That's why then.......

 

solar...its because you weight 1/2 what we do...lol...driver weight reduction FTW

 

AAAAARRRGGGGHHHH!!!!! Tire swaps do NOT change mileage. The tires change the final gear ratio which throws off your speedo. After going up or down on a tire height YOU MUST recalibrate the speedo.
[sm=chairshot.gif]

 

We knew that! We just didn't feel like explaining it. Somewhere I have a chart for guys that put monster tires on trucksand itexplains what ratio gears are best suited for the size monster tiresthey use. But someone who increases their tire size to 35", 37" or even up to 44" usually doesn't bother to change their gears. They just recalibrate the speedo.

 

Are you talking MPG? Larger tires generally are heavier tires with larger rotational mass. More mass being turned means more power to turn them, more gas used. It's one reason those econo-boxes have such little wheels. The mpg drop isn't as drastic as it might seem before the speedo recalibration but larger, heavier tires do hurt. Here's one example:

http://rockcrawlermag.com/techreport...iner/page4.asp
A good general rule seems to be that each pound of un-sprung mass = 10 lbs of sprung mass. How does this affect our Jeep? Each one of my stock tires weighed 51 Lbs. Each one of the new tires weighs 68 Lbs. This means that my Jeep has to move, stop and turn with the effect of 680 more lbs. of weight on-board. If you check a gear ratio chart you can see what gears you need to change to in order to regain your power and gas mileage.

More info here: http://www.tirerack.com/tires/tirete...jsp?techid=108

 

Your gonna make Impy cry in his Wheaties!!!

 

Here's the formula again... Thanks http://www.4lo.com/for their EXCELLENT real 4x4 information!


I'm not going to get into rotational masses or the whole tire weight thing. Of course if you have heavier tires the engine has to move more weight thus uses more energy. But the mileage difference is insignificant compared to changing your final drive ratio. Too many people fail to comprehend that when you put on a taller tire your final drive ratio increases.

Here's another
(New Tire Diameter / Old Tire Diameter) * Speedometer MPH = Actual MPH

New 35" Old 33" 55 58

Your vehicle now thinks your're going 55 when you'regoing 58. If your speedo is off then your odometer is off too so your mileage has not changed but your math has. Someone forgot to tell the speedo that the final drive changed which throws EVERYTHING off.

 

RealJeep,
Your equation in terms of Tire diameter seems right. In your example,however, wouldn'tthe vehicle think you are going 55 (the speedo reading) when you are actually going 58? You stated the reverse. -- Oh, I see you corrected it.

I would thinkthevehicle mileage is based on rotations calibrated to tire size. Hence a larger tire size would have you traveling slightly further than your odometer indicated. So if you calculate mpg by dividing odometer miles (e.g. on the trip meter) by gallons to fill up, you get a result that is less than actual -- hence an apparently lower mpg when that's not the case.

Actually it's a bit confusing, but correct me if i'm wrong.

 

All I know is when you get to 88 miles per hour, your gonna see some serious ****.



Cliff

 

I think my flux capacitor is broken...cause when I hit 88 I just see red and blue flashing lights...especially when i'm in a 40mph zone for some reason