Santa Barbara County and corrected.  So the 23 mpg number was a good number.  My current tires are the same brand but one size smaller (225 vs 235) so the odometer is not too far off.  My truck is a 91 H.O. 2 wheel drive with automatic and 3.55 rear end.  And I run the a/c all the time.

 

Your 19 mpg is more than most report. That doesn't mean you are not getting 19 mpg.  You should check your odometer accuracy, simply to know how close it is.  If the odo is close, then your numbers are good.

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if I know how far off my speedo is, let say 4 mph so if I'm doing 55mph speedo says 59.  how can I figure out the actual miles on my trip.  can I times miles on trip by .4 then add the 2 together?

usually it will be off by a fixed percentage, the tires on my charger for example where i think where 6% shorter than stock meaning my speedo was reading faster than i was going (this will be more noticeable at higher speeds) and falsely telling me i was getting better MPG as well because the reported higher where also increasing the reportage mileage.  I did some math and figured out what percentage i needed to subtract from my reported mileage to get closer to actual mileage, until i was able to purchase a tuner and correct my speedo.

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http://www.fuelly.com/car/jeep/comanche/1987/Oceanic815/102921

 

Above is my Fuelly tracking for my daily driver: average of my last 5 tanks as stated is 24.0 mpg, with a best tank of 24.8; however, I run 245/70/16s that are 4.6% larger than stock to result in a 25.1 mpg average for my last 5 tanks and 25.94 mpg for my best tank.

 

What have I done to achieve this? Thanks for asking. ;)

 

  • Light on the gas pedal, shift early (I'm in 4th by 25mph)
  • Maximum speed of 55mph (I can get away with this in Oregon without getting flipped off or cut-off much)
  • Start coasting a long way back from a stop
  • Over-sized tires aired to sidewall maximum suggested pressure
  • Empty bed, no canopy or tonneau cover, tailgate up
  • Antenna and bumper lights removed
  • Homebrew air dam and grille blocks:

 

The air dam, which was taken from a mid-80s S-10 pickup and mildly resized, is riveted to the stock chin that runs between the fender flares. It is then gorilla-taped to the underside of the bumper in order to seal it. I run angle iron to brace the dam to prevent it from folding back at highway speed. The dam could stand to be another 2" lower and extended to the sides to cover some of the frontal area of the tires... but that's a project for another day.

 

Our rigs have poor aerodynamics, mostly due to the fender flares and coarse underside- it is easy to improve fuel mileage over the EPA estimates with only a little work.

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if I know how far off my speedo is, let say 4 mph so if I'm doing 55mph speedo says 59.  how can I figure out the actual miles on my trip.  can I times miles on trip by .4 then add the 2 together?

 

No you really can not just do that and expect accurate correction.  The speedo can be off by X% and the Odo off by Y%, even if they are getting their information from the same original source.  Reason?  Two different instruments = two different errors.  You can use a 5 mile highway marker and a stopwatch to check the speedometer to get very accurate results.  Drive the 5 miles at an indicated 60 mph, start the stopwatch when you enter the 5 mile check, stop the stopwatch when you exit the 5 mile check.  If the speedo was dead on and you held the truck to exactly an indicated 60 mph, your stopwatch will say 5 minutes, Zero seconds.  If it takes 5 minutes 15 seconds to travel the 5 measured miles, the speedo is low.  Unfortunately, to accurately check the odometer, you need more than 5 miles.  I once used over 100 miles of marked highway signs - the Odometer was off by 3.45 miles/100.  Because I changed to bigger wheels, I need to do it again (one of these days).

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I ran errands all day today. I have 31" BFG A/T's, 3.55's, AX4, and a 3" lift along with my 2.5. I also have a VERY free flowing exhaust, a new Wix air filter, 8.8mm wires, and no carpet or insulation in the cab. I keep track of speed and miles via gps. In mixed (mostly highway driving), I averaged around 22mpg. This is on a fairly warm day, with no large hills and telatively little wind. I also keep it under 60 and drive accelerate and decelerate in a... leisurely manner.

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I found this page it gave me the percentages I needed I typed in the original tire size 205/75/15  and then size I have on there 215/75/15 and it gave me a 2.2% difference.  now with that said it had 30 inch tires on the truck when I bought it. I think that someone might have changed the gear already. it does read faster on the speedo then I am actually going.  this is according to the GPS on 2 different phones and with that, 2 different apps. 

 

 

www.miata.net/garage/tirecalcold.html

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