In warmer temps (50+) and not running winter fuel, I usually see about 10% higher mpgs. So that 18-19 will become around 19-20+. In 100% highway driving, I see 25-29 depending on terrian, wind, vehicle load, etc. I haven't done a 100% highway trip with the 3.5 gear, but I have no reason to believe my MPGs will sway much from what I've seen in the past 5 years.

There seems to be no downside to this mod. On Friday it was 70 and sunny, Friday night it was 33 and raining, and on Saturday it was in the uper 20s and we had 7" of snow on the ground. I really got to test out that VLSD

 

haha

 

About a month ago I reinstalled my slightly heavier 18" rims/tires. The summer tires are have about a 1" wider contact patch and are pretty worn which all increases drag and rolling resistance. I'm consistently seeing 19.2 to 19.5 mpgs in 70% city driving and that's with me getting on it fairly often. These numbers are basically what I saw before the 3.5 gear swap.

I still like the swap and the extra gearing is nice, especially in the warmer weather.

 

I imagine if you are doing alot city driving where the car is always going though the gears, one might not see a big diff in mpg.

But if you do a long highway drive where you are stuck in 5th + 3.5 FD, it might affect things.

It's probably well in the range of "benefits outweigh any slight diff by 10x" though.

 

I made a mini road trip to Houston last weekend from Austin and my mileage averaged around 22.5-23 mpg. I was getting 24.5 consistently on the highway before my swap. Like you said the benefits far outweigh the negatives for me. My around town mileage has remained consistent at 18.5 MPG.

 

My gas mileage has stayed around 19-20 around town. On the open road doin 80 my MPG dropped from about 27.5 to about 24.5.

 

So after reading through this whole forum it sounds lik e3.7 is the best?

 

oh yea i do alot of drag racing at the strip against my friends.
Im stock right now