When I swap my 3.42 to 3.73 how much if any mpg will I loose?

 

Minimal amount.

 

I used to have 3.73s and got about mid 15s with normal driving. I put 4.10s on mine, still drive 80 everywhere on the freeway and get low 14s, so 4.10s are kind of high. The best I got with the 3.73s was 17.8 mpg with carefull driving, and with the 4.10s the best is 15.8mpg. If you are going from 3.42s to 3.73s you are putting a little less strain on your system then mine, but you will get right about the same gas mileage, but the vehicle will feel more drivable. My friend got my 3.73s from me, and he loves the truck more, and he said that his gas mileage is stone cold the same. I did get .2 second improvement in my quarter mile, and my truck feels rowdy all the time now, like it is waiting to take off, so I love it, but to be honest, to do it again, I would stick with 3.73s for overall drivability on the freeway. My truck used to do 120 with no effort, now it gets there faster but it really feels like the motor is working when cruising that fast, just the higher rpms cause more vibration, and it seems just not tame at that speed like it used to. If you have 3.42s, you are loosing quite a bit of power, and not really retaining any great mileage gain from the gears, so 3.73s seem to be about the perfect gear for our 1/2 ton trucks.

 

You will gain mpg in city driving and you might loose at higher speeds but it should be minimal.