Rear Gear Ratio??

[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.

By calculation:
At 6000 rpm 1st=47mph, 2nd=67mph, 3nd=101mph, 4th=135mph, 5th=213mph

Lets assume my 5.0 to 347 cid stroker, when all is said and done, gets somewhere around 400 Hp and torque at the flywheel.

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?

[KevinM]

Anyone know if the .82 fifth gear can be retrofitted without replacing half the box?

Kevin

[mylesdw]

Do you do long road trips and if so, what speed do you like to cruise at? Most of the cars that I have owned that have been easy to drive over long distances have given 25-30 mph per 1000 rpm in top, so you get an 80mph cruise at around 3000 rpm. If you are limited to 55 mph cruise then you might at well go lower still or fit a four speed since fifth won't really be useful for anything.

[Ant]

Olddog,

I agree, fifth is a waste of time unless its in a commuting vehicle where economy etc will benefit.
By going down to a 4.10, this will make 1st sort of useless, or first would be just a launch gear or make it easier to pull away from the lights etc.
Fitting the 0.82 fifth would help significantly.

I am running a 3.27 with direct top gear this gives me around 172mph at 7000rpm even this is high enough for me as at cruising speed 65mph it will be doing 2750rpm, but bearing in mind my engine is designed for racing mostly.

[Tony Aprile]

A SB with a fairly high lift and long duration cam will not like low engine speeds below 2000 rpms in 4th and 5th . A better combination would be keep the 350's and change the 5th gear to .82. With the .82 you will not have a large drop in RPM's between 4th and 5th gear. I agree with kevin

Tony

[iwantacobra427]

0.82 fifth gear and the 3.77 with 0.64 fifth gear. I opted for the 3.77 w/ the 0.64. I figure I'll be doing about 2300 rpm at 70 mph.

[wtm442]

Olddog

I have a 3:73 rear gear with a 0.68 fifth gear and 15" tires. In fifth at 100 MPH its at 3000 RPM. I do not use fifth unless I'm on highway and all the traffic is doing at least 70 MPH. Its great for gas mileage on the highway, but I'd rather have a 0.82 OD gear.

Here is a great calculator for SPF's
http://www.secondstrike.com/Technical/GearCalc.asp

[iwantacobra427]

Richmond has a new 5 speed (USA made) with a 0.77 OD. I wish this was available last year when I was bought the TKO 600 (Mexican made boat anchor?).
http://www.richmondgear.com/07pdfs/RG22.pdf

[Rickd]

IWANTACOBRA. That Richmond is a nice looking transmission, and that OD gear is perfect. But $$$$. It costs about 60% more than a TKO, and I believe there are some other adapters etc. required.

[jwd]

351W, 420 HP, 420TQ, TKO 600, .82 OD, 3.27 gears, 295/50/15 tires. Perfect combination. Useable 1st AND 5th gears.

[WarrenG]

I have the 3.54 rear gears with the 0.64 TKO 600 and it's been a good combination for my engine. That said if I had it to do all over again, I'd probably go with the same tranny and 3.77 gears. From what I've seen on other threads, if you want to convert to 0.82 you're better off to sell what you have now and buy a new transmission.

[olddog]

I was aware that the .82 gear is available, and agree it would be a better fix. I think I have a pinion bearing going bad in the rear end - not sure yet. So, I was looking at changing the gearing as a kill two birds with one stone type fix.

As I said the cam 236/242 duration at 0.050" lift doesn't smooth out until 2000 RPM, which is 71 mph. Interstate speed limits around here is 65 mph. I do very little interstate driving so far.

I would like to know what it would cost to get 5th gear changed.

[Anthony]

Originally Posted by olddog

Rear Gear Ratio??

When picking a rear gear ratio, its more like picking a rear gear - Tranny combination.

What you need to ask yourself is

1) What will be my cruising speed (mph) and what rpm do I want my engine running? This will determine final drive overall gear multiplication.

2) What overall gear multiplication do you want for 1st gear to get a good holeshot?

3) How closely do you want your gears spaced? This will determine whether you want a 4, 5 , or 6 speed tranny.

If you can answer all these questions, figure out what the numbers are, then they will determine which tranny / rear gear arrangement will be best for you.


In your case, I'd say either swap the TKO for a wide ratio toploader and keep the 3.5 rears, or switch to a TKO600 with 0.082 5th, again keeping the 3.50 rears, but obviously that's something you need to decide for yourself.

[Dwight]

I have a 5.0/354 EFI 375 rwtq & 323 rwhp, TKO 600 .64 with 3:55 gears, 25.25" tires. First gear 2.87 is great. 70 mph at 2100 rpms. I can cruise at 1300, 1400, 1500 rpm with easy, no trailer hitch jerk.

Dwight

[turnpike boy]

I've got a Unique with Jag rear; 4.09 final drive, Tremec 3550, 0.68 5th. gear, 331" with cam specs 228*/236*, .605/.589 lift, AFR 185's, Victor Jr., Speed Demon 650 mechanical secondaries.

Very, very fine to drive. Anything under about 60 mph, like around town, use whichever gear makes you happy because the car pulls like a Clydesdale no matter which one used. 4th. gear, 3000 rpm is a hair over 60 mph - right up the cam's alley, so roll-ons with anybody are smile contests.

Wanna cruise? - no problem. Nobody runs the highways around here at under 75 mph anyway, which is 2500 rpm for me; again, right about where the cam really starts to work. 2700 rpm is about 82 mph, perfect for the left lane on Rt. 80 or the Penn Turnpike or.....you get the idea. Hi gear passing is a joy unlimited.

Anybody in here tells you the hi 5th. gear ratios are useless is wrong. It might be that the .82's work better with lower ratios, but around town that 4.10 will be the most effortless drive you ever had. And the pull 2/3/4 has to be felt to be believed. First is about worthless (mine is 3.27 first) but makes crawling traffic a no-sweat proposition - put it in gear, let out the clutch, and just idle along. No in/out with the clutch pedal.

No regrets whatsoever here.

[Excaliber]

1st and 5th is really the question here. If you calcs are right and 1st works out to 40 mph, thats not to bad, in the range of 'useable'. Therefore, a 4:10 ratio, which would make 5th work nicely, aint to shabby!

My Excal, 302 with the classic T5 and ultra low 1 st gear was about 25 mph with a 3:50 rear gear. While 5th was sort of reasonable, 1st was a joke!

[enyawix]

wow I had the same question. Thanks

[427SnakeSC]

Old dog, I had 4.11's in the Classic Roadster, I sold earlier this year. I am not sure of the gearing of the tranny, but it had a standard Ford Mustang 5 spd in it. I was running a 302, aluminum heads & intake, 750 Holley and fairly wild Hydralic Roller cam. With the 4.11, 1st gear was practically useless. It got so that I was starting off in second, just to get some "legs." At around 65 mph, I was running about 2,200 rpm, with lots of punch. I think with that tranny and motor, a 3.73 rear end would have been much better. IMHO

[Ron61]

I had 235/60/15s on my Cobra if I remember correctly and a 3:27 rear end ratio, with the .68 split in the transmission. I checked it with the highway patrol radar and the readings were right on. 70 MPH in 5th was 1900 RPM and 60 MPH I don't remember, but the engine was more of a racing engine and I didn't even use 5th on the freeway about 70% of the time as the traffic caused the engine to lug. But 2nd, 3rd, and 4th worked very well. However if I had another car I think I would go to 3:56 gears and a much milder engine.

Ron

[mylesdw]

Originally Posted by Excaliber

1st and 5th is really the question here. If you calcs are right and 1st works out to 40 mph, thats not to bad, in the range of 'useable'. Therefore, a 4:10 ratio, which would make 5th work nicely, aint to shabby!

I agree with what you say about 1st and 5th with the 4.1 rear. Zero to sixty dashes only need one gearchange which is nice and it will lope along nicely in between 65-75 mph in 5th. 'Ordinary' main road cruising in the 55-65 range might me a bit of a pain though, you'd be in 4th at around 3000 rpm all the time.