will changing the gears ratios in my truck get me better acceleration and mpg

 

If by "higher gears" you mean a higher number (i.e. changing from 3.55 to 4.10 or 4.56) you should get much better acceleration. Mpg will probably not improve, might be lower. Depends on gear, and driving style, etc.

 

what about lower gear ratio

 

"Low ratio" would give poor acceleration, and POSSIBLY better mpg. I don't think it would be a good idea.

Stock gears are pretty low ratio already. Probably designed for highest fuel mileage possible for average conditions.

By "low ratio" I mean lower numbers. i.e. 3.55:1.
High ratio = high numbers i.e. 4.56:1.

If you search, you'll find a lot of threads on this subject.

 

its like this with stock tire size

4.56 good in town bad on highway
3.23 good on highway bad in town

what size tires are you running?

 

im pretty sure theyre 225 75r16s

 

Those are really tiny tires, definitely don't go any lower with the gears (bigger in number). With those beefy meats you will murder your highway numbers. What gear ratio do you have in the axles now? You may benefit from taller gears (lower number).

 

stock 3.55s

 

My stock ratio of 3.92 was pretty good (IMO) for the 265 75R16s I had on when I bought it. It gave pretty decent economy and the power band was good.

If you look at this chart:
http://www.4lo.com/calc/geartable.htm

You can see where you fall with your ratio and tire combination.

You are actually in the economy band already with your combo. About 2600 RPM @ 70 in 3rd gear whereas I was slightly higher when stock near 2700RPM in the same conditions. You're about where you need to be gear wise already. You could go either way with the gears, but you're either going to sacrifice a little economy to get more power or sacrifice some power to gain a tad more economy.

Either way, its probably not worth the money to swap in gears.

 

at 70mph i get 2000-2100 rpms real world