more rear end gearing
Thread Starter
Cruising
Joined: May 2003
Posts: 14
Likes: 0
From: Alpine Ca
more rear end gearing
my 75 says that it has a 3.36 ratio and i am running 245 60r 15's it also has a th400. my problem is that it turns about 3500 rpm (or more) at 70 and 5000 or just short of that at 80. I like how well it breaks them loose but high speed mileage is killing me. Do you think that the ratio is really what it says. I can't afford a gear vendor but i was thinking that at least changind the ratio might help. I was hoping to get 75 at 3000 (if it is possible) and also where is the best place to get the ring and piniion, zip has them but they do not have all of the ratios that were originally offered. thanks, coel
Race Director
Joined: Dec 1999
Posts: 19,610
Likes: 778
From: Forked River NJ
Re: more rear end gearing (redrider75)
Something does not sound right. Either your tach is not right or you have some major torque converter slippage. My 3.70 gears turn 3500 at a little over 70. At 5000 I'm at about 95. I have a 4 speed so there is no slippage. Your tach I think is electronic. I would suspect something is amiss in there. My calculations with 3.36 gears at 3000 you should be at about 70.
Drifting
Joined: Sep 2001
Posts: 1,427
Likes: 1
From: Boxford Mass
Re: more rear end gearing (redrider75)
You can do a po' mans ratio check by lifting the back of the car, putting it in neutral, spin the tire once & have a friend or loved one count the driveshaft rotations.
You are probably just shy of 4 complete rotations, indicating a 3.70
oh yeah, dont forget about all that safety stuff when going jacking the car up, like blocking the front wheels...
You are probably just shy of 4 complete rotations, indicating a 3.70
oh yeah, dont forget about all that safety stuff when going jacking the car up, like blocking the front wheels...
Drifting
Joined: Nov 2003
Posts: 1,634
Likes: 4
From: Northern NJ
St. Jude Donor '04 & '05
Re: more rear end gearing (redrider75)
I have a 75 L-48 w/a 4 spd and 3.36 gears. At 70-75 mph, it's turning 3000-3100 RPM. At about 90 mph, it's still around 4000, give or take a bit. Even my Camaro (below) w/4.10 gears is only at about 4000 or so at 70-75 based upon the traffic around me. However it's hard to say exactly on this one until I get the speedometer fixed. :D
According to the 75 sales brochure I just looked at, if your 75 is a L-48 w/the TH400, the only gears originally available were 3.08 std and 2.73 optional. If it's a L-82, then 3.36 std and 3.55 optional were the choices with the TH400 trans. Only with a 4 spd could you get an optional 3.70 ratio. Of course over the 29 years since these things were new, anything could have been changed.
I suspect if your really turning 3500 at 70, it would be impossible to actually be turning 5000 at 80 "IF" everything is working correctly. I agree w/Gordonm that either your electronic tach is amiss, or you have an issue with the trans.
Hope this helps. And Welcome to the forum. Lots of good advise to be had here. Someone will jump in and add more to this I'm sure. :yesnod:
And, let's see some pics! :)
:seeya
[Modified by ACECO, 2:45 AM 12/31/2003]
According to the 75 sales brochure I just looked at, if your 75 is a L-48 w/the TH400, the only gears originally available were 3.08 std and 2.73 optional. If it's a L-82, then 3.36 std and 3.55 optional were the choices with the TH400 trans. Only with a 4 spd could you get an optional 3.70 ratio. Of course over the 29 years since these things were new, anything could have been changed.
I suspect if your really turning 3500 at 70, it would be impossible to actually be turning 5000 at 80 "IF" everything is working correctly. I agree w/Gordonm that either your electronic tach is amiss, or you have an issue with the trans.
Hope this helps. And Welcome to the forum. Lots of good advise to be had here. Someone will jump in and add more to this I'm sure. :yesnod:
And, let's see some pics! :)
:seeya
[Modified by ACECO, 2:45 AM 12/31/2003]
Thread Starter
Cruising
Joined: May 2003
Posts: 14
Likes: 0
From: Alpine Ca
Re: more rear end gearing (ACECO)
thanks for the advice so far. and by theway it is an L-82 I may have been a little high with 5000 it is more like 4800 and 83mph, the tack does seem a little funny especially one time a plug fouled and during that time the tach (when driving on the freeway) would drop 500 rpm or so and then jump right back up and it would just keep doing that over and over. The engine has been rebuilt and i kinda believe the tack because yellow line is at 5000 and the engine starts up a hum cycle like it is being reved to far at about that point . I haven't ever had it above 85 though. I have had the car fo about 9 months now and I bought it when i was 15 (my parents are really cool and i'm a good kid). I am making the payments and the main reason for concern with the rear end is that it makes gas mileage worse than i think it should be and i gotta pay for that too, along with insurance. I have some picture to post but i don't know how :confused:
