VCCA Home Forums TECHNICAL - GENERAL Mechanical - Engine, Trans, etc Engine RPM vs Vehicle Speed

I guess if a you know your rear tire circumfrence and your rear end ratio, it's easy enough to figure out what RPM your engine will be turning at any given vehicle speed, but I decided to do a field test on my 40 1/2 ton pickup with 4.11 rear end. I hooked up my old Sears dwell meter/tachometer and had my wife set the pace on the highway in our 96 Audi with the cruise control set at 50 mph. At that speed, the truck engine was turning pretty close to 2400 rpm. At 55 mph the truck was turning about 2600 rpm. The truck seems to be pretty happy at 50 mph, and is nearing the practical high end at 55 mph, although it will run at 60 with favorable wind and grade conditions, though I don't think I'd want to try to run at that speed all day long.

Mark Yeamans
VCCA #35724

Mark, I find about the same thing with my 1950, 3600. I have a SunPro tack temporarily installed to monitor the real rpm rather than estimating it from a chart. I don't know what size tires your running, that would be the only difference between the two, but I have the stock tires and just installed a 4.10 gear in the old gal this summer.
The feel of the engine at 55 mph now is quite comfortable and I have kicked it up to 60 and even 65 a few times to pass or just to keep up with traffic. And like you, I wouldn't want to keep it running there for very long, it’s just something you sort of feel in the seat of your pants. I'll feel a lot more comfortable after I rebuild the engine which has about 58k in it now and although she runs real nice and has fairly good compresion it does has its share of rattles, knocks and slaps.
Denny Graham
Sandwich, IL

The RPM/speed chart in the 1939 truck data books for 4.11 and 6.ooX16 lists-
2600 at 50 MPH, 3100 at 60 MPH. That would be with a corrected speedometer reading,etc.

Denny--Thanks for your input. I learned to drive in 1960 in my Dad's 1949 3600. What a great truck. After it threw a rod, we put in a later model 235 and thought we had a real hot rod.

Gene--I'm running 6.00-16 tires on my truck. I expect Chevrolet calculated the RPM and field tested before they released the figures for 1939. My Audi speedometer is probably pretty accurate, so the RPM difference is probably due to an error in my tachometer. The reason I did the test was to calibrate my speedometer. My original speedometer drive gears were incorrect (and by the way, stripped due to being mis-matched), and, unable to find the correct repalcement set, I installed a set for the larger utility trucks. I then checked the odometer error and bought an externally mounted ratio adaptor to correct the error. In the process, I found that one of the odometer tumblers wasn't rolling over, so I took the odometer apart to fix it, and in doing that, evidently moved the speedometer needle return spring tension slider, which threw the speedometer reading way off. I reset the tension slider about where I thought it was, and on the road test, the speedometer was pretty close, but I need to make a slight re-adjustment. Now that I know I'm going 50 mph when my tachometer says 2400, I should be able to set the speedometer so it's correct at 50 mph. The ratio adaptor I installed got the odometer error within 2%, but that wasn't good enough for me, so I sent it back for a slight adjustment to the ratio. When I get it back, I'll make the final adjustment on the speedometer needle tension slider.

Here's a link to a theoretical method of determining RPM, speed and/or differential gear ratio. All based on a perfect world, of course, but should get you pretty close.

http://www.richmondgear.com/101032.html