3.55 is ok with the .64 OD if you have plenty of low end torque. I can drive the Blue Ridge Parkway at 45mph and stay in 5th most of time. I have a 454 so don't know how a small block would do. Any taller gear definitely needs the .80

 

I have 3.70 gears with my T56 and mainly use 5th gear on the highway and 6th gear on the interstate. Sometimes while cruising on the highway I'll use 6th just to quiet the ride with the sidepipes.

As far as the size goes, the T56 is a little more work, but it fits.

 

I have a T56 with the .50 overdrive in my '79 C3 with 275/60/15 which are 28" tall. I went on a drive through the desert last weekend where I can keep it up over 80mph for a good 30-40 mile run. Used 5th. I have only found 6th to be useful when I don't want the engine to slow the car down when cruising down a hill at 60+mph. 6th is better than neutral or 5th in that case and it cools the engine way down.

This is all with 3.36 gears. 1st gear takes it almost to 60mph. I expect an entirely different world to open up when the rear end is re-geared.

If you have a 4-speed and like how it is and don't even intend to regear to get more out of what you have, you won't find much use for the 6th gear.

Of course, in my case, when re-gearing I'll have to think a little bit about the driveshaft speed. I don't intend to ever take the car past MAYBE 120mph ever so I think it will be okay. I had my driveshaft shortened and balanced for the T56 and did the rest of the work and fabrication on my own.

 

I have a much longer driveshaft and at 120 no whipping/vibes you should be Ok. Vette shaft is so short-
Dont be afraid of gear with a T56 have 4.11 26 in tall tire and 6th sitll have to have quite a bit of speed up to use it part of its the cam though it can cruise at lower rpm it isnt happy below 3k

 

If I try to hit or exceed 120 now, driveshaft wobble will be the least of my concerns!

 

Nice clean undercarriage. Looking at your crossmember(nice job), I see the two screws per side, pointing down. I'm guessing they go in the stock holes? I can't figure what the single screw on top, both sides, pointing fore and aft, is for.

 

If you would like both of best worlds a hot rod / cruiser then a TKO 5 speed is the way to go. We have a complete kits for a C3 Vettes. All kits require no body mods. For example you can run a 3:73 gear with a 26.5 rear tire and a TKO 600 w/.64 OD your RPM's will be 2,100 @ 70mph. Let me know if you have anymore questions.
Take care,
Steve
Hurst Drivelines
707-544-4761 EXT452

 

Like the .6 or .8 5th better than the .05 of 6th more versatile
If i hadnt done a T56 i think the tko500 would be the pick

 

Only tunnel cutting for your 78 is to get the shifter to the inside of the car. If you already have a manual trans this would be minimal. Almost the same hole as a TKO unit.

You can see the size of the hole when I went from a TH350 auto to a T56 here

https://www.corvetteforum.com/forums...-corvette.html

 

Here is my view on 5 speed vs 6 speed.

My car is a 3.08 with a T56 magnum 2.66 1st, .63 6th gear.

In town I use 1st - 5th gear. 5th gear anything above 90km/hr (55mph) 6th gear 65mph +. This keeps the RPM in the 1800 RPM range for normal driving.

Why most people think the extra gear is a waste is because most corvette owners like to drive in the 2500-3000 RPM range so when they put their foot to the gas it pulls hard (without down shifting). They only do this for about 2-3 seconds before they are way above the speed limit of the road.

Anyone that also owns a C6 manual car using a 3.42 rear and the same 1-6th ratio knows what a good driving car the C6 is. But will also fall in the same category (never use 6th) if they enjoy 2500-3000 rpm normal driving range. If you fit in this category you can use 1-5th easily therefor all you want is a 5 speed.

I also use my car for track days which only requires 1-3rd gear. Running high rear gears 3.73 make for more shifting and slower laptimes. Current setup is to maximize rpm range for the straights.

 

What you said definitely applies to me. I prefer to keep my engine in the 2,000-2,700 range when cruising. My cam's advertised range is 1,900-5,600 (although I pull it to 6,000 all the time) and my intake's range is 1,500-6,700. Although there is some grunt from 1,300-1,900 rpm, getting power out of it really dumps the vacuum which indicates poor economy to me. It doesn't even run cooler under load at lower RPMs. I have a choice of 5th or 6th on the highway, both run equally as hot but 5th gives me a higher vacuum reading. Just seems to make sense to keep the RPMs in the mid 2,500 range for my setup.

I wonder how different it really is with a modern engine. Even my '06 Expedition exhibits low vacuum conditions under 2,000 RPM under load as indicated by the fact that when I hit a long hill and it doesn't downshift my A/C starts blowing out of the defroster vents until I let off the gas or downshift. I think if I put a vacuum gauge on it, I would notice the same thing that if I ran it outside of overdrive at 65mph it would have higher vacuum and better economy.

Maybe I'm putting too much weight on the vacuum readings for economy, I'd like to hear a rebuttal to that if it exists. A friend of mine is convinced that lower RPM = better economy but that's just not always true.

I wonder if a stock L48 or L82 would give different vacuum readings than what I'm experiencing. My engine is not at all like a stocker.

 

I think that your friend wins hands down on this issue....Lower RPM's will trump vacuum readings all day for the best MPG.

Vacuum readings are most useful for the same engines, running the same gearing and transmission since the vacuum gauge is simple a throttle position monitor and load indicator. My 73 Nova SS back in the 70's had a vacuum gauge with the Turbo 350 and 350 engine. For best economy I would try and keep the vacuum as high as possible on the gauge.

MUCH of the big gains in fuel economy in the last 20 years on cars is simple, lower the revs at highway speeds. My 2001 Grand prix (3,600 lbs) with the 3.1 V6 turns 2,200 RPM @70 mph and gets 30 MPG...30 MPG on a level road with a steady throttle, with a 15 year old car (makes you wonder why I would want a new 4 cylinder car that gets 32 MPG on the Hwy?)! My 2012 Lexus IS350 F Sport with a 6 speed auto and 306 NET HP turns 2,500 RPM at 80 MPH and gets 29 MPG. My 2010 C6Z06 with a 505 NET HP 427 SBC turns 1,800 rpm @ 80 MPH and gets 28 MPG (think about that one for a minute). Take all the C3 owners that convert to 5/6 speeds and report 21/22 MPG leaps from 15/16 MPG just with the OD's on the highway, as long as the cam operating range starts at 1,800-1,900 RPM. There are other factors involved but the basic MPG gains come from lower revs, pretty simple. GM figured this out 20 years ago and makes you wonder how far have we really come with MPG gains with all the sophisticated electronics.....???

My GM v6 3.1 and 3.8 motors basically lug at highway speeds in OD but all of them deliver stellar fuel economy. OD's are just that OD's and NOT acceleration gears..they are designed for fuel economy and to reduce engine wear. If you have a 5/6 speed and need to accelerate quickly you need to downshift to 4 or 3 or have the auto do it for you. There seems to often be a comment about OD's with manuals about acceleration...that is not what OD's are for...on the street. If I raced my C3 and had a 5 speed, I would probably want the .82 OD, but on the street with my 3.70 gears, I would opt for the .64 OD all day long. As for lugging my 355 SBC (even the OEM l-82), I can lug it down to 1,000 RPM now with the 4 speed in 4th with zero stumble, hesitation, plug fouling etc....The 355 L-82 could run all day long easily at 1,800-2,000 RPM with zero issues if I had a 5/6 speed on the street. I think the sweet spot with my motor would be about 2,200-2,500 RPM @ 70-80 MPH instead of 3,500-3,800 RPM with my current setup.....which would probably net 20-22 MPG!

Hope that helps!

 

I think the rules are different when its carb vs EFI

 

EFI is more efficient absolutely BUT hard to argue with the dramatic increase with carbed SBC/BBC engines that go to OD manuals and autos and see a quantum increase in fuel mileage just by lowering the revs...lots of folks on this forum that will support that observation from what has been reported.....

Physics is physics...taking my 355 L-82 400+Gross HP engine from 3,500 RPM @70-75 MPH to 1,800-2,200 with an OD tranny is going to result in a giant reduction in fuel used to propel the car down the road at said speed. The major difference with EFI versus a carb on the same engine is that the EFI will maintain optimal air/fuel ratio over the rpm range and the EFI may hypothetically result in 25 MPG versus a carb on the same motor with the OD giving 21-22 MPG. The principles are the same....

 

Everything you are saying makes sense and I want to believe that more throttle with lower RPM means better fuel efficiency. I may just have to do some testing with my car to convince myself.

I really don't care as much about efficiency as I do about heat.

 

This is my test sample. 3 driving conditions.

Engine GM ZZ383 rear 3.08 T56 6 speed.

1. Highway 6th gear ~ 70 MPH = 27.8 MPG (CDN)
2. City Driving 1-4th varying speeds = 15.4 MPG (CDN)
3. Track day open lapping (2nd-3rd gears) = 9.66 MPG (CDN)

 

It's not so much having enough low end torque when you have "issues" in overdrive, but more about camshaft overlap. large camshafts don't like tall gears and overdrives.
I have some surging issues with my 496 if I get much below 2k rpm (around 1800) in 5th or 6th gears (.80 and .63 ratios). I have more than enough torque down there, its just not smooth operation.

 

The top of the crossmember bolts to the factory trans crossmember bracket that is welded to the frame. All factory holes used for the crossmember, no drilling.

 

I chose the tko600. Cost was reduced a fair bit for me by buying the box locally (avoiding import taxes/duties and freight) and buying just the conversion specific parts from the US (from Hurst in fact). This meant that I had to cut and weld the shifter with an offset/angle to locate it in the standard location, and open the hole in the floor (covered with a larger aftermarket boot) to allow the shifter through, but it saved a bunch of cash compared to importing the modified box, and looks identical with the console fitted.

 

Don't forget the 1st gear ratio of the 5 speed trans. You might end up with a 1st gear so low that it's never used. IMO, purchasing a 5 speed seems like an expensive solution to a rarely experienced problem. Most of my driving was done at speeds below 50 MPH around town. On the highway I kept to the speed limit and let the wanabe NASCAR drivers weave in and out of lanes far away from me. I had too much work invested in my cars to chance having a wreck.