My 81 has a super leaky C6 in it. I want the 3 gear gone. As I said, it's leaky and I go thru about a liter or so a week of Type F, and it's rather quick on shifts. The shifting I can fix once I figure out the kick down lever positioning. It's the acceleration and top end I want to change. I know already I need to get rid of the, also leaky (on the welds in fact) 2.75 rear end. I'm looking into a 3.00 or a 3.25 down the road.

What overdrive transmission can I put into this thing without modifications? There has to be something as I refuse to believe they used the 3 gear C6 on the 300 I6 all the way into the 90's.

 

An AOD will bolt up, and as shown here (Automatic Transmissions - ???Gary's Garagemahal) Ford used in in 1983 and later behind the 300 six. However, I don't believe it is a good combination with your current axle ratio as its .67:1 overdrive ratio would give you the equivalent of a 1.84 rear axle.

Oddly enough, if you had 4.10's you'd have exactly the 2.75:1 ratio you currently have when you hit OD. So, think through what you want to do before pulling the trigger.

 

My son's 1984 F150 with 4.9L and 4 speed manual O.D. transmission was "born" with a 2.47 axle ratio. In O.D. [.71:1], the equivalent rear axle ratio was 1.75. It was quite capable of 26+ mpg on level highway, BUT if you saw a small rise ahead, you needed to downshift!

 

David - How did it pull in 3rd, which would be 1:1? In other words, is a 2.47:1 effective overall ratio a good one for the 4.9L? Would an effective ratio of 2.75 with an AOD/4.10 combo be better? Or, is it still too high?

 

ZF5 or a NP435/T18/T19 with a Ranger overdrive unit in front. I went the ZF5 route on my latest truck but kind of wish I had gone with a Ranger box. ZF5 was certainly cheaper though.

 

To answer your question of "how did it pull in 3rd?"... a mechanic friend of mine and I took a weekend trip in this truck. He drove the truck, with me in it, for 10-15 miles @ 65 mph without even realizing that it needed an upshift. In his defense, we were chatting and his mind was not fully on driving. It was turning ~1650 RPMs. In O.D., it would have been @ ~1200 RPMs.

The rear gears have been changed to 3.55 and this essentially shifted the ratios down to what was previously 3rd gear with the 2.47 gear set.

3.55 X .71 = 2.52

The truck is quicker and more fun to drive, but gave up a little gas mileage on the highway.

 

So, something in the 2.5 to 2.75:1 overall ratio would be a good target when in OD - assuming stock ~29" tires. Good to know. Thanks.

 

In my 1986, the rear gears are 3.08:1. My idea of "perfect", for a 4.9L would be a 3.25:1 [9 inch] rear with 4 speed manual O.D.

 

Remind me of what your transmission is - the RUG?

 

I believe that is correct. It is not the SROD.

 

So, you are saying an effective ratio of .71*3.25 = 2.31 would be ideal.

 

With stock sized tires, I think so. The engine likes to run @ ~1650 RPMs, its maximum torque range.

 

Not to hi jack but I got to ask why you say this?
I would like to put a Gear Vender OD unit with my T18
Dave - - - -

 

I get to about 100KMH (60MPH) and things start to feel like the revs are getting too high. Cruises at 80KMH (50MPH) most efficiently tho. Best mileage at that speed. My bitch is not having the gumption to get up to highway speed withouy a semitruck slingshot lane to use up. It accelerates better than some cars. But even a Prius can leave me at a light.

Regardless of anything, I do need a whole new rear end. I put oil into it, it gets wet and nasty along welds on the back of the pumpkin. No leaks along the pinion shaft or 5th member bolts. One of the welds cracked at some point before I got it.

I'd like better acceleration, and maybe another 40K so I can cruise down the 120KMH sections of the highways here and not feel I'm overdoing it. That's why I'm wondering what OD autobox and rear gears would do for me. Thankfully, 9" diffs and housings are like dirt.Everywhere.

Now to contemplate 4W disc..................