I was talking to Dan "The Fastman" the other day, and he said the best thing I could do for my truck is change the stock 3.55's.
I have been mulling this over for some time, and about have myself talked into 4.10's.

What I am wondering is:

1) can someone give me a rough idea of how much this will cost me mileage wise at cruising speed (~75mph).
2) I have done some calculations, and it seems that if tell the computer that I have 27.5" tires, then the speedo would still be correct at
75mph. Would this type of correction work across the entire rpm/mph band?

 

When I went from 3.55 to 4.10 (before I changed my tire size to a larger tire), I saw very little mileage difference (2 mpg at most). You might see more then that though because you will get a bad case of leadfootitis because of the fun you will be having with your new gears.

Here is an RPM calculator that will give you some idea of what changing gears or tire sizes does to your rpms at various speeds.

http://www.differentials.com/calc.html

 

"When I went from 3.55 to 4.10 (before I changed my tire size to a larger tire), I saw very little mileage difference (2 mpg at most). You might see more then that though because you will get a bad case of leadfootitis because of the fun you will be having with your new gears."

Yeah, that's one of the cons on my list. At 40, I figure I might become a poster boy for mid-life crisis. Going around town lighting up the tires at every traffic light.

Was the 2mpg hwy, city, or an average of the two?

 

Highway mostly. Very little difference for city.

 

When I went to 4:10 my mileage improved it wasn't enough to brag about but any helps.

 

when you do swap them, I recommend getting the speedo and abs re-cal done at the dealer... for whatever reason... my milage jumped 1.0 mpg after the re-cal when compared to my old speedo & correction calculations for gas milage...

 

how much do 4.10's usually go for.. are they hard to do? and do you need to get them spaced so they dont hum when accelerating? if you do.. then i'd much rather pay someone to do that for me

 

Gears are a tricky little job that really requires some experience. Not something I would recommend you do if it's your first time. Find a good shop, and pay them. Prices are usually $400-$600 per axle, including labor.

 

thats not to bad i guess for the fun you'll be having afterwards with your new gears

 

Yup, really good investment, escpecially if you tow or haul loads, or off-road. Having that mechanical advantage makes a huge difference for grunt work like that.