I have 3.42s in my truck right and am thinking about going to 4.10s. My concern is fuel mileage. I cruise down the road right now at 65mph at 16-1700 and 80mph at 2000rpm. How much more will the 4.10s be?

 

Try this guy

http://www.csgnetwork.com/multirpmcalc.html

 

I went from 3.42's to to 4.10's and run 1900 rpm at 65 mph and 2000 rpm at 70 mph. Around town gas mileage suffered a bit (partly to my foot) but my highway mileage actually increased. I'm going to be around Kingfisher this weekend so flag me down and you can check out my truck and see if you would like the 4.10's...

 

I've got the 3.90s and the stock rims/tire height. My rpms at 70mph went from 1800 to ~2150. It actually helped my mileage 2-3mpg, I think because of my cam.