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MPG Increase?!!?

Urban Redneck

Urban Redneck

NAXJA Forum User
Location
Arlington Heights, IL
Ok, this is really strange. I was running 235/75/15's with a 3.55 gear, my mpg was 14.75. I switched to 30x9.5x15's with the same gear and am now getting 16.30! Thought the mpg was supposed to go down with increase in tire size. Any thoughts? :confused:
 
Kejtar said:
sis you change the speedo gear?? If not your miles traveled are way off....
Click to expand...

Not necessarily. In my 97, moving up to 30's actually corrected my speedo. GPS used to always show that my speedo was running to fast. With the 30's it's now right on the money. I wouldn't doubt that it's the same way for an '00.
 
ThePhantum said:
Not necessarily. In my 97, moving up to 30's actually corrected my speedo. GPS used to always show that my speedo was running to fast. With the 30's it's now right on the money. I wouldn't doubt that it's the same way for an '00.
Click to expand...

Are you really sure?? With my 98 I have had to change the speedo when I moved up to the 235's (although it might be slightly different for me since I'm stick).... In general I'd maybe agree how moving up one tire size might get you closer to the reality in regards to the miles, but not 2 or 3 sizes...
 
speedo error

I have to agree with Phantum, when I switched from 225 x 70 x16 (28.25" tall) tires to 31 x 10.5 x 15" (30.25" tall), I thought my speedo would be way off. I tested it w/GPS (within 1mph) and cop radar (speedo read 2mph faster than radar). Another way to test is to run 60mph down freeway and time the mile markers.

BLUTO :)
 
I agree also, I went from stock to BFG's 31x10.5x15's (And a 3.5" lift) and now my speed is right on. This was with a GPS unit in the Jeep. I think it was .5 MPH off, to be exact. I was doing research into what I needed to change the speedo gear, I'm glad I did the test first before changing it out. :)



Alex


Edit: Kinda wondering how many miles are actually really on the Jeep :)
 
Kejtar said:
Are you really sure?? With my 98 I have had to change the speedo when I moved up to the 235's (although it might be slightly different for me since I'm stick).... In general I'd maybe agree how moving up one tire size might get you closer to the reality in regards to the miles, but not 2 or 3 sizes...
Click to expand...

Oh I'm with you. I expected to have to change the gear when I put the lift and 30's on. But GPS has me right on the button after the switch. I've checked it multiple times on more than a few trips to the mountains and it's always right on the nose. Maybe it is a difference with stick and auto (I have an AW4), or maybe Cryco just ran out of the correct gear when they built mine and put in what they had.

Who knows....:rolleyes:

Steve
 
Mileage increase

You moved the cruise RPM down 100 - 200 and that aparently puts you into the peak torque RPM range of the new manifold. Operating at the peak torque gets the most efficient use of the fuel. My 89 with 3.73 AW-4 would get better mileage at 70 MPH cruise (2300) than at 55 MPH. Just depends on tire diameter - but if you go to larger cross section tires you will loose some MPG as they eat up more horsepower.
 
Kejtar said:
sis you change the speedo gear?? If not your miles traveled are way off....
Click to expand...

He is traveling further if the speedo is off, so his mileage is even better if this is the case. I am running 31's and stock gears and getting 17-21 mpg, but that is with a high flow exhaust. My speedo is reading about 3 mph slow, so my odometer is off as well. It is reading low, so I am getting a little better mileage than I report, but not enough for me to do the calculations.

Curt
 
If you are using the numbers exactly from the tripmeter without factoring in the percentage change between the two tires, your math is wrong.

True, you want your odometer to track mileage acurately for an accurate MPG calc but just factoring in the percentage will at least tell you if you MPG went down soley based on tire size change.
 
Urban Redneck said:
Yeah, I figured I had to be doing something wrong. I have to do the speedo test this weekend and see how off I am.
Click to expand...

What you should find, if your speedo was correct from the factory is that you are getting even better mileage than you calculated this past time. Going to a bigger tire makes the speedo read low, so you are going further than what the odometer says.

Curt
 
most car manufactureres now make the speedo show a little higher than your are actually going. it's a way for them to make people "drive slower and safer" without knowing it. when you add slightly larger tires the difference in circumfrence is enough to make your speedo accurate in most cases.
 
PS-UrbanRedneck

Where you live (dont' say AH)?? I grew up in Palatine and just moved away a year ago. My folks still live there.
 
Ok, what GPS units are you guys running? I've got a Garmin eTrex Ledgend that I use when I go turkey hunting. I didn't think that they were that accurate.
 
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