15 MPG and 35" tires
04-12-2008, 04:57 PM
#11
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the tach runs off the distributor if I'm not mistaken...unless they've changed how that works now. Speedo and odometer is off, but tach should be the same...right? I mean when I installed my tach in my 57 Chevy i ran it to the HEI... got nuthin to do with tires/gears...
04-12-2008, 05:33 PM
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04-12-2008, 07:41 PM
#13
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The engine tach has nothing to do with the speedo, gears or tires!
The tach in the dash gets its reading from the engine, it doenst care about anything else. How else do you think it works sitting still, in park or neutral and idling or revving it up?
04-12-2008, 08:14 PM
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Wth the higher axle ratio (4.88s) your MPG will go down due to the higher engine RPM's that will be required to attain vehicle speed. Think about peddling your bike 1st gear versus 5th.
Conversely, you attain the benefit of increased acceleration as the engine can build-up RPMs faster due to the mechanical efficiency of the higher axle ratio.
I'm sure others can express this more eloquently.
PS:According to my car's dash readings, I get anywhere from 14.8 - 15.3 MPG running 91 octane fuel. I have 35's, 3.73:1 axle gears, and an auto tranny. I also keep my foot firmly planted on the accelerator most of the time.
Conversely, you attain the benefit of increased acceleration as the engine can build-up RPMs faster due to the mechanical efficiency of the higher axle ratio.
I'm sure others can express this more eloquently.
PS:According to my car's dash readings, I get anywhere from 14.8 - 15.3 MPG running 91 octane fuel. I have 35's, 3.73:1 axle gears, and an auto tranny. I also keep my foot firmly planted on the accelerator most of the time.
04-13-2008, 04:06 AM
#15
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You cant regear a speedo on the JK, it uses wheel sensors to determine the speed of the wheels, but your right about the speedo being off and the dash reading being wrong on the mpg's.
The engine tach has nothing to do with the speedo, gears or tires!
The tach in the dash gets its reading from the engine, it doenst care about anything else. How else do you think it works sitting still, in park or neutral and idling or revving it up?
The engine tach has nothing to do with the speedo, gears or tires!
The tach in the dash gets its reading from the engine, it doenst care about anything else. How else do you think it works sitting still, in park or neutral and idling or revving it up?
Right, you need to hook into the computer to program the settings on the JK. The tach measures rpm of the crankshaft, tire size only affects the spedometer and odometer. On the JK, your fuel economy and distance left in the tank are also significantly affected by changing tire size.
04-13-2008, 09:26 AM
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I have the Hypertech programmer and now my speedo, odo and mpg's are right on the gauge, Ive verified everything with my gps and hand figuring my mpg's.
So, I have the programmer on the regular gas setting, Air Raid intake, relocated Magnaflow muffler, 5.13's with auto trans, 35's and heavy full size bumpers, winch, engine/trans skid, and heavy rock rails.....Im getting 14 - 15.5 average mixed driving and 16.5 on long highway trips.
So, I have the programmer on the regular gas setting, Air Raid intake, relocated Magnaflow muffler, 5.13's with auto trans, 35's and heavy full size bumpers, winch, engine/trans skid, and heavy rock rails.....Im getting 14 - 15.5 average mixed driving and 16.5 on long highway trips.
Last edited by Eracer76; 04-13-2008 at 09:29 AM.
04-13-2008, 01:25 PM
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I have a manual trans, 3.21s, airaid w/throttle body spacer, and am running 35's. 100% of my driving is around town and I rarely get over 60 mph. I have not used any kind of programmer on my speedo and I consistently average 13.7. I realize that I am actually getting a little better than that in reality but I don't think it's anywhere near what you guys are reporting. I only use 85 octane. I am now even more unhappy with my mileage than I was before. Luckily I only drive about 7,000 miles per year so it's doesn't really matter much as far as $ is concerned but I don't like having worse results than the majority.