Thread: Some contemplations on overdrive

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I have long wondered why the auto manufacturers use overdrive transmissions. Wouldn't a high gear that is direct 1:1 be a more efficient transfer of power? So what I see is that a 1:1 top gear with a taller differential would be more efficient that achieving the same overall top gear ratio with overdrive and a lower differential? For instance a .80 ratio overdrive with a 3.55 gearset compared to 1:1 top gear and a 2.84 gear to achieve the same final ratio of 2.84. It would seem the 1:1 with a 2.84 cog would be more efficient because the 2.84 might have less drag and the 1:1 definitely has a smoother power path than the .80 overdrive does.
Comments? Did Detroit just sell us a buzz word here?

If we all drove vehicles that were the same physical size that never varied in weight, we could probably go with the simple math approach. As it is, vehicles have been (and continue to be) manufactured with everything from a two speed Powerglide to 6-speed sticks. Trucks have long had multiple options for transmissions and rearend ratio with the goal being optimization of engine performance (a relatively small range) over a wide variety of driving conditions (a vast range). An engine runs at its best within a certain RPM range. By using a transmission, the engine’s power is delivered to the drive wheels while keeping the engine within that range. As such, various gear combinations/rations are required.

While OD may seem superfluous, it allows great fuel economy when driving conditions allow long periods of constant driving at a lower RPM. Corvette 6 speed ratios are .74 for fifth and .50 for sixth. Granted, the Corvette has a lot of power being transmitted, but the principle applies from a Smart Car to a 80,000 pound semi.

Regards,
Glenn

I can only imagine my V10, 7400# F350 without OD on the open road. At 71mph, the tach is at 1900-2100 rpm. Kick it out and into direct, and it's running 27-2900 rpm. It takes more fuel to maintain that level of engine speed and ground speed. Will it pull as much of a load in OD? No, which is why that particular trans has a load/haul switch.

You could run a 1:1 ratio in high gear if you have a five or six speed transmission with very low first gear. I think Richmond sets up their transmissions like that.

I went through this exercise after I bought a 8" Ford rear with 2.73 ratio and realized that the 1:1 high gear would give good mileage BUT (!) low gear would be horrible with a TH350. The point is that with a high (low ratio gear) ratio 1:1 in the transmission the low gear is likely to not have enough mechanical advantage to get the car moving well in low gear. I used:

http://home.tampabay.rr.com/1bking/c...calculator.htm

and sold my TH350, replacing it with an 700R4 and changed the rear ratio to 3.55:1. My design goal was to have a lower low and a higher high gear than my memories of a '47 Ford flathead with a 3.78 rear, although with a SBC 350 and this is what I came up with:

Table I Present Gear Ratios with 3.55:1 Rear Gear

Gear Transmission Overall 1947 Ford (3.78 rear)

First 3.06 x 3.55 = 10.86 2.82 x 3.78 = 10.66

Second 1.63 x 3.55 = 5.79 1.604 x 3.78 = 6.20

Third 1.00 x 3.55 = 3.55 1.000 x 3.78 = 3.78

Fourth 0.70 x 3.55 = 2.49 -


Maybe my low gear is still not low enough and maybe my OD is too stiff but with about 400 ft. lb. at 2500 rpm the SBC 350 should haul it at least as good as my old flathead with the 3.78 rear and only a three speed. The second and third gears of the 700R4 are stiffer than the flathead three speed but as I recall it took a very good flathead to make even 300 ft. lb. of torque at 2500 rpm so I should be OK with good mpg in OD and still have a good low gear. Maybe I should have gone with a 3.73 rear ratio but I favored mpg. You can use the site given above to design your own drive line. Another factor is the weight of the vehicle but I estimated a light roadster will still have good acceleration with a 3.55 rear gear; you may like a lower low gear?

Don Shillady
Retired Scientist/teen rodder

I think some of you are missing the point I am trying to make. Stay with me here, this is hypothetical. Ok lets say that instead of a 5 speed that has a .080 overdrive and a 3.55 rear end, you had a 5 speed with a lower first gear and a 1:1 5th with a 2.84 rear end. For the sake of the whole argument let's say that the overall ratio is the same in each of the 5 gears but the top gear is 1:1. The engine rpm would be the same for any given speed in any given gear still but instead of the power path going through a .80 overdrive it is a direct powerpath through the gearbox in 5th. There would have to be less parasitic drag in the transmission and also I think less drag in a taller rear end gear too.

Originally Posted by rumrumm

You could run a 1:1 ratio in high gear if you have a five or six speed transmission with very low first gear. I think Richmond sets up their transmissions like that.

Yes, I posed this on a couple other forums and that is the best answer I got. Another post on a different forum has the link to Richmond gear and they are saying exactly what I thought.


http://www.richmondgear.com/07pdfs/RG23.pdf
What has amazed me is the range of responses and the fact that very few people understood what I was postulating. Maybe I just didn't explain it right.
thanks rumm

Willowbilly---The best thing about OD is when you pull a trailer or other load you leave it in high and don't cause excessive wear on the gears. You are right about a direct drive top gear being a slight amount more efficient at highway speeds, but face it, most vehicles these days never leave city limits so the OD hardly gets used. To have the same breakaway gearing with a tall rear gear you'd need some steep gearing in Low.

I see where you're going. Move all the transmission ratios downward and the differential ratio upward to maintain the same overall drive ratio in all gears.

No reason it wouldn't work, I guess.

Well, probably would be better, but you would also have to totally redesign the differential. to be a 1:1 ratio the ring and pinion would have the same number of teeth, that's not going to work (mechanically speaking) in the rear end design we have now... I don't have any idea what the amount of drag is when in overdrive, but the new 5 and 6speed transmissions are well designed and I'm sure it's minimal loss.

Best case scenario would be a constantly changing drive ratio and no gears similar in theory to the way a snowmobile drive works. Think Chrysler tried this in the 50's. Perhaps replace the transmission as we know it with a hydraulic pump and drive assembly that would have a constantly changing drive ratio based on the power demanded to accelerate to or maintain a given speed??????

Dave, I didn't mean a 1:1 differential, I meant a 1:1 top gear in the transmission instead of overdrive.
The constant variable transmission like a snowmobile is a great idea and I have that rattling around in my brain too. I read about a guy who used a 17 horse Kubota in a 1200 pound car like an old triumph or something. He kept the 5 speed and clutch somehow. Around town he just left it in 3rd and got 67 mpg and 100 mpg highway. The whole theory there is it takes a much smaller engine to maintain cruise than it does to accelerate. Plus a constant engine speed is much better too.
Subaru did have it in the Justy too but I don't think there is anything like that for a pickup or larger vehicle.

Oh, ok... gotcha now. Sure, could leave the top gear 1:1 and lower (numerically) gears in the back... But with the od in the trans, I get to run my 4.11:1 rear gears and with a .62 overdrive I can still cruise the highway with it... I hate compromises on cars, but in the case of an od tranny I can come close to a dual use car. Besides, if it's for a Hot Rod, it doesn't have to make sense or be logical!!!!!!!

It's one of those things that would be interesting to see the real world numbers comparison on a dyno. Maybe the difference would be minute.
According to my calculations your 4.11 with a .62 overdrive would produce the same total ratio in top gear as a 1:1 tranny top gear and a 2.54 rear end.

I have had a Few GM's with the OD Transmissions . What I can Say I did not Like is . When your at Low Speeds 35-40 and it goes into OD . The Car will Lug and the RPM's go to a real Dead spot . Now on car's with 3.23 or Larger Gear gears Like the 1988 F-Body 3.27 Gear and 1987 442 3.73 Gear . They would go into OD at 30-32 MPH and what it felt Like to me was that the Rear end was trying to push the car . Making it even slightly harder to stop fast at that speed . Now this would be more of a Problem on a Car Like Mine that was driven more in the City then the Highway . My Friends that run these OD Trans Build's use a Switch under the dash to Lock out the OD When they are Driven in the City . Yes you could Shift down into 3rd But on my Pick up with the 700R-4 the Lock up Converter jumps in and out at 35 MPR . I said the heck with that and Un-Pluged that Trans Plug . It works Great Around Town now . Yes when your over 50 MPH They are Great . And you can get unreal Power off the Line and Top end . That 442 with the OD and 3.73 Gear Felt Like it never ran out of the Right gear .

Originally Posted by willowbilly3

It's one of those things that would be interesting to see the real world numbers comparison on a dyno. Maybe the difference would be minute.
According to my calculations your 4.11 with a .62 overdrive would produce the same total ratio in top gear as a 1:1 tranny top gear and a 2.54 rear end.

That would be correct, but with a 2.97:1 low gear and a 4.11:1 rear gear, it launches quite well!!!! The lowest ratio low gear I know of in a stock box 4 speed would be a 2.20:1..... When you figure the ratio in low gear, then it becomes evident which rear gear will give the hardest launch and quickest 60 ft. time......a four speed with a 2.20 low gear, 1:1 top gear and a 2.54 rear gear will not launch near as hard as a 5 speed with a 2.97:1 low gear, 1:1 fourth gear, .62 overdrive 5th gear, and a 4.11:1 rear gear....(and we won't even talk about how much fun the five speed is with a 6.00:1 rear gear!!!!)