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Old 06-15-2005, 10:33 PM   #1
cwilkie
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Highest Gear Ratio with TH400

What is the highest gear ratio that can be put in a rear end using a TH400? I would like to lower my RPMs a little. Currently I have a 396BB with a TH400 using a 3.07 rearend. Some would say to go to a 700r4 but I want to keep my TH400 because of sentimental value. Anyway, all tech advice is needed.

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Old 06-15-2005, 10:42 PM   #2
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Are you wanting to go with a smaller numerical gear? How tall of tire are you running? What rpms are you turning at what MPH?
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Old 06-16-2005, 12:10 AM   #3
Robert1970C20rstbukt
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I always thought that 3.07's were as high as you could get?
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Old 06-16-2005, 12:21 AM   #4
67chevemall
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ya youd probaly have to get a custom rear end.
That set up is great for long hauln and pullen though.
email the Nascar tech's if you want to go Fast.
Are ya buzzn for speed?
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Old 06-16-2005, 12:41 AM   #5
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2.73's in a 1995 Camaro with a 4L60e auto. 2,000 rpm's at 77 mph. Cruisin'.

Quote:
Originally Posted by cwilkie
What is the highest gear ratio that can be put in a rear end using a TH400? I would like to lower my RPMs a little. Currently I have a 396BB with a TH400 using a 3.07 rearend. Some would say to go to a 700r4 but I want to keep my TH400 because of sentimental value. Anyway, all tech advice is needed.

Chance
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Old 06-16-2005, 10:03 AM   #6
cwilkie
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Gears

I want to decrease my fuel consumption and I am buzzing for speed. Actually, I would mind getting a heavy duty 700R4 but it's not cost effective in my current situation; however, I found one I liked at Mad Dog Transmissions (The God Father) but I do not have that kind of ca$h at the moment.

I have mid 1980s Chevy truck 15x8 really wheels. I do not know how fast I am going since the speedo gear is wrong; however, I would guess at 2500rpm I am going 50-55mph.

Are 2.73 the next step higher to 3.07? Suggestions are more than welcome...they are needed.

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Last edited by cwilkie; 06-16-2005 at 10:08 AM.
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Old 06-16-2005, 10:31 AM   #7
kwmech
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The 2.73 ratio fits a later model 10 bolt axle. Has a carrier all to itself.
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Old 06-16-2005, 11:15 AM   #8
lloyd k
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i have a 2.73 ratio in my 67 converted to leave spring rear,it works well with my combo because i have a big block cadillac in it.my carrier has a 12 bolt cover not sure what year it came out of,i would assume a mid 80s truck,i changed from a 3.42 and it made a noticeble difference all the way around.not sure you will see a big difference from a 3.08.
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Old 06-16-2005, 11:40 AM   #9
67ChevyRedneck
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Hate to say it, but the only way your gonna get what you really want is a 700R4, like what has already been said, you're not going to see a big drop in RPM's. Check out the link below. You input tire height, a speed, and rear-end ratio and it will give you rpm's for and auto tranny and an AOD tranny. If you input different rear ratios you will notice there isnt much of a drop from a 3.07 but the700R4 will give you a noticable drop. My 67 with a 350/700R4/3.73 runs about 2,400 rpm's at 75mph and my 2000 Z71 with 327/4L60/3.73 runs about 2,000 rpm at 75 mph. With a 700R4 and 3.07 I wouldn't recommend any heavy hauling, might want to swap to a 3.73 for that......

RPM calculator
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Old 06-16-2005, 10:01 PM   #10
67chevemall
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Get a standard 1:1 4 gear tranny and play with the gears?
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Old 06-16-2005, 11:56 PM   #11
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Check out link below, they seem to have a 2.78 gear that will fit the 3.07 carrier, that will drop your rpm about 200 rpm. Going from say a 28 inch tire to a 30 inch will drop another 150 rpm. New gears, setting them up and new rear tires are going to set you back serious bucks. I think it would take a LOOOOOOOONG time to pay that back.



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