I know that the final drive output ratio should be 1:1 - but I am not convinced my C-6 has that.
Setup: 460 in front of a C-6 out of an '83 F-350 dually fifth wheeler, all in front of a 2.75 rear end gear ratio.
Symptom: At 65 MPH it hits 2500 RPMs, which seems pretty high for a 1:1 and a 2.75 rear.
The transmission shifts firm, no slippage, but hits 3rd at less than 20 MPH. (Note - the vacuum modulator is fine, but the downshift rod is not in place, is disconnected at this time.)
I am thinking that this transmission was built by Ford with a final ratio different than 1:1 since it was going into a heavy duty truck designed for heavy towing. Is it possible that this transmission has a lower output ratio? Did Ford put different ratios in C-6 transmissions?
How would I be able to tell what the final drive ratio is?
Is it possible to change the final drive ratio, and if so, how?

I drive rather sedately, and don't use the truck for heavy towing, so improving gas mileage is more the issue here.

Thanks in advance for any help and information you can provide!

 

2.46/1.46/1:1. Every one they made. NO different final ratio.

How can you have a 2.75 ratio in an F350?

The tallest ratio that's been talked about for that type of axle (actually for Dana 60s, not sure that's what you have) was 3.54

Stock tires should be either 215-85x16 or 235-85x16. That means either 30.6 or 31.4 inches in diameter.

Using this calculator:

Novak Conversions - Gearing & Gearing Math for Jeeps

it shows that a 30.6" tire, 3.54 axle, & 1:1 trans would give you 2526 rpm at 65 mph.

With 31.7" tires, you get 2449 rpm at 65, so again, not far from 2500.

With the 30.6" tires and a 2.75, you would only turn 1963 rpms at 65.

 

Hey, thanks for the reply!
1. I understand that some 460 built C-6s in ambulances had an OD ratio for the final gear - can you speak to that?
2. I probably should have made it clearer what I have:

'66 F-100, 9-inch rear 2.75 ratio, '86 460 and the C-6 out of an '83 F-250.
Tires are nothing special, just what would come as stock on 15 inch rims.
(Plus all the other modifications, like '76 power disc brakes, '76 power steering, '93 serpentine belt system on the '86 460, and more. All Ford, and all bolt-up.)

 

By the way, the 65 MPH figure is not coming from the speedometer, but from pacing another vehicle.
The speedo drive gear is not correct, so it is off from what is correct, which is why I need to use a pace vehicle.

 

No OD C6s. It could be the E4OD you've heard of, which is a 4 speed OD. Sometimes I see them referred to as "a C6 with an OD". Not true--completely different trans. I think you can swap E4OD 1st & 2nd gears to the C6 for wider ratios, but that's about it.

Ok, if you've got 15s on the vehicle in question, they could range in overall diameter by quite a bit. But let's say they are 225-75x15s.

Those would be 28", which should give you 2145 rpm at 65. Add a couple hundred for TC slip....still not that close to 2500.

3.25s, a very common ratio, would make it 2535 at 65.

Are you certain the truck still has 2.75s?

 

The tires are definitely 235s.
Yes, the rear end is a 2.75, but not original to the truck, as I put it in there.
It originally came with a 3.50.

And your comment about the E4OD sounds right, it was mentioned as a C-6 with an OD, but found only in ambulances.

 

235 75s?

E4OD was in all trucks, not just ambulances.

 

There is no possible way to get other than 1:1 in drive. When in direct the whole gear train is locked up. To do so would require a totally different mechanical setup.