Thread: Some contemplations on overdrive

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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.