a few weeks ago i changed to a 4th gen 3.42 rear end from a 2.77 9 bolt. about once or twice a month i take a 2 hour trip and with the 2.77 gears and cruise control set on 70 i would always burn 3/8 of a tank. then in the mornings i would always go fill up then drive home and always burn around the same 3/8 of a tank. well last night i made the trip the first time with the new rear end, and i made it there burning just over a quarter of a tank. this was at a verified 70 mph even though my speedo is off, there is a radar thing right by my house on the highway that shows your speed. when i arived there i parked the car thinking that the gas gauge was probably off so i just went on inside to where i was going. this morning i went to my car thinking maybe the gauge was going to be reading different, but it was the same. so when i went to the gas station, the pump verified that i did only burn that much, it didnt take much gas at all. i remembered the rpms the tach was reading for 70 the night before, and set my cruise off of that. well when i arrived homed i have burned the same just over a 1/4 of a tank. theoretically this isnt supposed to be happening, is this really possible?

 

Yes it is possible, depending on the engine parts combination.
Usually any engine will be very efficient at a given rpm not all the way from idle to redline. It seems you are now cruising closer to the rpm range where your engine is at or close to peak volumetric efficiency.

 

YES This is so hard for me to convince my friends that it can be true. the reason is because that 2.77 or whatever high gear ratio is doing is causing the motor to run at low rpms. they tend to "bog" and hang in overdrive. The computer sees this bogging and richens up the fuel mixture, causing poor gas mileage. A more resonable gear, like a 3.42 or 3.73 will keep the engine in a stronger part of it's powerband while cruising, keeping the fuel mixture normal.

tom

 

thats what i was thinking, the engine mustve been put more into its "sweet spot" with the gears. man what a mod, gas milage increase, and acceleration increase.

 

Hey I'm doing the same mod next month, I'm putting a 3.42 10 bolt in my car in place of the BW 2.77. How much of a difference did you notice with the rear change? Was it worth the time and effort?

 

yes it was very worth it. it completely woke my car up. second gear is now as powerful as first gear used to be. its alot more fun to drive to. i have to nail it at least once every time i drive it now

 

that's the funny thing, you put the better gear in it and notice a fuel economy improvement, then you have to punch it and you lose what you gained!

 

I've experienced this myself. The key is that the engine wants to run where it makes power and highway gears, while keeping RPM down can also keep an engine below it's power band.

 

Same here with me. I had 2:73s and making a 200 mile trip to college I would have to fill up right when I got there. When I switched to 3:73s, it was the same, fill up when I get there. Now I wouldn't drive as fast as I did with 2:73s which was 80-90mph. With the 3:73s I wouldn't go over 70. Considering I was going faster with the 2:73s however the car was running longer with the 3:73s so I figured it equaled out.

 

sofa king, what mods do you have aside from the 4th gen rear?
Did you have to modify anything to put it in? any problems?

 

mods in sig.

yes i modified the stock brakes to work on the 4th gen rear since the rear i found didnt have any brakes included. otherwise its a direct bolt in.