Hey all!

I am planning to put the flathead Merc out of my dad’s old truck (fitted with a 4-barrel carb, and maybe newer heads) into my ’52 F1, put an AOD out of a ’91 Mustang and a 9” rear diff out of a ’70 F100.
I am going to go thru the rear diff and want to know the best ratio for a truck that will be comfortable driving on the highway and around town. It has a 3.00:1 ratio now. I was thinking somewhere in the 3.7:1 after using the calculator.
Thoughts? And where is the best place to purchase the ring and pinion and rebuild parts?
Thanks folks!
Doc

 

Does a flathead make enough torque to warrant using OD? While I have not run a flathead in anything the consensus here always seems to be they are happy running around 2500 at cruise. Tire size and desired cruise RPM will dictate the rear gear you want.

 

With an overdrive type of transmission a 3.70:1 ratio would still give you good highway driving. Did you calculate the engine RPM at the highway speed and with the tire size you plan on using? What RPM are flat head motors comfortable running at for long time periods? My 1995 F150 has a 3.73:1 rear axle ratio. With 235/75-R15 tires and the E4OD auto trans, the tach indicates in the low 2400 RPM range at 65 MPH. Obviously a shorter tire will increase the engine RPM.

For ease I purchase my ring and pinion and axle parts through Summit Racing. The tough part is getting a good quality brand ring and pinion gear set. Sometimes two brands are owned by one company and each brand has different quality R&P sets. For example, Yukon sells their lower quality stuff under USA Gear. Stay away from Richmond gear brand, they have a reputation for gear noise more often than not. With that said, Yukon brand has a fairly good reputation, Motive brand in their Performance series are also one of the better gear sets. US gear claims made in the USA but is unclear if all of their lines are USA made or simply the high end racing stuff.

For the installation hardware, get kits that contain Timken brand bearings.

Lastly, the higher end gear sets are being manufactured in USA or Italy. I know Motive Performance line of gears are made in Italy. I'm not sure about the others. You usually have to speak with the gear company to get that information.

I was going to use a Yukon gear set a short time ago, purchased them, they looked to be good quality. No mfg place of origin. My only issue was Yukon no longer provides a pinion depth spec and instructs using the trial and error method to obtain a good tooth contact pattern. I don't have enough experience so I like a pinion depth spec and still also check the contact pattern. I returned them and went back to a Motive Performance gear set.

Hope this helps a little.

 

It helps a bunch! The info on ring and pinion sets alone was worth the read. I have only done a couple of rear end rebuilds-my good fortune is my brother was a prototype engineer for for rear differentials for 30 years and is fixing to help me with it.
I agree that the pinion depth is way easier with specs to aim for...
I understand the flatty can handle about 2500 rpm.
Yes, the calculator I used had spots for tire size, and each gear ratio of the transmission.

Doc

 

IMHO, a 3:70 gear with an AOD is good choice. I would not use anything less (numerically lower) than a 3:50.

 

'53 F-100's were the first to be available with OD, with nearly the same OD ratio as the E40D, and those trucks came with either a 3.92 or 4.27. With a 3.92 axle, the effective gear ratio in OD is 2.75. I doubt that is good for anything but steady speeds on flat ground with 100 HP on tap. (depends on tires but not a whole lot) That's what BobbyTNM on here runs with his 500 c.i. Caddy engine....

 

My truck had a 2.75 rear behind the flathead. It did not like hills. I don't recommend it. I've switched to a 3.50, and it is much better.