I got a 99 xlt 3.0 v6 flex, I have 33s on there and I was wondering and also it's a 5 speed manual but will swapping a 8.8 with 4.10 be better and help relax the engine so it's not trying so hard to move those tires, will I gain some gas mileage and more power for doing this?

 

MPG is related to HOW you drive
Power is the same regardless

The 4.10 will make it feel better starting off, with 33" tires 4.56 might be better, but if you have the more common 3.73 axle now the 4.10 will "feel" much better

The 3.0l Vulcan is a high RPM engine, has best torque at 3,600rpm, so you need to run this engine at higher RPMs than you may think is good, but that is its DESIGN
The 4.10 will allow higher RPMs at say 65MPH, which will help MPG, because engine LOAD is less if you stay at 3,000-3,100rpms with this engine, within 500rpm of best torque

Look at you drivers door label and get stock tire size and rear axle ratio
Look at the bottom for "AXLE", under it will be a 2 digit number, last digit is Ratio, 2 = 3.08, 5 = 3.55, 6 = 3.73, 7 = 4.10, 8 = 4.56

When you have stock tire size and stock ratio, go here: https://www.therangerstation.com/tec...ioChange.shtml

This will tell you what the 33" tires make the stock ratio "feel" like, and what "new" ratio will do

i.e. if you had a 3.73 stock ratio and 28" tires and changed to 33" tires it would "feel" like you have 3.16 ratio, and "new" ratio of 4.40 would make it "feel" like you had 3.73 again