OldCarBum

Like many of us on this forum, I am interested in reducing my highway driving rpm's by replacing my TH400 for an overdrive automatic transmission or by replacing my TH400 for a TKO or other 5 speed manual. I read in a recent thread about Gear Vendors Underdrive/Overdrive units, went to their website and it appears from what I read, that his is the best route to take. The units are basically a bolt on unit with minimum, if any, clearance modifications to the trans tunnel, the drive shaft needs to be shortened, but other than that its a bolt on overdrive that gives me 6 forward speeds over the 3 I have with my TH400. The standard units will except 1200hp and come with a 2 year warranty, even if used strictly on the drag strip. Using this unit I can retain my stock TH400 and keep my C3 with the transmission it came with. The cost from Gear Vendors is $2695.00, which is less than replacing my transmission with a new overdrive automatic or switching to a manual.


My next step is to contact their tech department and start asking questions.

I'm not looking for everyone's opinion, my questions are simple:
1). Has anyone purchased and installed one of these units?
2). Did you have any installation issues?
3). Did it perform as advertised?

claysmoker

I don't think it will fit the tunnel without some serious mods.

hugie82

GEar Vendors is a very strong unit and you can get a good buck if you decide to sell it but it is a very expensive option that may not fit. I don't know anyone who is running one. The go to overdrive is the 700R4 , 200R4 and the tremec 5 speed OD.
IMHO the 700R4 is the easiest convertion because GM already did it for you. The stock shifter can be used, just notch the shifter gate for (low 1) . Order an 82 corvette drive shaft and a torque converter lock up switch . Add a few minor parts including the short tail shaft 700r4 and you are done.

Seasoned80

Do a search on gear vendor. 2004R is easiest.

widowmaker221

I would love to do this install too but all the research I have done on this says it will require alot of modifications to make it work thus I am buying a 2004r in the Spring.

jb78L-82

I looked at gear vendors as well and decided against it since it requires modifications and the gear reduction in 4th is only 22%...I prefer the .64 OD of the 5 speed....Still way too much money for me to spend on an occasional use toy....whether GV or the Tremec....At $4,500-$5,000 for the tremec all in...no way...$2-2,500 would be another story. Honestly, I think the money it takes to do a manual conversion is really high

OldCarBum

I agree, that's why I'm still looking into all the options available. I know TCI builds a 2004R that is rated for 525hp which is a simple swap for my TH400 and Summit Racing offers it for about $2,200.00. I found that going to a manual TKO 600 has basically the same final gear, however the cost and the modifications involved does not make since with a final price tag of over $4,000.00. At this point I plan to keep my "73 for a long time once it's finished and the type of driving I plan to do with the car and bringing the highway rpms down to a reasonable rate is worth the investment. Plus I want to run in the fast lane at 70+ mph and not plug along in the slow lane at 60.

molsen

There were some posts on this subject, gv can be done, but you will need serious right side tunnel modifications.

OldCarBum

Thanks everyone. My hunt continues.

Richard Daugird

If you don't drive in stop and go traffic, I'd seriously consider going manual, maybe from a wrecked Camaro/Firebird. I originally wanted an automatic, based on when I used to commute an hour+ each way to Houston for years. I ended up with a four speed. I am really glad I did, because shifting a V8 sports car on a weekend cruise(the purpose I bought my Corvette for) is WAY more fun than fighting rush hour traffic in a stick shift econobox. Now I don't think I would even WANT this car if it had an auto. Totally made a convert out of me. Now if I only had one more gear on the freeway...

7t9l82

An overdrive transmission is nice but apparently we have all driven newer cars that ran lower rpm on the highway. And for some reason it bothers us to hear our engines running at high speed. The fact is that's how these cars were built.before that lunatic Jimmy carter gave us thee 55 mph speed limit we drove these cars at 70 and 80 mph all day long they were made to do it,sure fuel mileage is less and engine wear might be more but synthetic oil helps alot on the engine wear and the $2000 + dollars for an overdrive can buy a heck of alot of fuel. I ran up the interstate in the left lane running 90 the other day and the old girl felt better than putzing around town. I may bite the bullet at sometime and change it but it's no longer a priority.just another perspective.

OldCarBum

You are totally missing the point. My 454 has 3.36 rear gears. If I throw in an overdrive transmission I can cruise down the highway at 70 mph @ 2057 rpm, I can easily run at 80 mph @ 2351 rpm, at 90 mph @ 2645 rpm and I can run all day at 100 mph and I'll only be @ 2939 rpm. The 200r4 has the same third gear ratio (1.0) as the TH400 and with the second gear ratio of 1.57 it will be great for running on a winding road. I'd like to build my 454 to around 450 ft lbs of torque. With the right trans, suspension, steering and brake upgrades I should be able to run pretty well with the big boys. Yes it would be a little more fun with the 5 speed manual, but the additional $1,500.00 to $2,000.00 will pay for the new forged crank, rods, pistons, and the new Holley.

mrvette

My '72 was a muncie 4 speed , and when putting in a 700, the floor pan/tunnel was cut/glassed in about 2" wider on pass side.....to clear the pan, your car having a 400 already should fit it fine, but one thing, in traffice the drop from first to second gear was very noticeable and my trans had a tight converter and Trans Go shift kit....I finally tired of it and went for a 200 4r trans....I built that one myself and have maybe a grand into it.....far better gear selection for street driving....same TGo shift kit mod.....

69ttop502

Why not just change the rear gears to 3.08's or even 2.73's?

jb78L-82

A 2.73 gear with a 27 inch tire in 4th 1:1 @ 2,500 RPM would yield 74 MPH BUT would really kill acceleration versus a 3.7 rear.

A 3.08 rear with a 27 inch tire in 4th 1:1 @ 2,500 RPM would yield 65 MPH. At 75 MPH the same rear 3.08 yields 2,900 RPM which is really still too high for mileage and engine wear.

Really need to have a .64 OD to achieve relaxed revs at 75-80 MPH...no way around it

stingray2

I installed a Th400 with a gear vendors unit in my 1970 with a modified 502 ramjet. Both sides of the tunnel have to be modified especially the passenger side but not that bad. The car was a manual origonally so I made my own removable cross member. I can't tell you how well the unit works yet & now I plan on pulling the motor & trans out to use in my 1968 C10 project. See pictures.

OldCarBum

Thanks for the photographs. I'm pulling the body off and re-doing the interior so the tunnel mod is no big deal. I'm still leaning toward going with the 2004R and all this information helps. Let us know how the unit works in your other project.

c3_dk

Agree it is like a lot wants to drive a v10 diesel in 8th gear, with a 2.0 rear gear

chevymans 77

I did a good amount of research on the Gear Vendors unit beck when I decided I needed an overdrive to make my car more friendly for the street. I didn't like the gear split on the 700R4 so I removed it from the list of options. The 4L80 was another option but the weight and the amount of HP to make it work was a concern along with trans tunnel mods needed to make it fit, I had no plans to pull the interior nor was I planning to pull the body any time soon. The computer needed to make it work in our cars is a little expensive but comparable to the total cost involved with the Gear Vendors unit also. I settled with the 200 after doing the research the GN guys have been abusing them for a good while and if built correctly they can handle a good bit of abuse. My personal feelings are the Gear Vebders unit with a T400 is the best option for someone throwing a lot of power at the trans., the 4L80 is probably the next best option but both require trans tunnel mods. The 200 is a compromise but can be built to handle the abuse and requires little work to make it fit and function in the C3.

Neal

c3_dk

That is why you need a 3.70 or 4.11 rear gear, and a TH2004R