660 ground speed

Nebraska tests report 15.39 mph with 18-26 tires. Two tests showed that speed. The third test is not reported in CH Wendel's book. Seeing that the speed is carried out to two decimal places, I would have to assume that the speed is calulated using loaded radius, planetary reduction, bull gear reduction, differential gear reduction, and engine RPM.

My 660 has 18.4 x 34 tires and probably stands a bit taller than the test tractors. If your tach works, your should be going 15-16 MPH at 2400 rpm. If somebody replaced the ring and pinion in the differential, you would go alot faster. I don't think there is a gear set available that would make you go slower.

I'll let soemone else comment on the TA.

Farmall 560 15/49 teeth (slower)
IH 560 16/49 teeth (faster)
IH 660 15/49 teeth (slower)

The main shafts (pinion gear at end) all have different part numbers.

This appears opposite of your intuition. The 660 is geared slower at the ring and pinion than the IH 560. To get both tractors runing a top speed of approximately 16 mph, the 660 is made to run at 2400 rpm vs 1800 rpm(?) for the IH 560.

If you have the time, you can do the math that I outlined previously. Suffice it to say that the reduction of the planetaries is more than offset by the higher engine rpm.

Other differential gear sets will fit the 660. They come from the SMTA, 400/450, and corresponding W models. They will all dramatically increase the speed of any 560 or 660. Top speed might be in the 19 to 22 mph range. I am not going to look up teeth counts on them and do the math unless you beg me.