who's good with math?

Too little information. What's the gear ratio of the transmission and what's the gear ratio of the differential and what is the final gear ratio at the wheel?


Edited by DougS - 11 Oct 2015 at 2:05pm

The owner's manual should give speeds vs engine speed. The shop manual I have says speed in 4th gear is 9-1/2 mph, but it doesn't show what engine speed that is. It says full load engine speed is 1300 and high idle is 1577 to 1622. 1800 is 1800/1600 times the high idle speed so would put the ground speed up to 9.5 x 9 / 8 = 10.7. 1800 is 1800/1300 times the full load speed and the ground speed would be 9.5 x 18 / 13 = 13.2 mph. The shop manual says the differential gears are 4.7 for standard tires and 6.7 for 36 inch tires. From the transmission cross section it looks like 4th gear is straight through but it doesn't mention the final gear box drive were there is considerable speed reduction.

Working backwards the 12.4-28 tire from Goodyear/Titan handbook has a rolling circumference of 148 inches, which means it turns 428.1 revolutions per mile. At 9.5 mph that's 4067 RPH or 67.78 RPM. The transmission output shaft is turning 1300 RPM, the ring and pinion 4.7:1 puts the axle at 276 RPM and the 67.78 RPM of the wheel puts the drop box gear ratio at 4.08:1.  Close to what I guessed of 4:1 but 4.08 would be better engineering. Its not a great idea to always have an even gear ratio so the same gear teeth mesh each revolution. Gear wear is more even when the ratio is slightly off of an even number.

Again the operator's manual should give speed vs gear and engine speed and the parts book might show the number of gear teeth on the final drop box to show how far my calculation is in error.

Radar can be easily confused by a bigger vehicle behind you giving a stronger echo and the low speed calibration is probably not very precise because it doesn't result in speeding tickets. Then if you are the only moving object the radar shows you slowing as you get close to the radar but off to the side of the radar beam because the relative distance from you to the radar is changing slower than your ground speed.

Gerald J.

The '37 WC motor was not designed to turn at 1800 RPM. You won't find the speed listed in an owner's manual.


Edited by DougS - 11 Oct 2015 at 4:44pm

I think all speeds in the book are at 1400 rpm. Need to know the rolling radius of the tire they tested it with.
AaronSEIA

In other words... steal the Garmin from your wife's car and take the WC for a spin.

By my math presuming full speed is full load, not fast idle, 15 mph in 4th gear means the engine speed was 1300 x 15 / 9.5 = 2052 RPM probably not healthy for that unbalanced engine.

Gerald J.

Not only are there small differences between tire makers, the standards for tire measurement changed drastically between 1937 and 1980 or so. In the old time tire width was the bead spacing, and after that the tire width is the widest part of the tire and the cross sections could be different height too depending on the tire model and manufacturer.

Gerald J.

Now if the township had a camera in that radar contraption, and reviewed the tape, they'd be sayin' boy, there's an awful lot of Tractor traffic on Pequest Road. Wait a minute!! Its the same guy on all those Tractors!! Who's this knuckehead with all the Tractors wizzin' by? LMAO!! Well, Big Al, its 25 mph in that zone past your house so you still weren't speeding! LOL! The things we do to entertain ourselves... And people bust me about my molecular structure in relation to the ground... sheesh... LMAO!!

Approx. 14.6 mph at 2100 rpm. May be different based on tire diameter error but should be close.