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I have 33x10.5 and 4.10 on my BJ42 3B diesel and I can get 28 mpg highway.

I keep it around 2500-3400 rpms, the engine seems to like it there. I also think you under geared your truck with the 5.29s. Get a hold of a GPS and see if your odometer is off any. Using a gear/tire ratio chart you should be off 12%. For every 100 miles you put on the clock, you are really going 88 miles. If you have not figured that into your MPG already, then sadly you are getting even worse millage...
Perhaps your truck needs a good tune up? Or you just need to adjust your driving style.
22re with headers, block decked .040" and bored over .050"
33x12.5x15 super swamper radials on steel rims
18.5MPG mixed driving heavy on the pedal
19.5mpg when I am not so heavy on the pedal
21mpg highway from the one long trip I did have with them
I also had some 35x12.5's on aluminum rims, I saw no difference when I dropped a tire size.
I always compare my odometer readings to a GPS on a drive that is at least 50 miles. I will then take the distance on the GPS and divide it by the odometer readings. The number I end up with, is what I use to calculate my true millage when I fill up.
With the 35's I multiplied my distance by 1.18 and the 33's by 1.13

You're looking at about 17-18% faster and farther than whats' showing on the speedometer and odometer.
So if you go 200 miles so says the odometer, times it by 1.17 and you get 234. divide that by the gallons used and you get your mileage.

Well, it looks like I may be getting 19 mpg according to your formula, and that's after some mild wheeling over the last weekend..A lot better than I thought..
If I go from 4.10's to 4.88's what mpg difference might I expect?
Thanks again.
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