SladeX

When I had my 88 car, changing gears did very little to the gas mileage, and that was going from 2.73 to 3.23. A change from 3.42 to 3.89 (in my case I'm going to a 3.90), will yield little change as well.

I've tested highway driving at various speeds, tried holding 65mph the whole time, and another tried 75-80mph. The result was nearly identical gas mileage for a full bolt on/cam car.

As long as you aren't constantly accelerating and braking and holding a consistent speed, our engines are fairly efficient. The higher speed may be spinning more rpm but you have more speed. The only thing you are changing with speed is really an increase in friction from the air in the form of drag, which is still not a big change. Aero doesn't come into play until much higher speeds.

What WILL lose you mileage is the fact you are going to a 9". The size of the gear + physical weight is more rotating mass and every time you accelerate it will require more energy to gain speed than the 10 bolt.

So don't worry too much about highway mileage, it'll be your city mileage that will see a bit of a drop.

supernaturalta

iTrader: (6)

x2
Almost any modification to a car is going to be a trade off. You just need to decide what's more important to you and go with that.

I lost 2-3mpg on the highway just going from 3.42 to 3.73 gears, so I figure at least 3-4mpg would be lost going from 3.42 to 3.89 gears. Again, if gas mileage is more important, go with a lower numeric ratio.