Re: Over-Drive vs Gear Change for a '34 Coupe

Posted by DavidM On 2016/11/8 17:56:44

Discussion about this topic are more meaningful if we compare miles per hour per 1000 RPM instead of just the drive ratio, as this does not take account of the effect of different wheel sizes. For example my 1929 model 633 has a 4.38 drive with 6.50 x 20" wheels. This gives about 22 mph/ 1000 RPM.
A 1934 Packard with 7.00 x 17" wheels and a drive ration of 4.69 would be travelling at about 20 mph / 1000 RPM. For a 3.05 ratio, as mentioned above, this would increase to around 30 mph / 1000 RPM which would obviously make a very significant change to the cruising versus hill climbing compromise.
It is a simple calculation:
MPH / 1000 RPM = (3.14 x outside diameter of tire in inches x 1000 x 60) / (12 x 5280 x drive ratio)
So for the 633: (3.14 x 33 x 1000 x 60) / (12 x 5280 x 4.38) = 22.4 mph/1000 rpm