Rear end ration
Thread Starter
Racer
Joined: Oct 2021
Posts: 258
Likes: 26
From: NY
Rear end ration
My 1968 has 411 gears, (4 speed manual) the engine is a 454 tricked up to a Dyno'd 450hp, as it was built for drag racing (it never saw the strip). On the highways at 65-70 I'm turning 3500 rpm, so I'd like to change the rear end gears to something that would get me below 2000 rpm at 75 mph. There is plenty of power so I'm sure this can be done without degrading performance. Can you recommend the proper ratio that would be readily available? Also I would have to change the speedometer gear in the transmission to match the new rear end.
Thanks
Thanks
Popular Reply
Jul 5, 2023, 10:42 AM
Le Mans Master
Joined: Aug 2005
Posts: 6,541
Likes: 1,975
From: Temple Terrace Florida
If you trade up to a rear that will give you 2K RPMs at highway speed, your car will be a dog off the line and a pain in traffic. Why not swap in a Tremec overdrive transmission. You will have the best of both street and highway. I have 4:11 in my 66 with a small block L-79. On the highway my engine is around 2200 RPMs and a rocket off the line. Jerry
Le Mans Master
Joined: Aug 2005
Posts: 6,541
Likes: 1,975
From: Temple Terrace Florida
If you trade up to a rear that will give you 2K RPMs at highway speed, your car will be a dog off the line and a pain in traffic. Why not swap in a Tremec overdrive transmission. You will have the best of both street and highway. I have 4:11 in my 66 with a small block L-79. On the highway my engine is around 2200 RPMs and a rocket off the line. Jerry
Le Mans Master
Joined: Aug 2017
Posts: 8,336
Likes: 2,810
From: Cool Northern Michigan
Lots of forum members like their 3.73 gears. Some opt for more conservative 3.55s.
No matter which route you decide on, rear-end or O.D. trans, both are big projects. Especially removing the differential.
And yes the speedometer gear or gears will have to be changed. The driven gear behind the speedo cable is pretty easy to swap. But, you can only change speedometer gear ratios so far until you reach a point where the driven gear has to be swapped out as well. Lot more labor involved with that.
In the past, the entire diffy could be strapped on a pallet as a "core charge" and shipped to a Vette vendor. In return and $1200 later you received a rebuilt unit of your choice ratio. IDK if that is still an available option.
OR
Send the diffy to our forum rear axle man. He can set up a new ratio, check things over, return it to you.
No matter which route you decide on, rear-end or O.D. trans, both are big projects. Especially removing the differential.
And yes the speedometer gear or gears will have to be changed. The driven gear behind the speedo cable is pretty easy to swap. But, you can only change speedometer gear ratios so far until you reach a point where the driven gear has to be swapped out as well. Lot more labor involved with that.
In the past, the entire diffy could be strapped on a pallet as a "core charge" and shipped to a Vette vendor. In return and $1200 later you received a rebuilt unit of your choice ratio. IDK if that is still an available option.
OR
Send the diffy to our forum rear axle man. He can set up a new ratio, check things over, return it to you.
Thread Starter
Racer
Joined: Oct 2021
Posts: 258
Likes: 26
From: NY
It looks like my question is put to rest. I'll live with the 411 gears and drive it no more than 60-65 cruising. That's a lot easier than changing the gears, I didn't know it was such an undertaking to change them. It ain't broke so I'm not going to fix it.
Thanks for your responses.
Thanks for your responses.
Thread Starter
Racer
Joined: Oct 2021
Posts: 258
Likes: 26
From: NY
Thanks.
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just look fo r a used rear end that has 3.08 s in it. They are on the Marketplace shouldnt be more than $500-$600. They arent a sought after ratio and the big blocks mostly came with them......that being said I have 4.11s and a 5 speed...it is the correct answer, just save up and look fo used ones. I got a deal on a used McLeod Muscle car 5speed. ITs a good tranny and would be perfect for you. With the TKX out the tko 5 speeds tko 500 and 600 are on the chopping block and would be a cheap alternative. They dont like to shift at high rpm thats why the hotroders want to swap them pout
Thread Starter
Racer
Joined: Oct 2021
Posts: 258
Likes: 26
From: NY
just look fo r a used rear end that has 3.08 s in it. They are on the Marketplace shouldnt be more than $500-$600. They arent a sought after ratio and the big blocks mostly came with them......that being said I have 4.11s and a 5 speed...it is the correct answer, just save up and look fo used ones. I got a deal on a used McLeod Muscle car 5speed. ITs a good tranny and would be perfect for you. With the TKX out the tko 5 speeds tko 500 and 600 are on the chopping block and would be a cheap alternative. They dont like to shift at high rpm thats why the hotroders want to swap them pout
Race Director
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5 speed is definitely the best choice to have your cake and eat it too. And that Muncie can be sold to help offset the upgrade.
.68 O/D will be like 2.79 in 5th....idling down the road.
JIM
.68 O/D will be like 2.79 in 5th....idling down the road.
JIM
Thread Starter
Racer
Joined: Oct 2021
Posts: 258
Likes: 26
From: NY
That would be perfect. So what exactly should I look for?? Will this require a change in the driveshaft?
Thread Starter
Racer
Joined: Oct 2021
Posts: 258
Likes: 26
From: NY
Tech Contributor
Joined: Aug 1999
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From: Connecticut, USA
correct. The only way to drop cruise RPM under 2500 is with an OD trans, like the TREMEC
Melting Slicks
Joined: Jul 2006
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From: Salmon Arm, BC
As Rescue said in post #9 above that the Tremec TKX is the latest trans out, and some people are replacing their TKO 5 speeds with the TKX.
The TKO has a issue with not wanting to shift from 2nd to 3rd above 6000 RPM. I have one of these in my car and absolutely love it.....and I've shifted 2nd to 3rd at 6300.....you just have to be precise with it....not rammmy.
You might be able to get a good priced used TKO, just make sure it has had the shifter placement modified for installation in a Corvette.
The TKO has a issue with not wanting to shift from 2nd to 3rd above 6000 RPM. I have one of these in my car and absolutely love it.....and I've shifted 2nd to 3rd at 6300.....you just have to be precise with it....not rammmy.
You might be able to get a good priced used TKO, just make sure it has had the shifter placement modified for installation in a Corvette.
Race Director
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There are several vendors out there with complete TKX kits.....Silver State or something? I don't know them all...but search around..there are several.
JIM
JIM