just changed gears, went to 3.73s, drove around for nearly 2 weeks w/ original speedometer gears. i expected a decrease in MPG but what i was getting was much worse than what it should have been (could damn near watch it go down as i drove). since then, i have the new speedo gears in and it seems im getting much better MPG.
so i was wondering if it was MPH instead of RPM that determines fuel pressure in the 305 tbi? i was getting comments from people that they could smell it pretty bad, now that seems less noticable.

 

fuel pressure has nothing to do with any change in the drive train. it is set mechanically at the throttle body.
you mileage didn't go down much because you just made it easier for the car to move itself around.

later
tim

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What was happening was the car was moving faster that the speedo thought it was. So in essence you were driving more miles that were registering on the speedo. SO like say for ever 10 miles you drove the speedo only registered 8. This would make you gas milage look really worse that actual because after runing out 10 gallons say you went 200 miles. should have been 20mpg, well if only 8 out of 10 miles registered on the speedo then 160 miles would show up and you would think you are getting 16 miles per gallon.
The Milge is calibrated to the speedo gear.
I hope this clears things up for you.
And yes lower gears cause gas milage to go down.

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89 Trans Am Turbo 3.8L All stock 43,000 miles #1053 of 1555

Past Thirdgen:
86 Trans Am w/ built 355TPI with SLP goodies and too much other stuff to List. One sweet *** car, wish I would have had a good enough Job to pay insurance on three cars so I could keep it, but for a 89 Turbo Trans Am w/ Low miles, I think I made the right choice!

 

or wait do I have that backwards?
I dont have time to think about it right now. Lets see the wheels are turning slower due to the gear at the same RPM so the car is going slower than the Speedo sayes right? Um just reverse what I said above LOL.

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89 Trans Am Turbo 3.8L All stock 43,000 miles #1053 of 1555

Past Thirdgen:
86 Trans Am w/ built 355TPI with SLP goodies and too much other stuff to List. One sweet *** car, wish I would have had a good enough Job to pay insurance on three cars so I could keep it, but for a 89 Turbo Trans Am w/ Low miles, I think I made the right choice!

 

You shouldn't be able to smell gas...means you're too rich. Have you played with the fuel pressuer at all?
Anyways, NJSpeeder and Kyle are giving you a fast answer. I'll be a little more indepth since I'm imbetween classes right now and have nothing better to do.
MPG is really dependant on both your RPM and your engine load. If you have tons of engine load like driving a loaded pickup...you aren't going to get great gas milage even if you do have 2.73 gears. With higher gears it might be better for many reasons.
Now if you have a light car like a 3500lb camaro, then any v8 has enough torque to move the car around with very little engine load. With my engine right now I can go up a huge gradient in overdrive at high speed and not have to go beyond 1/2 throttle!
I get great gas milage as long as I stay below 80mph. I have 3.73s now and the gas milage has been better around town but worse on the highway. I get about 20 on the highway when I'm cruizing at 3000rpm.
I still haven't changed my speedo gear on my tranny yet. Note to self: When changing speedo gear, change motor mount AND install that 2800 stall locking tcc!
So you can see in a lighter vehicle you don't need as much gear to get good gasmilage. The best is probably 3.08 for highway and city driving. When I had my 3.08s I got 28mpg on the highway. Big difference! At 3000rpm my engine has very little load (not much extra fuel added) and at 2000rpm (with older gears) it was the same way so obviously the difference in both rpm and engine load will effect your highway milage. As for city, I don't know, I've gotten better gas milage with the higher gears. On light throttle the car shifts from 1-2-3 all below 2000rpm, an Lo3 might be slightly higher.

 

ok, thanx guys.
i realize about the the extra miles and the whole bit about showing faster than going. but when i say i could damn near see the gauge go down, i mean it. it was so pathetic, im basing this on distance traveled. it would take me nearly a 1/4 tank to go from home-to-school-to-home. thats not even 15 miles round trip! it was that damn bad. i used to go the same distance to high school, 5 times a week on less than half tank.
anyway, since i got the speedo gears in i've gone to class twice now and still havent used more than a 1/4 tank.
doesnt make sense to me. havent touched the motor or anything in a couple months now. so i dont know what was going on.

 

BTW, the ecm uses the vehicle speed to so a few things. One important thing is engaging the tcc at the correct time. If you're telling the ecm you're going faster than you really are you'll bet extra stress on the engine (heavy load) and that will hurt your mpg. I compensated for it and just changed my tcc lockup speeds to ~60 since my speedo still thinks I'm running with 3.08s!