Rear Gear Ratio??

[RICK LAKE]

Olddog Where is your cam power come in?? you want to be 500-750 rpm inside that power band. Sound like the motor is lugging along at highway speed. You forgot to take into the formula of resistance of the car and how much added HP is needed to push the car through the air. 3.73 min gearing is needed. If you could get a 3.90 gear this work be better. You would still get to use 1 and 5th gears alot more without quick shifts and motor lugging. You are not the first to under gear your car and will not be the last. Rick L.

[Anthony]

Originally Posted by olddog

Today I have 3.50:1 rear gears, 295 50R 15 tires, and a TKO 600 5 speed. Fifth gear is 0.64:1 or about 71 MPH at 2000 RPM. The cam doesn't like steady speed much bellow 2000 rpm. At 1800 rpm depending on load it may start to surge. So basically 5th is useless. On top of this the gears seem a little tall for best performance.

With your combination, I think the problem is not the tranny / rear ratio, its your engine, its got a too radical cam for your tranny/rear combination. You need to put a milder cam in it, ? about 204/214 duration.

[Madmiles442]

Originally Posted by olddog

...
By calculation:
At 6000 rpm 1st=47mph, 2nd=67mph, 3nd=101mph, 4th=135mph, 5th=213mph
By my calulations a 4.10:1 rear gear would give:
At 6000 rpm 1st=40mph, 2nd=58mph, 3nd=86mph, 4th=115mph, 5th=182mph
At 2000 RPM 5th=61mph
That too low? What say you?

Those calculations look correct. I wouldn't say it's too low. 60 is such an odd speed these days. I either find myself more around 70 (in a 65mph zone) or down around 50 (in a 45 mph zone). I have 3.73 with the .62 5th gear and love it. I only use it for highway driving and get better mileage that the Explorer. (20.8 vs 19.6 on a 350 mile trip) 4th gear is all I need for any spirited driving or most track events.
That calculator that WTM442 posted is dead on. Play with that.
It only confirmed for me that 1st gear is from here to the mailbox....

[olddog]

To answer some of the questions asked, here is some of the car's history.

Origonal owner put a relatively mild 5.0 with auto trany in the car. The 3.50 gears likely servered his intent well.

2nd owner put the TKO 600 and more radical 347 stroker in it. He shortly there after came across a great buy on another Cobra with 427 SO and bought it.

So the car has evolved to this point and I am just continuing the evolution.

The engine runs glass smooth from 2000 rpm up. No dyno done yet, as I'm still sorting out the EFI. By the seat of my pants, it is pulling fairly strong by 2500 rpm, peaks by 5000 rpm, and doesn't seem to be dropping off at 6000 rpm.

[enyawix]

what was the stock 5.0 rear end gear?

[Rick Parker]

It was 2.73 with automatic or 5 spd in 5.0 Mustang or a 3.08 was optional with 5 spd only. That may have had something to do with the limited slip option or requirement too.

[JAM1775]

I'd move up to 377 rear gears as the cheapest, easiest way to go with a slightly added bit of acceleration. At 61mph, from your #'s, you would probaly add another 2K RPM to make it 2200 which your motor should like.
Now you can cruise highways. First gear would still be very manageable .
I have 373's with the 600's .287 first gear. That comes out almost exactly as though I had my old tremec 3550 with 3.27 first gear and converted 327 rear gears. You'd get similar results moving to 377's and your 2nd/3rd/4th gears would get more spark to boot.