Changing Rear End Gear Ratios: How to calculate Engine RPMs,

Most old flathead powered vehicles don't feature an RPM tach. In this video I discuss how to calculate your engine RPM at various road speeds. You can use this same formula to learn your engine RPM if you change your rear end gear ratio. Or perhaps you want to add an overdrive gear system to your transmission? You'll be able to know exactly how various changes will effect our old flathead driving RPM speed.

[rollie]

Cool. My Grandfather also subscribed to Skinned Knuckles. Your calculations confirm these engines were pure torque monsters

[bazmarty]

Fantastic video.Another way to calculate rolling tyre distance travelled (instead of using tape measure directly on wheel) is to mark your tyre and where it touches the pavement with chalk.Roll the tyre one full revolution until chalk on tyre retouches the pavement and mark pavement.Then measure between the 2 chalks marks on distance traveled on pavement.This can be useful in working out how different sized tyres/wheels effect distance travelled per revolution.

[mikesilva]

Thank you for this. My project car is getting updated rear end. This will be very useful.

[tombob]

Great lesson. I know of a guy that had a 50 chevy 216, 3 speed. He put in a 323 rear end and a 5speed from a S10. He claimed it resurected the car. 1st was a steep and 5th was like 70% overdrive. Cruise all day at 75

[jimh.]

I have a '61 VW bug that I converted from a 1200cc engine to a newer 1600cc engine. People often install a newer transmission that will drop the RPMs and take advantage of the extra power. But instead I calculated what diameter rear tires I'd need for the same result. It worked great. Now the rear tires are a little bigger than in the front, but it's not very noticeable.

[joseluisramirez]

Gracias por los videos me gustan mucho los del plymouth 1938 por el motor de 6 en linea

[FlatheadTerry]

Thanks for the lesson Keith. Good stuff! I think I have 4.11's in my '37 so this will help make it easier to calculate. Now I need to fix my wavering speedo needle! Not sure if I'm going 48 or 53 mph. 😃😃

[jesusoz]

My neighbor has a 1946 Chrysler we are try to change the freeze plugs don’t know how many are they and how you remove the can over the top rear plug may be you have a tip

[JB]

Why is your final RPM at 60mph? What determined 60? I have a final transmission gear of .75. How do I adjust the numbers? Would I reduce my final rpms by 25% Thanks.
Edit: I think I take .75 of 4.1 which is 3.01

[mathewanderson]

I have a 50 dodge B2 and was told it shouldn’t do more than 45 mph. It has fluid drive but I’ve never successfully used it. 3 on the tree. Can you verify it shouldn’t go above 45? Should I install a tach?

[midnightraiin]

how did u determine 4:11 differential ratio