I have a nice overdrive 4-speed in my 67 slant six convertible. I was prepared for the big rpm drop between gears, and it's not a big problem. The tinkerer in me is wondering whether it's possible to mix in some gears from other 4-speeds, to tighten up the ratios a bit. According to the slantsix.org 4-speed article, the O.D. ratios are: 1975 thru 1987 Overdrive 3.09:1 - 1.67:1 - 1:1 - 0.73:1 (Trucks used a 0.71:1 fourth). I immediately am interested by the truck 4th gear for a little more top end.
The other 4-speeds are:
1964 thru midyear 1966 A-body 3.09:1 - 1.92:1 - 1.40:1 - 1:1 = G/RG and LA engines. (These all used a 4.35" IBR.)

1964 thru 1970 B/RB engine 2.66:1 - 1.91:1 - 1.39:1 - 1:1 = (The "close ratio" V8 gearbox)

1970 T/A & AAR 340s 2.47:1 - 1.77:1 - 1.34:1 - 1:1, = 1970 thru 1974 high performance
(A 2.44 first gear was also produced in this time period and some units had 1.91:1 second gears.)

Looks like Mopar 3 speed combinations are:
..3 SPD..............1ST....2ND....3RD
- '72 A903..........2.95....1.83....1.00
.........................3.22....1.84....1.00
'73 & UP A250......3.18....1.83....1.00
SLANT SIX ONLY

'73 & UP A230.....2.55....1.49....1.00
V8 AND SLANT 6..3.08....1.70....1.00

After that, my knowledge starts to run thin. Questions: are any of these gears compatible with the overdrive? If so, what combination could get you to the 1:1 with the best manners? If you could mix in from other 833's, maybe 2.47..... 1.77....1:00....0.73, or 0.71 for more top end.

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/6 '67 Barracuda convertible, electronic ignition, 4-OD, street cam, SBP KH discs, 3.55 SG 7.25" 1" t-bars. Bilstein.
340 '67 fastback, Doug Nash 5-speed.
1988 Toyota pickup work truck

It's important to understand that there are no actual 1st, 2nd, 3rd or 4th gears. Rather, they exist as part of a gear cluster. So, you can't mix and match them as if they were in a bucket and you just picked out which ones you want to use. There might be some rare instances where you could mix/match clusters and have it function but even then....your choice of ratios would be very restricted.

GregCon is 100% correct. You cannot change the gears on the cluster so there is no mixing and matching of ratios. If you want closer ratios you are changing the transmission.

You already have an overall high gear ratio of 2.59:1 (3.55x.73). Unless you are seriously supercharged you will run out of horsepower long before you are out of RPM. You are drag limited in high gear, not RPM limited. Reducing the torque multiplication, RPM and developed horsepower further will lower your top speed in high gear. If you want to achieve the highest possible road speed then you are adjusting your gearing to have peak power RPM coincide with the theoretical top speed. Your top speed is always dictated by drag and horsepower, but you have to gear properly to achieve peak power. I'll bet that your car reaches a higher speed in 3rd/direct than in overdrive.

What most people want is to have good acceleration from a stop, low RPM drop between gears and a low cruising RPM. You have items 1 and 3 now. More gear ratios than you have are required to achieve all 3 goals. For something that bolts in you're probably better off with a 3.09 1st gear 4-speed and a numerically lower rear gear like 3.23 or 2.93. No you will not have the same off-the-line punch, but you will have gears available that more closely match the road load and make much better use of the power you make.

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Joshua

The real downside to the OD 3:09 trans is the big drop between 1 and 2. Otherwise, it's quite good.

Just for fun...here's my latest acquisition...a Lenco 5 speed. Ratios 2:99 / 2:17 / 1:57 / 1:25 / 1:1 No overdrive but also not a lot of drop between gears....

You dog!
Man I love that Lenco, seriously cool part.
What are your plans for the trans?

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Hyper_pak

With the 3.09 1st gear the drop to 2nd is 62% of the pre-shift RPM. With the 2.66 1st it's only 72%. But with the OD it's a nasty 54%. I ran the 2.66 1st gear 4-speed with 3.55s and short tires. That worked fine.

Those are nice close ratios. Lenco's are bitchin' so long as you're on the throttle. For those who don't know, there's no engine braking. I'm more into road racing so I'd rather have a sequential dog box or a dual-clutch. I'm not willing to spend that kind of money so my ZF 6-speed will have to do.

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Joshua

D

Duster Idiot,

Something doesn't check with your gearing and MPH. 5,000 RPM with a 3.55 axle ratio, 0.73:1 overdrive and 26" tall tires works out to 149 MPH. If you can pull 6,000 in direct with 3.55s and 26" tall tires that's 131 MPH. If I had the Duster aerodynamic drag numbers I could give you the horsepower required, but I gave away my Direct Connection Chassis Manual with that information.

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Joshua

D

5000 RPM, 3.55 x .73 high gear, 25.3" tire = 145 MPH

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Joshua

Parsons builds a 3 speed/OD for the big block crowd with a better gear spread. Maybe they can put a 23 spline input shaft on one.

Well, as the saying goes "If it can't be done, we won't do it." Thanks for the highly informative responses.

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/6 '67 Barracuda convertible, electronic ignition, 4-OD, street cam, SBP KH discs, 3.55 SG 7.25" 1" t-bars. Bilstein.
340 '67 fastback, Doug Nash 5-speed.
1988 Toyota pickup work truck

Can't be done?? I was watching Oprah the other day, and she told us never to say you can't do something. The only thing that is holding you back is yourself. Keep reaching for the stars and you can do anything. I gave that a lot of thought, and after I was done thinking I thought some more. Then, I started running. I ran all day and night, across many states. I had a huge crowd of people following me, and I grew filthy and my shoes fell apart many times. Suddenly, I stopped running. I just stopped. It finally sunk in - Oprah is an idiot. There are a lot of things that can't be done.

Dusteridiot mentioned that:
"A long while back doc had talked about how to setup a splined countershaft and gears with splined centers to allow the gears to be swapped, but the cost wouldn't be worth it."
Which is what I was envisioning, so maybe it can be done. My concern is that it could be an almost not worth it amount of effort and then have it not work. Getting tired of that scenario.

It just takes money. But if you do it within the case [A833] you have you're still only going to have 4 forward gears. You're money ahead to do a T-5 swap and pay G-Force for a .80:1 5th gear.

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Joshua