Plz Help with Trans Gearing

[Davidless]

Hi All
Another SF question so I’m looking for your opinions plz.
I need to order my trans and having a tough time with the overdrive gearing choice.
It’s a TKX 2.87-.68 or a 2.87- .81. Rear end is 3:45 (standard with a new roller).
427 sb with hp in the low 500’s

I’m running 18” wheels and a 110 LSA cam which I believe will need to run at 2200 rpm minimum. It’s mostly street driving with maybe some auto cross if I feel daring, not any track straights right now that I know of, maybe a yearly play day at the Glen but no racing.

So with that said if I use 2400 rpm as the bogie I’m looking at 90mph with a .68 OD 5th gear which is 32% over drive compared to a .81 which will cruse at 76mph 19% OD.

It’s hard to really get the feel without test driving to compare the difference.

Thanks for any thing I may be missing

David

[MKS427]

Do you know the number of revolutions per mile for your rear tires?

[MKS427]

Assuming 801 rotations per mile for the rear tires and 3.45 R&P.
0.81 2200+ @ 60 and 2600 @ 70
0.68 1900- @ 60 and 2200 @ 70

[Tommy]

I have three comments. First, if your engine is high strung (i.e. doesn't like low RPM like mine) I don't think you'll like the 0.68 gear very much. Second, the jump from 4th to 5th with 0.68 means you've got to be carrying a pretty high RPM in 4th to keep from lugging the engine after the shift. Last, unless you plan to do some track days where you'll be exceeding 90 MPH, the 0.81 should be sufficient. Even if you do plan to do track days you may want to go with the 0.81. If you shift from 4th to 5th at 6000 RPM with a 0.68 gear, your next RPM at that speed will be about 4,000 RPM. That may be so low in your horsepower curve that you can't accelerate much faster. A 0.81 gear will put your RPM after the shift at about 4700 RPM in a stronger part of the horsepower curve.

[Davidless]

Originally Posted by MKS427

Assuming 801 rotations per mile for the rear tires and 3.45 R&P.
0.81 2200+ @ 60 and 2600 @ 70
0.68 1900- @ 60 and 2200 @ 70

I used the Tremac calculator on their website, pretty sweet

[eschaider]

David,

You might want to attempt to search out some of these answers before posting. If you do you will find a number of webpages like the Cobra focused Second Strike Gearing Calculator, <= clickable, that will provide answers to the questions you have.

[Davidless]

Originally Posted by Tommy

I have three comments. First, if your engine is high strung (i.e. doesn't like low RPM like mine) I don't think you'll like the 0.68 gear very much. Second, the jump from 4th to 5th with 0.68 means you've got to be carrying a pretty high RPM in 4th to keep from lugging the engine after the shift. Last, unless you plan to do some track days where you'll be exceeding 90 MPH, the 0.81 should be sufficient. Even if you do plan to do track days you may want to go with the 0.81. If you shift from 4th to 5th at 6000 RPM with a 0.68 gear, your next RPM at that speed will be about 4,000 RPM. That may be so low in your horsepower curve that you can't accelerate much faster. A 0.81 gear will put your RPM after the shift at about 4700 RPM in a stronger part of the horsepower curve.

Makes sense
That was my original choice but after talking to MDL he was suggesting the .68 if no track days or straights and a .81 if I'm doing a lot of track straights.

That's why I was second guessing my original choice of .81

David

[patrickt]

Tommy is right on everything he wrote. Now, with that said, let me tell you why I went with a .64 OD with my 3.54 rear. That TKO 5th gear ratio gives me 34 miles per hour per 1000 RPM. I use 5th gear for nothing other than pleasant highway cruising and there is nothing I enjoy more than easing along at 2200 RPM, which is 75 MPH, out in the countryside, and the engine is at its quietest and is smooth as glass. You can actually hold a conversation with your passenger. That is the only thing my fifth gear is good for, but that's why I chose it. Now, it would also come in handy if I made a run to see what my top speed was, which I've never done. I think she'd top out somewhere in the 160's, but I don't know. But, that little item is on my bucket list... probably the last item, especially if the ol' girl blew a tire at that speed.

[jhv48]

I had the .64 overdrive in my TKO600 and absolutely hated it. 3.54 rear gears. I’ve got a street friendly 550 horse engine that pulls just fine from 1800 rpm in top gear. I found myself rowing back and forth from 5th to 4th during freeway driving. There was such a big jump from the fourth to fifth gear. Almost 1000 rpm.
Swapped out the tranny for one with a .81 fifth gear and couldn’t be happier. Now I actually have a usable fifth gear. I’m turning about 2500 rpm at freeway speeds and can accelerate smoothly if needed without dropping a gear.
I highly recommend the .81 fifth gear. Based on 35,000 miles of all kinds of conditions.

[Davidless]

Originally Posted by MKS427

Assuming 801 rotations per mile for the rear tires and 3.45 R&P.
0.81 2200+ @ 60 and 2600 @ 70
0.68 1900- @ 60 and 2200 @ 70

I'm getting from the calculator:
.68 @ 1900rpm =72mph 2200RPM 83mph

.81 @ 1900rpm 60mph 2200rpm 70 mph

[Davidless]

Originally Posted by patrickt

Tommy is right on everything he wrote. Now, with that said, let me tell you why I went with a .64 OD with my 3.54 rear. That TKO 5th gear ratio gives me 34 miles per hour per 1000 RPM. I use 5th gear for nothing other than pleasant highway cruising and there is nothing I enjoy more than easing along at 2200 RPM, which is 75 MPH, out in the countryside, and the engine is at its quietest and is smooth as glass. You can actually hold a conversation with your passenger. That is the only thing my fifth gear is good for, but that's why I chose it. Now, it would also come in handy if I made a run to see what my top speed was, which I've never done. I think she'd top out somewhere in the 160's, but I don't know. But, that little item is on my bucket list... probably the last item, especially if the ol' girl blew a tire at that speed.

Good to know what RPM is nice for conversation at cruising highway spd. What is you cam LSA ? mine is 110, I'm calling tom at ford performance and see what the lowest cruising RPM the engine will handle, hopefully its 2200 also and not 2500.

With a 0.81 @ 2200 cruising in 5th would be 70 MPH
0.68 83 MPH
I made up a spreadsheet so I can message the numbers but I'm leaning towards the 0.81 OD

Thanks

[Davidless]

Originally Posted by jhv48

I had the .64 overdrive in my TKO600 and absolutely hated it. 3.54 rear gears. I’ve got a street friendly 550 horse engine that pulls just fine from 1800 rpm in top gear. I found myself rowing back and forth from 5th to 4th during freeway driving. There was such a big jump from the fourth to fifth gear. Almost 1000 rpm.
Swapped out the tranny for one with a .81 fifth gear and couldn’t be happier. Now I actually have a usable fifth gear. I’m turning about 2500 rpm at freeway speeds and can accelerate smoothly if needed without dropping a gear.
I highly recommend the .81 fifth gear. Based on 35,000 miles of all kinds of conditions.

Great info, I'm now leaning back towards the .81 OD

Even Prestige Motors spec'd the same OD (.81), its just the the guy at MDL through me off when he said the 0.68 would be better for mostly street driving.

I agree too large of a jump between 4th and 5th!

Thanks loads

[patrickt]

Originally Posted by Davidless

Good to know what RPM is nice for conversation at cruising highway spd. What is you cam LSA ?

Here is my cam -- it's an old school cam that has a wonderful sound and idle when tuned perfectly along with decent manners down low. Remember is a solid flat tappet. Now jhv48's comments on the RPM drop are valid. Unless you're really sure how you're going to drive the car, the .81 ratio is the safer choice. But you should ask your builder what's the lowest sweet spot in your rpm curve just so you can do your math. Remember though, regardless of what you choose it will be better than an original Cobra with a four speed.

[KarlzEE Bebout]

David, in addition to the O.D. ratio, also consider the transmission's first gear. My TKO 500's first gear is a 3.27, which would be fine for drag racing but since I don't bother doing that with my tiny engine, I do wish I had a lower number first gear ratio. First gear is SHORT! All that being said, I do like my .68 fifth gear even though I don't have a lot of high gear torque. I like to do road trips that have me cruising 75-85 MPH and in my high gear, the engine is just loafing along and I have enough torque to make a gentle pass. If I need to be spirited, fourth and sometimes even third gear come in to play. Love to hear my sewing machine engine sing. My 2¢

BTW, I run a 3.50 rear end ratio and 26" tires.

[Davidless]

Originally Posted by eschaider

David,

You might want to attempt to search out some of these answers before posting. If you do you will find a number of webpages like the Cobra focused Second Strike Gearing Calculator, <= clickable, that will provide answers to the questions you have.

the calculator seems outdated, no TKX or the proper ratio's the new TKX has available.

The calc. on tremecs web sight is quite good.

I could do a search but I'd rather have new information from people that care to share, plus the search engine on here doesn't like me very much

David

[eschaider]

The TKX and TKO have the same effective fifth gear OD ratio at 0.82 vs 0.81. Gear ratio-wise, the two transmissions are otherwise identical. The .01 difference in final overdrive ratios amounts to a 1% difference in engine rpm. At 2000 engine rpm that is 20 rpm. There is no measurable difference in the driving characteristics of either trans in one of our cars beyond shifting quality. The TKX synchros are what Tremec should have used in the TKOs years ago.

[patrickt]

FWIW, the original was not particularly highway friendly. And you didn't get much of a choice for that 5th gear overdrive.

[patrickt]

davidless -- note the "overall" numbers on the gear ratios from that C&D chart. The number 8.2 has always been kind of a magic target for pleasant street driving. But don't get terribly hung up on any of this, you can always have your gears changed out either in the trans or the rear. Like anything else with these cars, just keep throwing money at them and you'll eventually get what you want.

[patrickt]

As discussed 13 years ago. Note Mike Forte's comments on 1st gear ratios for the street. Toploader trans wr or cr?

[8litercobra]

I ordered a T56 Magnum 6 speed from Rockland Standard Gear some years ago. An optional gearset just about exactly matches the Ford close ratio toploader in the first 4 gears, and gives both a 0.8X and a 0.6X overdrive gear. Sorry I don't remember the exact ratios, but this tranny has it all covered!