I have 4.88's and 35's on my rig. I get 18-21 with my 35's and a best of 23 also. Its just where I drive, I dont stop much, 55-65mph.... go look at the 22re MPG page, most people get upwords of 21...

He also has NO lift, so less wind resistance really helps.

 

I've had the guys in the shop tune my truck. I hate the 3.0 so I try not to work on it, I had the keys off to one of my techs

My odometer is off by 10%. If my speedo says 65 mph, I'm doing 72 mph.. according to the police

 

If you would like to know for sure, check the calculator on this site. Many useful calculations here.
http://www.4crawler.com/4x4/4LoCalc.shtml#GearRatio
and here
http://www.4lo.com/

 

Those calculators assume your speedo was right in the first place, making them a waste of time in my experience.

Only way(s) are GPS and timed distance. Get a handheld GPS and compare it to what the speedo says or use a stopwatch to time between mile markers.

 

I prefer GPS, go 50mph on the truck, and read what the GPS says, double the difference...your % off... be sure to stabilize at the speed for 10 seconds or so, this will make the GPS more accurate.

I just bought the dakota digital speedo correction kit, gonna us my GPS to set it.

 

The calculators are a really nice start if one can not afford a GPS at the moment.

 

details..... I have found the mile markers on hwys to not be acurate enough..unless they are a check station.

with 5.29's and 35 you are probally going slower than you read....with 4.88's and 35' I am w/in 2%, but when I have my 33's on my speedo is about 5% fast....

 

Yeah, see mine was within 1MPH at 70 (1.5%) with the 4.56 and 33's - not enough to get real concerned about. (I never checked it 4.56 and 31's)

Now, with the 4.88's and 33's (which should be back to stock) it's 5% off.

 

Its weird like that. My 95 runner was really close with 4.88's and my 285's, but the truck isnt even close. And they were both 235/75/15 and 4.10 stock....must be the difference in elec and cable speedos....

 

Or difference in brands of tires. ie some 33x12.50s are smaller than what their measurement says, and some are right on. The amount of tread affects it also.