Guys. Just got a parts truck with a 4 speed manual and a straight 6. I was thinking of selling the straight 6 but I am thinking of possibly rebuilding it and using it versus my 302 EFI. The 302 EFI runs okay but for a truck I am not impressed with it. I have 3:1 rear gears, C6 and the 302 EFI. If I did a straight swap 302 for the 6 would I regret it? Would I be able to get better mileage because of having 2 less cylinders or is that just bologna?

The parts truck has 2.75:1 rear gears. What should I do with those gears? I drive the truck 20 miles one way to work 5 days a week. Would 275 rear gears put my C6 and 302 into a bad rpm range and actually hurt gas mileage? Im not sure if lowering the rpms is always a good idea. Don't ask me what my rpms are at what speed because I don't have a tach.

However if I had a calculator I could figure it out with the given info and tire size. Tires are 255/70-15's

 

The name of the game for fuel mileage is low rpms, but that's not good for towing and hauling. Thus they started putting overdrive trannies in these trucks to make them more versatile. You can use a higher number gear in the rearend (say a 3.50) and still get good mileage by using the overdrive when empty.

With what you have, the 4 speed will give you the best mileage. As far as engines, it's a toss up, I believe they will be close. The 302 is going to be better for driving to work at highway speeds empty, and if this is what you mainly use it for, I believe I would stick with it.

 

Okay, looks like at 65mph, with 3.00:1 gears, and 255 tires (29inches), I will be running the engine at 2259 rpm. That assumes a final gear ratio from the C6 is 1:1. Which I believe it is.

If I swap in the rear end only I will achieve final engine rpm at 65mph of 2071rpm.

My question is this: Is 2071 rpm too low of an rpm for 65mph cruising with the 302? What about the straight 6, doesn't it have better low enf torque than the 302?

Although the speed limit is 65mph or 70 mph where I live I pretty much stay at 65mph most of the time.

 

Franklin, we must be typing at the same time. Thanks for the advice. Sounds like I should keep the 302, and swap in the 4-speed. Is 4th gear in the 4-speed OD, or is it 1:1? Which 4-speed came in a 1980 f150? (remember I am a chevy guy who knows little about fords).

 

They did make a 4 speed with the overdrive, you will have to look to see what you have. The other 4 speeds where heavy duty units with a granny 1st and a 1 to 1 4th gear.

 

the number on the trans says "RF-D9BR-7006-AB" I know its a top loader, but what does that mean? Is this a desirable 4-speed transmission? Do you have any good links to pages that helps me identify it better?

 

Some people on here know how to identify some of the trannies better than I do. If it is the overdrive, they are considered somewhat light duty, which may be perfect for your use. But if it has a lot of miles on it, you may need to go through it before you use it. If that's the case, you might investigate how much a 5 speed would run you from the junkyard.

What year is the parts truck? If it has a mechanical clutch linkage, you will need to consider this in your tranny choices, since all the 5 speeds are hydraulic clutch. It's not a problem, it just means more parts you will need.

 

Ray - You should have looked at the NP435 sitting on the floor yesterday. But, for your edification pictures thereof are attached. That tranny is 1:1 in 4th. Note that it has the 4x4 tail housing extension on it so would look a bit different than a 2wd version, but you can figure that out.

On the engines, the 6 has a lot more low end torque than the 302, so can handle numerically lower differentials. And, IIRC, RW/Paul's truck is a 300 with 3.00 gears and the C6, and he's hauled another full pickup on a dual axle trailer with it, so it can be done - with the 6. I don't think the 302 would do nearly as well with those gears.

So, that is an option for you.

 

Mine is alot different than yours gary. maybe mine isn't a top loader, I dunno. I just rebuilt the 302. I will keep it in there and possibly put this 4-speed in if its any good or isn't too much to rebuild. I also need the clutch pedal assembly. This is a mechanical linkage setup. Does the clutch/brake pedal assembly just unbolt?

 

1980. Gary can tell you ALL about it. Right Gary?

 

Maybe I can tell more when I get home and have access to the catalog. Yes, the pedals unbolt and look vaguely similar to those you saw in the shop yesterday. Actually, exactly like.

 

When I said you can tell everyone ALL about it, I meant that you wanna kill the guy who took it apart. lol.

 

That's a light duty overdrive trans, Ray.........I think an SROD

 

Doesn't look anything like a NP435 or a T-18/19 to me. It looks to me like the 4 speed over.

EDIT:

Posted when he did. Beat me to it.

 

Interesting. How can I figure out the 4th gear ratio?