Let's Talk Gear Ratios

[zainyD]

Moderator, if this needs to be moved to the 4x4 & Off-Road section I apologize.

I'm in the parts collecting phase of returning a '72 F250 4x4 to the road. It currently has it's original drivetrain 360/NP435/Dana 24 and D44/D60 axles with 4.10 gears.

I intend to eventually convert the 360 to a 390 along with some possible other motor upgrades, but nothing crazy. Basically, a reliable motor with good horsepower and torque. Tire size will be in the 35-37" range. I intend to keep the NP435 and Dana 24, although, if I find a divorced NP205 for a reasonable price I might swap that in.

Anyway, this discussion is about gear ratios, so let's get to it. I will be swapping out the factory closed-knuckle, drum brake front axle for either a HP D44 or a D60. I am considering swapping to a numerically lower gear ratio of 3.55. With a 36" tall tire that would put me in the 2,300rpm range at 70mph. If I swap to a 3.73 ratio with the same tire I'd be at a little over 2,400rpm at 70mph and if I keep the original 4.10 ratio I'll be at almost 2,700mph at 70mph. I don't intend for my F250 to be a commuter, but I'd like to be able to take it on the interstate without running high in the rpm range either.

What is an efficient rpm range for a 360/390 motor? What ratio would you recommend given the perameters I've laid out?

Thanks for your input and experience!

[Lee]

Hi,

First to answer your question concerning RPM..... It depends on your comfort level..... while 2800 at 70mph is spinny right along.... It won't hurt anything except yer wallet for gas money.

Also to consider.... do you tow heavy loads?

I basically have the same setup you have in my 1970 F250. I have rebuilt the 360 to a 390 and added headers, RPM intake with a Holly 4bbl and an RV cam. I run 37 inch tires. I also regularly tow my 6000lb RV trailer.

I thought of swapping in some parts as you describe but,.... I came across a smoking deal on a new Advanced Adapters overdrive unit that I installed in front of the NP435.

This allows me to run in 4th gear and Overdrive while on the highway to lower RPMs.

While the Overdrives are expensive.... It's probably less than doing axle swaps if the primary goal is to lower RPM..... The advantage the overdrive has is its selectable with its own gear shift...... If towing a load.... I leave it in "direct" drive ..... If not towing I shift to OD which is 27%.

[stpackard]

A month ago I installed a 3.54 gear set in a 78 dana 44 8lug. I had it bolted in and had a set of tall/skinny 35’s. I was just about to bring the rear end to the shop to get the 4.10 changed to a 3.54 when I got reasonable and thought that I’d be running a tall tire on a tall gear ratio.
Long story short I took the d44 back out and had them regear it back to 4.09. I was originally wanting to do the 3.54/35’s to get rpms down, but I realized I’d compromise power and maybe drivetrain wear. It runs pretty well, I put the Sanderson FF427 Headers on.
If I were running 32’s I’d have gone with the 3.54’s.
I don’t know if this helps, but Lee’s suggestion for the overdrive is probably your best bet but with some labor and expense.

[Ranchero50]

OD trans makes everything else moot.