Cannysage

iTrader: (29)

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

Jason@Performance

iTrader: (11)

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

and vice versa...

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

Fairlady_z33

iTrader: (56)

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.

Cannysage

iTrader: (29)

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

Z1 Performance

iTrader: (564)

my 3.9 has not affected my driving range 1 bit

helldorado

iTrader: (4)

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.

guy121

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