How do I get a higher gear ratio with lower rpm on a 312?

 

'66 what, F100?

312 what, Y block engine?

IF F100, you should have a Ford 9 inch axle with a drop out carrier. The gears can be changed to whatever ratio works for your load and tire size.

You are in the wrong forum BTW. (reported)

Provide more info and perhaps more help can come your way.

 

No 312's were ever factory installed in F Series trucks.

312: 1956/57 Ford/Mercury cars / 1958 Mercury cars / 1959/62 cars: 292 2V.

1958/64 F100/350 = 292 2V.

No 292's were installed in F Series trucks after 1964.

 

It's F-350 66'. Gear ratio is 4.88 with a 390 ford c-6. Got a parts truck is same year but has a 312 in it. thinking about swapping motors but can't figure out how i am only getting 50 mph @3500rpm with the 390.

 

That is mostly due to your final drive gear ratio 4:88 is really short so you wind the engine up ...make it easier to move heavy loads, kills your top speed and mileage too if you push it beyond ....45-50 mph.

It's not your engine or your transmission...it's the differential..

That's my $0.02.

TH1567
Run a Ford or run home.

 

Got it. The post was actually for my father in law. He has a few 66' 1 tons flat beds, he keeps complaining about speed, rpms, etc. I am trying to get his questions answered, but he has a hard time explaining it to me so I can post a question. I apologize for the first post, I am simply the messanger. hopefully this helps. Thanks for your patience!

 

What size tires are you running?
Assuming your torque converter is not slipping like a greased banana and your speedo and your tach numbers are accurate, then it's because your tires are DANG small (short).

If you have 4.88 rear gears, a C6 (which has a 1:1 highway 3rd gear ratio), at 3500 rpms, and at 51.17 mph, the math says you must be running 23.5" tall tires.

If you change to a 31" tall tire, your speed will pick up to 66.16 mph @ 3500 rpms. It doesn't matter if it's a 390 or a 312 engine turning it all. However, if you'd like to drop the rpms and gain some mileage (as th1567 indicated) change your rear end differential gears AND your tires. If you swapped out the 4.88 for a set of 3.80 gears, then at 66 mph with 31" tall tires, your engine rpms would drop to 2725 rpms. Realize that the lower you go with the overall gearing ratio, the less work you'll be able to do with your F350.

There are many gear ratio calculators that can be located on the web, some good & some not so good. This one seems to be pretty accurate and is the one I used above.

Stan Weiss' - Automotive Performance Software / Interactive JavaScript to Calculate Rear End Gear Ratio needed

Go to the above link and plug in some different gear/tire/rpm combinations & see what works best for you.
Good Luck!
BarnieTrk

 

Bobby, Don't know if this will help, but when in high gear the transmission is turning 1 to 1. Every rev of the engine the tranny makes one rev.

With a 4.88 rear the engine/tranny rotates nearly 5 revs for each complete rotation of the rear tire.

With the 3.12 it is a touch over three.

So the engine has to turn much faster to go 50 mph with the 4.88 than with a 3.12.

Problem is with 1 tons changing the rears is much more complicated than with a half ton with a 9". I can swap a 9" chunk out and have the truck back on the ground in an hour. I have no idea how long it will take on the big Dana, prolly all day.

My F100 has a FE/C6 combo turning a 3.00 rear and a 55mph the engine is turning 2050rpms. Reaching 100mph is a piece o cake for my truck, I have run it up to that in second gear. I doubt that your engine turning the 4.88 would reach those numbers, but on the other side of the coin, will pull a hig house down the road, where mine wouldn't.

hope I didn't confuze you.


John

 

What size tire (how tall) are you running, John?


BarnieTrk

 

P 235 75 15




John

 

Ok, so that calculates to a 28.9" tall tire (9.3" section width; 7" section height).

I don't know what to tell ya, John, how sure are you that your speedo or tach is reading correctly?

The calculator says if you're running at 2050rpms, in high gear (C6 is 1:1), rolling on 29" tall tires, at 50mph, you must have a 3.5 rear end gear set.

BarnieTrk

 

Hey man the truck is 43 years old and I am 60, hell we both could be off a little.

I thought I had checked it against another spread sheet and it pretty much agreed. I believe that if I was running 3.5 rears the engine would be turning faster at 55. What can I say?


John