Iowaboy Posted September 25, 2017 Report Share Posted September 25, 2017 I am trying to calculate what rpm I have to run to hit 60 mph. My rear end is stamped 41F, I assume this means 4.1 ratio. My rear tires are 28.25 inches in diameter, if 4th gear is 1:1 then I have to hit 2926 rpm to run 60 mph. That seems kinda high rpm for a flathead 6. Quote Link to comment Share on other sites More sharing options...
59bisquik Posted September 25, 2017 Report Share Posted September 25, 2017 As far as I know, the 4th is a 1:1 ratio. Gotta remember these trucks were never designed to go the highway speeds of today. Probably not too many cars and trucks traveling at 70+ back in the 50’s. With shorter tires and 3.73’s, I calculate about 3200 for 70mph in mine. I drive the freeways of SoCal all the time and hold her at that speed with no issues. Quote Link to comment Share on other sites More sharing options...
greg g Posted September 25, 2017 Report Share Posted September 25, 2017 Got a 4.11 in my coupe, 65 indicated is 62 over the road, calc with 225 75 15 works out to 3280 rpm. The old fellow who did my machine work told me 80-85% of peak HP rpm (3600) makes for happy motoring. Quote Link to comment Share on other sites More sharing options...
55 Fargo Posted September 25, 2017 Report Share Posted September 25, 2017 4 hours ago, greg g said: Got a 4.11 in my coupe, 65 indicated is 62 over the road, calc with 225 75 15 works out to 3280 rpm. The old fellow who did my machine work told me 80-85% of peak HP rpm (3600) makes for happy motoring. This of course would be qualified by a sound engine in good internal condition. I know this has been your experience for a number of years to date. Mine I run at 2400 RPM for 62 mph or 100kmh but I have 3.23 gears. These old brutes can handle this far better than any Stovebolt splasher or Banger engine that's for sure. Quote Link to comment Share on other sites More sharing options...
greg g Posted September 25, 2017 Report Share Posted September 25, 2017 2925 rpm according to http://www.4lo.com/calc/gearratio.php Quote Link to comment Share on other sites More sharing options...
Iowaboy Posted September 25, 2017 Author Report Share Posted September 25, 2017 I knew that engineering degree would come in handy. LOL Quote Link to comment Share on other sites More sharing options...
TodFitch Posted September 25, 2017 Report Share Posted September 25, 2017 19 hours ago, Iowaboy said: I am trying to calculate what rpm I have to run to hit 60 mph. My rear end is stamped 41F, I assume this means 4.1 ratio. My rear tires are 28.25 inches in diameter, if 4th gear is 1:1 then I have to hit 2926 rpm to run 60 mph. That seems kinda high rpm for a flathead 6. Mopar engine designs of that era had to survive 50 hour tests running at max BHP. That would be at 3600 RPM 9 hours ago, greg g said: Got a 4.11 in my coupe, 65 indicated is 62 over the road, calc with 225 75 15 works out to 3280 rpm. The old fellow who did my machine work told me 80-85% of peak HP rpm (3600) makes for happy motoring. I've seen that 80% or 85% number in a number of places and believe that was (and perhaps still is) a pretty good rule of thumb. So basically the engine, if in good condition, should run happily all day at 3000 RPM. I have a personal preference for not exceeding 3200 RPM on long "high speed" drives which works out to about 60 MPH in my older car (190 cu.in. fitted with the stock 4.375:1 rear end and skinny 17" bias ply tires). Above that speed the car begins to be a bit uncomfortable with respect to its suspension, etc. With your tire diameter, my calculator shows you running 66 MPH when at 3200 RPM (assuming a 4.1 rear end). Quote Link to comment Share on other sites More sharing options...
55 Fargo Posted September 26, 2017 Report Share Posted September 26, 2017 Interesting enough these engines can run in the 2500-3000 RPM range and be happy with it. Try this with a 216 Stovebolt ,babbit pounder splasher Chevy engine, not going to be too happy and might self destruct. I do not think these engines like to be run below what 2000 rpm, necessarily, and are comfortable in a higher RPM band Quote Link to comment Share on other sites More sharing options...
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