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Discussion Starter #1
Ok here goes guys. One of our members just bought a C/3 350c/in. with a 350 auto. He Loves the car ,hates the gas mileage. Seems he's running 65mph and the tach says he's turning 4600 rpm. I suggested he change the rear end to a different ratio. Anyone have any ideas.
 

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wow yea id say hes probably got some very low gears in there 4:11 maybe ...but im not gonna embarass myself by trying to go into how to check its been so long since ive done that.

there are some people with some insane levels of know-how on rear ends that will probably chime in but stock is normally in the 3:08 range (i think)
I'm pulling 2000-2300 or so at that speed with a 350,factory 4 speed and stock gears i get 23-26 mpg in my car ..only a mild cam and not much else at all.
 

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I would suggest changing out the rear end to a higher gear like a 3.08. Another would be to swap out the trans to an overdrive. The OD trans will show the biggest difference.

:agree: Overdrive transmissions are awesome. I put a TH2004R in my Vette that has factory 3:08 gears.

I'm expecting 2,100 - 2,300 RPMs at 70 MPH.
 

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i would recommend a 700R4 swap, its a great upgrade and not much needed to install it just a driveshaft modification and a crossmember, I will actually have a crossmember for sale soon for a 700R4
 

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something is amiss if he's accurate with his rpm and speed description. Even with a 4.11 rear gear he should be going 90 mph at 4600 rpm (assuming the tranny is 1:1 and 27" diameter tires). Either the tranny isn't shifting into third or he's got some other issue.
 

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just crunched some more numbers and interestingly enough, a TH350 in 2nd gear (1.52:1 ratio) with a 3.70 rear gear would run 65.7 mph at 4600 rpm with 27" tires. He needs a tranny rebuild :thumbsup:
 

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:agree: Overdrive transmissions are awesome. I put a TH2004R in my Vette that has factory 3:08 gears.

I'm expecting 2,100 - 2,300 RPMs at 70 MPH.
I did the same thing to my 76 that had the 3:08 gears. No driveshaft mod or cross member mod. Got my 2004r from BowTieOverdrives.
:thumbsup:
 

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Hope you have better luck with yours than I did mine. I bought a 700R4 from Bowtie, stage 2. It lasted 10000 miles and cratered. The torque converter went out and when the local shop took it apart, they found all kinds of inferior parts inside. Even if the converter hadn't cratered, the tranny wouldn't have lasted much longer.
 

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I enjoy my 2000rpms at 70 with my six speed;) and 4.11s
 

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Discussion Starter #11
Thanks guys,I knew I could count on the guys here to have some answers for my friend. Seems to be agreement that a transmission replacement and a gear swap would be the way to go .
thanks again and Please keep the comments coming.
 

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ok here goes guys. One of our members just bought a c/3 350c/in. With a 350 auto. He loves the car ,hates the gas mileage. Seems he's running 65mph and the tach says he's turning 4600 rpm. I suggested he change the rear end to a different ratio. Anyone have any ideas.
if i remember right, to check the rear end. Mark the drive shaft, spin it one full turn, count how many revs the rear wheel makes. That should get you close.
 

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What year is his TH350? If memory serves, the 79-81 TH350s have lock up torque converters. Sounds like his may not be locking.

:thumbsup:
 

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Discussion Starter #14
What year is his TH350? If memory serves, the 79-81 TH350s have lock up torque converters. Sounds like his may not be locking.

:thumbsup:
It's a 79. He bought it from a private party and the car while is looking good, has had no work done on the drive line, so the converter and trans could be in need of a flushing and overhaul.
 

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If his tach & speedometer are correct, he would have to have at least a 4.88 rear gear for the numbers to work.
Even for that to work the converter would have to be slipping at least 10%, which would be pretty horrible.
With average converter slip it would take a 5.13 gear for the numbers to work
Something sounds a bit odd. Maybe the tach or speedo is just badly off.
 

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Discussion Starter #16
If his tach & speedometer are correct, he would have to have at least a 4.88 rear gear for the numbers to work.
Even for that to work the converter would have to be slipping at least 10%, which would be pretty horrible.
With average converter slip it would take a 5.13 gear for the numbers to work
Something sounds a bit odd. Maybe the tach or speedo is just badly off.
The problem is that the numbers are general. I was not in the car to see for myself. The guy who is my friend is not mechanically inclined. So your guess is as good as mine. I suspect the truth is he needs to have the trans check out for proper operation. Then go for a gear change as well as a trans change.
 

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If his tach & speedometer are correct, he would have to have at least a 4.88 rear gear for the numbers to work.
Even for that to work the converter would have to be slipping at least 10%, which would be pretty horrible.
With average converter slip it would take a 5.13 gear for the numbers to work
Something sounds a bit odd. Maybe the tach or speedo is just badly off.
unless he's stuck in 2nd gear. Then a 3.70 rear gear would give him the engine rpm's described above at 65 mph.
 
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