Thread: Some contemplations on overdrive

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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