Thinking of putting a 9" rear end my 51 Ford F1. What gear ratio do I need to run. Will have the flathead V8 and 3 on the tree.

 

How are you planning on using your truck?

 

I'm planning on a DD

 

Tire size makes a big difference.

Calculator Jules sent me:
Engine RPM / Engine Speed Calculator

If you want to get crazy with it:
Gear Ratio Calculator

 

not sure what size tires ... May go back with the 700 16 that are on it now

 

You may be aware that there is a replacement gear set that is now available. Eliminates the need for the differential swap.

Welcome to Randy's Gear Supply | Athens, GA

 

In in town (stop-n-go), freeway (55mph+) and/or country roads? You looking for performance or MPG? Speeds you expect to operate at? What size tires are you going to run?

 

in town / country roads no more the 55 mph .MPG would b nice Thinking of going back with 700 16 tires

 

I dont know what the flathead is happy at as far as cruising RPM, but with my modern OHV engines I use the calculators to shoot for 2000-2200 at 60mph. I like to be able to hit the highway for out of town car shows when I can. If 55 is all you want to run I would shoot for the same 2000-2200 if the engine makes enough torque to run there. If it needs more RPM to not be lugging then move the RPM number up.

 

I like the 3.7

 

7.00-16 tires are a poor choice for a DD, in my opinion. You'd be a lot happier with 215/85-16 radials, and tubeless. Your stock wheels would need to be in good condition to fit them, but they ride better, track better, and brake better.

My truck has the flatty and 3-on-the-tree. I'm running a 3.50 9" and it is about ideal. There's no reason to lug a flathead down, they are only 239 c.i. and low compression, so they need to spin a little to make power. Our trucks have a lot of wind resistance, and it takes HP to overcome that. I'm at 2500 RPM at 60 and it's happy running there all day.

I tried a 3.25 axle at one point, and with the 3-sp, taking off required a lot of clutch slipping, as did going around city corners in 2nd gear. It was a total dog on anything but flat roads at higher speeds, no responsiveness. Even the 3.50 is noticeably less peppy off the line than the stock gears. '48 - '50 F-1's had 3.73 gears and that strikes me as a nice compromise. You might find one of those axles in good shape that someone has taken out to go to a 9".

 

What is the guideline for determining what RPM is ideal all around for a given engine, like a flathead, or Y or FE? Or is this the wrong way to look at it? Can see it's a big can of worms.. "on the one hand ..."

 

http://www.jalopyjournal.com/forum/t...ng-rpm.312099/

Here's a thread that references RPM

 

My truck had a 2.75 gear when I bought it. It wouldn't pull a hill in anything higher than second gear (4 speed crashbox). On the the plus side, I got really good at the double-clutch downshift.

I switched to a 3.50 last year, and I'm very pleased with the results. Also I'm running 215/75R16 tires.

 

Not sure if I'm doing this right I counted the teeth on my ring gear had 47 there is a stamp on the ring gear 47-12 so I'm assuming the pinion has 12 and if I divide that I come up with 31 does that sound right. And if so is that a good gear ratio to run with a 239 Flathead with a 3 speed on the tree