I have encountered something interesting regarding an association between 4th gear and fuel economy. Seems mine and several friends tranny is not going into the final gear. We are all getting poor gas mileage so here's the question.

What RPM are you spinning when you are going 60MPH?

This is the first question of my investigation.

 

That all depends what gears you are running my car with the stock 2.73 gears was running at 1600-1650 rpm at 60 MPH some cars came with 3.23 geras so it would be slightly higher probably around 1900 rpm, and now since I switched to 3.73 I am reving about 2300-2350rpm at 60mph.

what rpm are you seeing at 60??

 

Tire diameter can make a diffrence too. Visit the calculator at www.f-body.org/gears/

 

About 22-2300 rpm, with 3:73's..

 

2K RPMS with 3.42's... but I'm also a V6

 

Mine is around ~2300 RPM while doing ~60 mph, but that's with 4.10's.

When you say "...it's not going into the final gear..." that seems odd. I've never heard of just 4th gear failing in a 4L60E. As a rule, I think 3rd and 4th go out together...could be wrong though? Maybe there's a problem with the lock-up on the converter though? If you drive "nice" (I mean, accelerate normally, not aggressively) then you should feel the 3 shifts (1-2, 2-3, 3-4) but then you should also feel (or at least see via the tach) the RPM drop when the converter locks up. If the converter fails to lock, then the RPM's will read roughly ~300 higher than they should be.

BTW, fuel mileage is more a function of throttle position than it is engine RPM. Going from 2.73's to 4.10's, the mileage isn't really that much worse on the highway, but it's a lot worse if my foot is further down than it needs to be.

 

is your converter stock??

 

Yeah, and I desperately need a stall. Even with the low 4.10 gearing, the car bogged really bad at the track when I ran with my set of BFG drag radials on it. I got my best time (and speed) on street tires...go figure!?

 

I had a stock stock stall with my 3.73 and was running 2300 rpm, maybe my lock up was F*cked and that contributed to my trans faliure? other wise I cant see how you would run 2300rpm with 4.10 gears,

 

Well, I rarely do just ~60 mph...normally I'm doing ~75 mph on the highway, but I'm sorta going off memory, as well as that gear calculator posted above (I've used that calculator for a long time, and I swear by it...I think it's quite accurate ) but I do believe that ~2300 sounds right for ~60. I know that it's around ~2800 to do ~75 mph. I also had 3.73's myself 2 summers ago, but I can't quite remember what RPM's the car would do with those? I know that they jived with what the calculator said though.

If your car revs higher to do ~60 mph than what it should be, either A) the factory tach was off (some people say they can be really inaccurate) or B) maybe your lock-up didn't work? The RPM's will be roughly ~200 - 300 higher than when the converter locks.

 

I dont have the link to the calculator that used to use but I specifically remember that for 3.73 in a 4L60E it was supposed to be 2350Rpm at 60mph.

what tire size are you running?? I have 245-50-16s

 

Same - 245/50/16.

www.f-body.org/gears - that's the one posted above, and the one I've always used. Like I said, with every gear set I've had (2.73's, 3.73's, 4.10's, and even my Dad's '73) it's worked out to be perfectly accurate as far as I can tell.