Question about driving RPM

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79MaliCrazz

G-Body Guru
Nov 6, 2010
577
28
Lake Wylie,SC
My question is i had my rear changed from a 1 legger to a posi my rpm at 70 mph is about 2800 to 2900rpm is that normal my rpm at 70mph before was at 2500rpm my 1 legger was 3:73 and the new one is 3:73 posi from what i was told can the change of 1 rear end to another rear end change my rpm or do you think i might have a higher gear ratio.
 

-83MONTESS-

Moderator
Moderator
Nov 4, 2010
4,570
113
Bellevue, Ohio
2800-2900rpm with 3.73s and an overdrive trans is way high. I did the calculations awhile ago trying to figure out what gear I should run and with an overdrive trans you should be around 2200-2300 IIRC. Do you know if they put in 3.73s when they did the work or did they swap in some lower gears? If your not sure see how quick you accelerate and compare it to before.
 

79MaliCrazz

G-Body Guru
Thread starter
Nov 6, 2010
577
28
Lake Wylie,SC
Thats what i thought my rpm should be lower when i had my other rear i was about 2500 maybe a little lower i cant remember cause i didn't drive the car much due to other work i was doing.I bought the rear from another person who had it on his Monte and he said he had installed a Auburn 3:73 unit.Im going to look and see if they disconnected my lockup cause they were doing some work under my car around the transmission.
 

pontiacgp

blank
Mar 31, 2006
29,280
113
Kitchener, Ontario
with one T5's with the .68 5th which is the same ratio as the 4th in an overdrive I'm reving 2300 at 75 mph with a 3.08 rear
 

79MaliCrazz

G-Body Guru
Thread starter
Nov 6, 2010
577
28
Lake Wylie,SC
i will checking my lockup tomorrow to make sure they reconnected it.
 

79MaliCrazz

G-Body Guru
Thread starter
Nov 6, 2010
577
28
Lake Wylie,SC
There is no plug there for the lockup so either they removed it or I have bigger gears then 3:73.
 

Phoenyx

Royal Smart Person
Jun 27, 2007
2,392
0
Alberta, Canada
Or maybe the seller didn't really put 3.73s in?
 

GBODYLS3

Apprentice
Feb 11, 2011
74
6
JACK UP THE BACK OF THE CAR AND ROTATE THE TIRE ONE FULL TURN WHILE COUNTING HOW MANY TIMES THE DRIVE SHAFT ROTATES. IF IT IS A 3:73 THE DRIVE SHAFT WOULD TURN 3 3/4 TIMES.
 

79MaliCrazz

G-Body Guru
Thread starter
Nov 6, 2010
577
28
Lake Wylie,SC
GBODYLS3 said:
JACK UP THE BACK OF THE CAR AND ROTATE THE TIRE ONE FULL TURN WHILE COUNTING HOW MANY TIMES THE DRIVE SHAFT ROTATES. IF IT IS A 3:73 THE DRIVE SHAFT WOULD TURN 3 3/4 TIMES.

if it turns more then its a bigger gear?
 

pontiacgp

blank
Mar 31, 2006
29,280
113
Kitchener, Ontario
if you don't know what gear you have then you don't know how old the oil is or what condition it's in.....fresh oil in the rear frees up some HP and gets you better gas mileage, plus you'll find out what gear you have.
 
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