The Ford Barn

Shoovel · 09-28-2021, 04:41 PM

Folks,
Just wondering what RPM everyone is turning at 50mph.

My 39 is turning 1500 to 1600RPM at approx 50 mph. I don’t know what the rear axle ratio is. I thought it seemed low, so I connected up my MAC tester tach and it reads the same as the tach I installed. 🤔

Zeke3 · 09-28-2021, 07:22 PM

Sounds like a pretty tall, low ratio, rear end. What size rear tire are you using?

tubman · 09-28-2021, 07:36 PM

My '51 has 225/75R15 rear tires and a 3.55 rear end. I run 2700 rpm at 65 mph. A little math yields a bit over 2000 RPM at 50 mph. If I remember, I'll check what I actually run at 50 mph tomorrow.

I will agree with Zeke that it seems you are a little "over-geared".

Hotrodfil · 09-29-2021, 07:25 AM

I'm assuming your Columbia is engaged for those RPM figures. Be about right for 600 x 16 tyres and a 2.72 ratio which is overdrive on a 3.78 final drive.

JSeery · 09-29-2021, 10:12 AM

Sponsored Links (Register now to hide all advertisements)

You can back figure using the RPM to determine the rear axle ratio. All you need to know is the speed(MPH) and tire diameter. This is normally calculated in 3rd gear, so that is 1:1 and not a factor. If you have an overdrive engaged you do need to know the ratio and include it.

mph = (rpm x tire diameter) / (gear ratio x 336)

rpm = (mph x gear ratio x 336) / tire daimeter

gear ratio = (rpm x tire diameter) / (mph x 336)

tire diameter = (mph x gear ratio x 336) / rpm

09-28-2021, 04:41 PM	#1
Shoovel Member Join Date: Mar 2021 Location: Milton Ontario Canada Posts: 83	RPM check Folks, Just wondering what RPM everyone is turning at 50mph. My 39 is turning 1500 to 1600RPM at approx 50 mph. I don’t know what the rear axle ratio is. I thought it seemed low, so I connected up my MAC tester tach and it reads the same as the tach I installed. 🤔

09-28-2021, 07:22 PM	#2
Zeke3 Senior Member Join Date: Dec 2014 Location: Columbus, IN Posts: 1,406	Re: RPM check Sounds like a pretty tall, low ratio, rear end. What size rear tire are you using?

09-28-2021, 07:36 PM	#3
tubman Senior Member Join Date: Dec 2013 Location: Minnesota, Florida Keys Posts: 10,304	Re: RPM check My '51 has 225/75R15 rear tires and a 3.55 rear end. I run 2700 rpm at 65 mph. A little math yields a bit over 2000 RPM at 50 mph. If I remember, I'll check what I actually run at 50 mph tomorrow. I will agree with Zeke that it seems you are a little "over-geared".

09-29-2021, 07:25 AM	#4
Hotrodfil Senior Member Join Date: May 2010 Location: Norfolk UK Posts: 213	Re: RPM check I'm assuming your Columbia is engaged for those RPM figures. Be about right for 600 x 16 tyres and a 2.72 ratio which is overdrive on a 3.78 final drive.

09-29-2021, 10:12 AM	#5
JSeery Member Emeritus Join Date: Nov 2012 Location: Wichita KS Posts: 16,132	Re: RPM check Sponsored Links (Register now to hide all advertisements) You can back figure using the RPM to determine the rear axle ratio. All you need to know is the speed(MPH) and tire diameter. This is normally calculated in 3rd gear, so that is 1:1 and not a factor. If you have an overdrive engaged you do need to know the ratio and include it. mph = (rpm x tire diameter) / (gear ratio x 336) rpm = (mph x gear ratio x 336) / tire daimeter gear ratio = (rpm x tire diameter) / (mph x 336) tire diameter = (mph x gear ratio x 336) / rpm