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Old 01-01-2008, 11:35 PM   #1
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MPG difference between stock, 31' and 33'

Hello all,

I think I want the beefier look; not for performance but sheer hotness. I want to switch from 31' to 33'.

Sorry if this is a repost but I couldn't find it anywhere.

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Old 01-02-2008, 12:32 AM   #2
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It will definitely suffer, how much depends on how heavy your foot is.

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Old 01-02-2008, 12:35 AM   #3
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i went from a 225-75-15 to 32-11.5 it was averaging 15 with the 225 and now I gat 13 with the 32's. not much, but then again I did not redo my speed sensor wheel thingie.. maybe it remained the same.. O I would jump for joy if I can average 20mpg.
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Old 01-02-2008, 01:00 AM   #4
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no change in mine i still average 16mpg. If you dont change your speedo gear it will look like your getting less mpg becasue you calibration is off.
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Old 01-02-2008, 06:21 AM   #5
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well i dont know how you're getting as low as 13 with 33's because i get 12 with 35's, but maybe its all in how you drive!
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Old 01-02-2008, 12:05 PM   #6
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correct me if im wrong bigger tires = less rotation = less mpg if no speedo gear is installed.. so I might be getting the same MPG.
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Old 01-02-2008, 12:19 PM   #7
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33's with 3:73 gears, I get 13-15 in the hills and 15-18 on the hwy. I don't have a city mpg.
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Old 01-02-2008, 03:12 PM   #8
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correct me if im wrong bigger tires = less rotation = less mpg if no speedo gear is installed.. so I might be getting the same MPG.
You also have to consider mass and weight. Two other very important factors. It take a lot of extra power to move that extra mass & weight.
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Old 01-02-2008, 03:36 PM   #9
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I had 31's when I got the jeep and it got bad fuel mileage.
now I have 33's and it still gets bad mileage. I get 200 miles per tank when I am in town or on the highway or wheeling.
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Old 01-02-2008, 03:39 PM   #10
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At least its consistent. lol
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Old 01-12-2008, 01:56 PM   #11
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Quote:
Originally Posted by bpowa View Post
correct me if im wrong
you're also putting more strain on the engine to push them forward unless you got new gears otherwise you're correct.

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You also have to consider mass and weight. Two other very important factors
Also gears, so 3 very important factors..

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At least its consistent. lol
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Old 01-12-2008, 08:29 PM   #12
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Math question:

I passed several highway digital speed readers and it was always 5-7mph faster than my in dash speedo.

So, when my speedo says 60, I'm actually doing like 65-67 mph. Now, I calculated my gas and it was 15.1 miles per gallon with 33s and a 4 inch lift. Now, am I ACTUALLY getting worse or better mpg? (I assume no calibration was done hence the mph discrepancy.

Hmmmm.
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Old 01-12-2008, 10:43 PM   #13
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10% error factor in the true speed from the radar set vs the speedo, try this, use the mile markers and see what your odometer says. Also try this site
http://www.4lo.com/calc/gearspeedo.htm, just plug in your numbers, the gear is easy to change, it's in the transfer case where the wires go in on the drivers side.

Just as a side note, my 98XJ came with P215/75R15 GSR's, I never checked the speed or stuff. When I went to 235/75R15 Pirelli Scorpions I was curious and checked using the mile markers on the two interstates I used daily. Those mile markers were dead on with my trip odometer, in other words my STOCK setup was rolling on miles faster than it was actually doing. I also had a friend of mine who is a state trooper in NJ clock me, with the 235's it was dead on from 30-90mph. Jeep got out of their warranty early, I did not change the stock tires till about 50,000miles.
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Old 01-12-2008, 10:47 PM   #14
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if you havent changed your gearing then to find how much you're off by do (new tire size)/29 so for 33's it is 1.14, and for 35's its about 1.21 so you get "better" mpg then what you know now

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