Les, I don't have a 3.5, but I will say that my mileage doesn't change that much (in some situations its better), but it really depends on what kind of trail I'm running. I've often thought that an hour meter would be a good little gauge to have, since wheeling is often spent at idle, looking over an obstacle, lining up to pull someone out, etc...

1991 HZJ77, 1999 KZJ90, 1999 UZJ100, 1992 M101CDN2
https://www.thegearheadproject.com/category/jdm-journeys/