I have 3.55 rear gears, lots of torque, and a .68 OD. In regular street driving this is a nice combo. I can cruise down I-25 at 80mph at a reasonable rpm (2000'ish). easy to drive, and plenty of torque for hills and passing.
But, on the track, it sucks. Drop it into OD and I might as well throw out a parachute. Can't accelerate much at all. If I were to do it over again, I'd go with .82.
I'm sure I will have the minority opinion on this but here's my take on it. The cam you are running is a big factor. Where does the power band start? I went with a .80 because the power band for my cam is 2400 - 6000. The .68 would have me cruising below the power band. The other factor I considered was the kind of driving I do in the Cobra. I spend very little time on the interstate and a lot of time on back roads. The .80 puts me in a perfect RPM range (2200 - 2700) at 55 - 70 mph on twisty, hilly back roads. I made a spread sheet to assess RPM vs. speed for the various gears and picked the one that would work best for my style and engine. (P.S. I had .68 w/3.55 with a B-cam 5.0 in my MKII and hated it because it lugged the engine in 5th below 70 mph.)
Have 345hp crate motor w/ carb, 275's, 3.55's and a TKO II w/ .83, we have shorter tracks up here and have not yet needed 5th (very close though 4th @ about 5800 - 5900 at the end of the longest straight, with a little more motor I will need the 5th, I think, will have to see what the future brings) on track which makes me wish I had the .62 when I am on the highway running 2700 r's plus to keep up w/ the poky traffic ... depends on your use ... a 6 speed might be even better ????
Bob G., if your accelleration is sluggish in 5th with .83, then .62 will be even worse! The .83 is closer in ratio to the 1:1 4th, so you lose less rpm's shifting to 5th. For me, the .62 is almost useless under 75-80, unless I'm on a long, flat interstate cruise, in which case, it saves some gas by keeping the car at idle. If I buy a new trans, I'll definitely go for something in the .80 range, as I hardly ever go on long interstate trips.
I'm going through the same dilemma.
I have a TKO600 with a .64 5th gear so I decided to sell my 3.55s before I put them in and I'm now looking for a set of 3:73s.
The cruising rpm's would be too low for the cam profile I want to use.
It really depends on where/how you will drive .
I have 373's/ 351W/ 15" wheels with .68 OD. It tops out in 4th around 125mph @6200rpm. Three guys in our group went with the .82 5th gear. On road trips with sustained 60-70mph cruising the only ones revving higher at speed than the .82's are the toploaders. With 500fp torque you might enjoy a 2000rpm 60mph cruise now and again.
I chose the .82 with 3.46 in back and 500 for HP and torque. I am quite happy with it. Go to the second strike gearing calculator. Put in both sets of numbers. When I did that, I found that I had multiple different rpm/mph bands where the .82 was reasonable. The .68 was very limited in my driving situations. My motor is happy at 2200 but less so at 1800. I run 2300 and change at 70mph. I am never going to wear this motor out, so I chose the flexibility of the .82.
I had Forte recommend that for me when I purchased my TKO600. At the time I had a 408 stroker and he recommended a 3.27 rear gear and the 0.82 OD of the road race version of the TKO. The best thing is to forget the TKO500 and it's 3.27 first gear. The 2.87 of the TKO600 flys by so fast as it is. Certainly with the torque of your motor you can chug around at lower rpms and be fine in terms of power. I find that if you have even a moderately aggresive cam, the lower RPMS are not appreciated by the large strokers. For me, cruising around at 1700-1800 RPM was not tolerable with some bucking and such from the motor/cam combo, so the 0.68 overdrive gear would have been useless. The 0.82 was perfect though. So it depends on what you are looking for and how well your engine will run at those lower RPMS. Don't overlook the fact that your motor might not like chugging around that low. HTH
I have a BB with the exact same setup (3.55 IRS, TKO 600 and .68) Really the .82 is a better choice. In 5th at 55 mph the motor runs around 1800 rpm and too low for fun driving. When I get on the interstate and can go 80 without getting caught then it's usable at about 2100-2200 rpm. I was doing 115 mph on Roebling Road Racetrack last week in 4th at 4400 rpm. So top end is not a problem, there is plenty of gearing to go fast if you wanted.
What kind of driving are you going to do the most of, back roads at 55 mph or interstate at 70 to 80 mph? That is the real choice. Your choice but if I had to do again I would pick .82 just to get more use out of 5th.
I contacted Tremek and a gentalman named jeff sent me a calculator to figure out rpm's/ and speeds given different gear ratios, rear end ratios and tire sizes. Very helpfull! All of the TKO 500's have a .68 fifth gear. They offer a 600 with either a .82 or a .64 fifth gear.
I appologize, I am not computer savy enough to figure out how to add the calcultor file to this responce. Bottom line is it all depends on what you want out of your car and what you plan on doing with it the most.
A forum community dedicated to Factory Five owners and enthusiasts. Come join the discussion about performance, racing, conversions, modifications, troubleshooting and drivetrain. We discuss MK4, GTM, 818, Type 65 coupe, roadsters, and more!