...axle ratio.

Pickup is a '54 F100. 351W and FMX from a '74 Gran Torino Elite.

I have obtained a complete 9" rear axle from a '66 F100. It is equipped with 3.70 gears, according to the axle tag.

The Torino has a 2.75 ratio.

I live 30 miles of highway from the nearest anything, so most of my driving will be at highway speeds. I drive conservatively (like the old man I'm turning into), and I'm not prone to spinnin' the wheels, but I might be tempted from time to time.

My question is, should I leave the 3.70 in there, or swap in the 2.75? Is 2.75 too tall? I figure it was good enough for the Torino. I will be running bigger tires, though (15" vs 14").

I know the FMX starts out in 2nd gear, which compounds the problem.

Will the 3.70 result in too high rpm's on the highway?

I realize if I had an AOD this wouldn't be an issue, but that's not in the cards. I gotta use what I got.

Which would you choose, and why?

Thanks for your perspective.

 

We take some pretty long trips in our truck so I used a gear for the highway, 2.72
In my panel I installed a 3.50 but it will have O.D. Don't know if that helps.
brian...

 

3.70 gears

 

I usually only consider a rear gear above a 3.50 if using with an overdrive tranny. You will be turing some rpms with the 3.70. You can use the following link to see what your rpm's will be with different gears and tire diameters.
Calculate Speed(MPH) for given RPM, Rear Gear Ratio and Trans Gear Ratio

 

If you plan on hauling anything I'd say 3.70's If you plan on just cruising it, I'd say 2.75's Best of both worlds would be to find 3.25's, which means continuing looking for another rear of similar years.....easier said than done, as I have found out the hard way....no luck for me yet.

 

When I bought the 9", the seller told me it was a 3.25. I didn't verify it until some time after I got it home. Oh, well...

 

I may be mistaken here. I was thinking of the Fordomatic, which starts out in 2nd gear.

 

you said your thinking about bigger tires..... actually the rim/wheel size don't matter, just the overall tire diameter. it acts just like changing the gear ratio. If you running 29" tires and change to 31" you would like a higher gear also.

good news, it's 9"...... you can alway pull the axles and throw in an different ratio center section (as long as it's the same spline etc....)

I don't believe there is a magic number, automatic without lockup torque converters slip a few more hundred RPM's away at high speeds,

I'd go with the 3.70's i drive 63mph on the highway, and enjoy taking off....... maybe swap to alittle taller tire later? or when 3.20ish's come around go do another swap...............

 

Mr. Mayo I like Joe's truck, that is really special when you can save a family treasure and keep the good memories alive.
Best of luck with the build
Mike

 

Well,

You need to know your rolling tire radius.....

Once you know the rolling radius (and circumference) you can then calculate the rear wheel RPM at 60 mph. (5280 feet/min)

Once you know the rear wheel rpm, you can calculate the direct drive engine rpm.

I'll get 2050 RPM in OD at 60 mph with my current tires and 5.83:1 rear axle ratio.

perfect for running 'on the flat'.......

The slower you turn the engine on the highway, the better your mileage will be running those 30 miles at "old man" speeds......



Cheers,


Rick

 

Hey Blaine,

Here my opinion. A 3.25 would be the best of both worlds or a happy medium. Since you have neither I would go with the 2.75 and play with different tire diameters to go up. In the meantime you might find someone willing to trade 3rd members, a 3.25 for a 3.70. If you are not interesting in light to light hole shots and such the 2.75 or 3.25 will be good and your gas mileage should be decent as well. The FMX while a little heavier I believe is easy on gas. Keep us informed.

 

Try the 3:70's they aren't too bad. I ran them for years in a van and running 70 didn't seem to be pushing it any. If they seem too slow try to find some 3:25's. I think the 2:75's will be a little fast.

 

Did you get to drive the Torino, I bet it was sweet out on the road...I vote 2.75's.....Rod

 

Since you do a lot of old man highway driving go with the 2.75 and save some gas money. I'm running a 2:26 and do a lot of that old man driving too because I am one...

 

I just gotta ask... Did you think out the title to this thread before you posted it? I chuckle everytime I see it.

Later Man...