Need info on Tremec rations/ and trans type selection
 

I know, my bet is that this has been asked more than a few times, but I need some educated person to fill me in on something.

With any luck at all by mid Aug I'll be taking delivery of my new (to me) slighty used SPF and will be seeking engine and trans.

Question. With the rear end ration of 3:73 (I think stock on a SPF) and a 351W at approx 400 HP what final drive would be best? I am looking for some low end to make it fun around town, but also want to be able to hit the hwy and cruise. Ideally 2000-2500 at 70, 75. I am interested in a Tremec 3550, but then perhaps a Tremec is not what I need at all. (open to ideas).

Here is what I think I know:


1. From what I can figure with the 3:73 rear end and a .82 my final drive would be 305, right? (.82 x 373). Would this not be a nice gear for the hwy?

2. Are there different ratios in the 1,2,3,4th gearing between the .68 and the .83. I don't want a large "gap" between 4th and 5th, but I do want a decent 5th for hwy cruising. As indicated above.

3. Also, are there differences between the lower gears on the TKO and the 3550?

Anyone been down this road before? Anyone have a 3:73 rear end with a 351 and any of the above rations that can tell me first hand how they like the set up? :confused: :confused:

Here are the 3550/TKO ratios

The 3.73 rear gear is going to be short of your expectations as far as highway cruising (2700 rpm at 70) and IMHO will give you a pretty useless first - too low. 42-44 mph at 6000 rpm.
Consider a dif ratio in the 3.3-3.5 range with the .82:1 fifth.

Roadwarrior,

I run an SPF with the 385hp/351 crate motor in front of a 3550 with the .82 fifth. The car does have a rather low 1st gear as previously mentioned. 40 ish mph @ 6000 is correct. However with no trouble the car will launch in 2nd gear with only maybe 20 to 30 feet of wheelspin then hang on as it will pull to around 70 mph. At 60 mph in 4rth gear is around 3000, in 5th is around 1800. Engine has enough torque to pull down to 12-1300 in high gear and just drive away with no hesitation. This works for me but, as they say, your results may differ. Hope this helps. ... Gary

If your interests are more sportscar than racecar, here's some food for thought.

Consider a T-5Z as an alternative to a Tremec. For 351W service in a 2400#, street driven car, the T-5Z should give you years of trouble free service. With street tires, it's difficult to put enough torque through the trans to break it. The tires spin first. If you go screeching it from stoplight to stoplight, your drivers license will likely be the first thing to go, then the trans.

The "Z" is a close ratio box with a 2.95 first, and your choice of a .62 or .80 5th. Stock is the .62. With 3.73 gears, leave it at .62. That will give you your cruise where you want it. And 1st isn't so tall as to be useless. None of the Tremecs have close ratio gearsets (yet), and you're stuck with 3.31 1st gear. (Great in a 2.73 geared Mustang, but not in a 3.73 geared Cobra).

Have 3.55 gears in mine, and went with the .80 5th. 60 - 75MPH cruise turns the engine 2000 - 2500 RPM. (And gets me 17MPG in the process - with a roller cam, HP aluminum heads, DP carb...not an economy 351W motor).

Bought the trans from Liberty Racing Transmissions who installed the .80 5th gear, and blueprinted the trans. Put a Hurst shifter in it as well. Shifts like a Honda.

Am considering adding the 393 stroker to the engine, and wondering myself how far I can push the envelope with the "Z". Unless Tremec comes out with a close ratio gearset, the "Z" goes back in, and we'll see.

I have a "built" 396 stroked Windsor with an aggressive cam, and the .68 fifth in my Tremec 3550, pushing a 3.73 rear. I cruise at 60 mph at 1900 rpm, easy and relaxed. i can putter around town in fifth at 1000 rpm without any shudder or shake, and even accelerate without protest in fifth. I like the tall first, as i never have to slip the clutch a bit at a start. I upshift out of forth at 6K rpm at about 130 mph on the track. For my driving, this is perfect. I have driven another SPF with the .82 rear in front of a 3.08 rear, his gears are more "long-legged" than mine, we are pulling the same rpm at cruise, too. But you have to slip his clutch a tad from a stop. He likes his road gearing, i prefer mine.

I think a lot depends on your cam and heads, and how slow an rpm you can run your engine with reasonable drivability. If you want to play with gearing, you can pull up Mike Stenhouse's gearing comparison on the Second Strike web page.

try www.secondstrike.com and plug in your choices