3.5, 3.7, and 3.9 can't be good for gas mileage. How bad is it?

 

when you are normally crusing in 6th gear, go in 5th...

and vice versa...

it will be like that but not as bad...

 

Well if im not wrong since your auto from 3.3 to 3.5 wont be a huge difference but from their up it will.

 

I was thinking about 3.9, but yeah.. I would predict that it'll crap out my mpg..

 

my 3.9 has not affected my driving range 1 bit

 

Your driving habits will probably affect it more. As with most mods of this nature, you'll be throttle heavy for a while from sheer enjoyment of the acceleration.

I'm anticipating an approximate 400-600 rpm increase at highway speeds in 5th on the 5AT. 1st - 4th should have negligible affect, because you'll have to sit at a higher engine speed for a given amount of time for the extra fuel consumption to really start to add up.

 

What is really odd is that I get better gas mileage when I'm going 85-90mph most of the way(Long commutes almost every weekend) rather than 75-80 the whole way. Don't know why or how but I figure the engine is most efficient at that rpm(never wrote down what rpm). So if I changed over to the 3.5FD then going 75-80 should be right where I get the best gas mileage. I've checked this several times and once I was going 100mph most of the way(everyone else was going 95+ on memorial day) on 580 CRAZY! and afterwards my gas mileage still improved vs 75-80mph.

This is on a 2004.5 AT