VCCA Home Forums TECHNICAL - 1945 AND EARLIER 1933-1936 RPM vs MPH, 207 35 std

I remember seeing a chart of rpm vs speed in G&D some time ago but can not now locate it. Therefore I measured the tire circumference (on the spare) divided by the rear ratio (4.11) to obtain the travel per engine rotation. Divide this into 5280 feet travel at a mile a minute and obtained 2900rpm at 60 mph. I then used a dwell tach in the car to check the speedometer accuracy. Have I proceeded corectly? The 207 in my manual shows max hp at 3200 rpm so I think this might be my red line. Actually 50 is less than comfortable.

this comes in handy

http://www.angelfire.com/fl/procrastination/rear.html

From GM Proving Ground figures for a 1935 Standard. With 5.25X17 tires which have not been available since 1942.
RPM at 35 MPH - 1805, 2575 RPM at 50 MPH, and at 60 MPH 3090 RPM
Top speed ( 2 way with 600 pound load) 78.1 MPH.( almost 4000 RPM)
I always drove my 1934 Master at 55 to 57 MPH with no problems.

I am close and the only factor we cannot verify is the actual tire size at the proving ground. Checking Coker Tire catalog different brands vary slightly. The link to a calculator starts with tire diameter rather than circumference and adds the possibility of calculations for other than high gear. It was a fun exercise and I still would not try to get up to 78mph. Thanks for the comments.

Looking at the figures for the 1935 Master which came with 5.50 X 17 tires the RPM at 60 MPH was 3042.
The slightly larger 5.50 X 17 tires on your Standard will slow down the RPMs a little but will also slow down the speedometer reading bt rhe same amount. That being said you should be getting close to the 3042 RPM at an actual 60 MPH if the speedometer is accurate internally.

My measurement of tire circumference may be off a bit but is, I think, more accurate than using tire diameter which can change with air pressure and deflection under load. Since my speedometer is admittly low reading I am going to make a scale for the dwell tach and do some calibrating.

I find a stretch of road with mileage markers at the side. Interstates are one but in many states there are mileage markers on bridges and other structures. Determine the time between markers and then use the fact that 60 mph is one mile in one minute or 60 seconds. Or 45 miles per hour is one mile in 80 seconds. Of course those techies will tell you that the GPS will give instantaneous readout of mph. Can't justify the cost just to calibrate my speedometer.

I hinted to my kids that I was thinking about getting a GPS prior to Christmas and it worked. Better than things I don't need.

Will not be taking it on the Early Eastern 6 Cylinder Tour next week in western Wisconsin, Iowa and Illinois. Getting lost is more fun.

On tours with my clubs we feel that getting lost only increases the mileage for our passports.

Hey,
Any ordinary idiot can follow the directions in the tour book but it takes some real creativity to find the destination by an uncharted route. And if you arrive within an hour of the intended time you get bonus points. Double points if you don't stop and ask directions. At least points in the attaboy system.

...But if you take a shortcut and run out of gas you get points deducted!