want to go from 3.08 to 4.10 gears
 

Hey guys i have a 1997 f150 2wd 4.2 v6. It has a 3.08 open diff and this is now my project truck. i want know if any one has a v6 with 4.10 gears and alittle bit bigger than stock tire (235/85/16). this is not goin to be my daily driver since i only get 13-15mpg no matter how i drive it or on the freeway. i pull a trailer a bit so i was hoping the gear change would help.

Going from 3.08's to 4.10's in your truck will make a HUGE difference in performance and will probably give you better gas mileage if you keep your foot out of it. You should see close to 20 mpg on the freeway if you keep it down to 65 mph.

Please correct me if I'm wrong but I thought if you went from 3.08 to 3.73 or 4.10 that would make the RPMs higher? If thats the case wouldn't the motor use more fuel??

Yes, the RPM's will be higher, but you will be running closer to the most efficient, your throttle opening to maintain the same speed will probably be LESS.

You can test this - with the 3.08's, set the cruise at 65 in OD and see what the gas mileage is. Then do the same thing with OD locked out and see what you get.

If you tow with 3.08's, OD will pretty much be useless - good chance it will spend more time in 3rd than OD anyway. I have a stick and 3.55's in mine, and I get better mileage towing at 65 mph in 4th than in 5th.

Not really.

Looking at the specs I found for the newest version of the 4.2L v6, I find 202 HP @ 4350 RPMs and 260 TQ @ 3750 RPMs.

That's the beef of the engine's powerband. While fuel efficiency may not jump by leaps and bounds (big truck + physics, ya' know...), it will put that power to use the closer you get to that RPM range.

With 3.55s and 255/70/16 in my SCab, I run about 1500 RPMS at ~55-60 MPH. Now I am putting that O/D OFF button to use now and the extra few hundred RPMs and less gear shifts don't give me reason to complain.

I don't touch 2000 RPMs until I'm doing 70 on the highway. The engine doesn't seem to start exercising until then. An occasional ~4000 RPM punch from a stop keeps it in shape.