I just purchased an 86 f350 with a 6.9 and a t-19 4speed tranny. I wanted some feedback on what I could do to raise the RPM;s at highway speeds. Right now it seems very high in fourth gear! RPM's are at about 2700-2800 at 65MPH.... would I have to do a gear change to lower the RPM a little? I sure do not want to change the tranny!

I also have no idea what gear is in it right know. I do know that the truck has 16" wheels and are about 29"-30" total height(ground up).

Please Help.

 

Look on the door jamb sticker and see what the axle code is. that should give you a clue of what gears you have unless they have been changed...

The most common gear is 4.10, 3.55's will lower the RPM. You can also get 3.08's but are costly.

-Enjoy
fh : )_~

 

Most of the F250s came with a 3.55 rearend and the F350s came with the 4.10 as standard.

If you want to drop the rpms, you can either change the gears in the rearend (I believe that you would have to change the carrier in the front end if you want to go up to the 3.55s in it), go to larger tires (my 33s dropped my rpms by about 300 over the stock 235/85R16s, or swap in a 5-speed tranny from an '87 or newer with the 7.3.

The front ends are usually the Dana 44 in the F250s and the F350s had the Dana 50. The simplest way to figure it out is to check the size of your lockouts. The smaller ones are the 44s and the really large ones are 50s. I thought that I had a 50 until I had to replace the lockouts and then found out that I was wrong.

BTW, if you have a corporate rearend that leaks oil from around the seals, Ford made an updated kit to eliminate that problem. After replacing 6 sets of seals and rear brakes, I figured that out.

My '86 has about 454,000 miles on it and I have been from one end of it to the other a couple times over but I found that putting the Gale Banks turbo on it was really made a difference it. BTW, I bought it new with only 4 miles on the odometer and wouldn't give it for anything.

 

Axle code is D5 and tranny P

With that being said what gear would lower highway RPM's? Also what would lowering the RPM's doing to towing?

 

I found this gem in the country from an 80 year old man! It has 92,000 orginal miles and has been stored indoors most of its life. I mean everything works from the factory. AC/Heat/cruise control/no leaks no where in the engine/etc...

I plan on changing all the fluids and maybe going to bigger wheels/tires! I want to use this thing to pull my race car to races that are no further than 600 miles away.

 

It would drop your rpms but as far as towing at a lower rpm, that would kind of be a crap shoot. It wouldn't be as "goosey" with punching the accelerator at a lower rpm.

One thing to remember is that the stock engine developed, if I remember correctly, the most torque at between 1800 and 2100 rpms.

Mine with the 3.55s and 33/12.5 x 16.5s will run about 2200 at 55 mph. You can do the math to figure out but I would think that you would probably be around 2550 at the same speed.

 

Sounds like you may have found a winner. One thing about it, it will be something that you can work on....not many electronics on it and you don't have to bend over to a mechanic everytime that something goes wrong with it.

And if you can't find the parts for the engine from you local Ford dealer, the IHC truck dealer can get you everything at a significantly lower cost.

 

yes, at 55mph its at about 2500rpm! at 65mph it jumps to about 2700-2800rpm which sounds like this thing is screaming or ready to jump out of the truck! I drove this thing back 300 miles @ 65mph with everybody blowing there horns on the highway....

I have thought about putting some 19.5's on her which would keep the same rpm's but give me more speed?

 

Tire height (which translates into circumference) is what will lower your rpms. Basically the taller tire is travelling more distance per revolution than a shorter tire. The circumference is equal to diameter (from outside edge to outside edge) multiplied by pi (or 3.1416). By travelling further which each revolution of the tire, you can reduce your engine speed to get the same distance travelled because the tire is covering that distance with a lower number of rotations of the tire.

If it has not been altered, the 33s are about all that you are going to get under your fender wells without some mods. And if it has sat for a long time, odds are that the 6.9 has taken some of the arch out of your front springs.

Some of the F350s had the IFS which is expensive to lift and some had the solid axles.

Anyway that you look at it, it is going to be expensive to lower the rpms unless you can do the work yourself.

Those trucks were built in an era when the max federal speed limit was 55...not the current 65.

It was built when trucks were meant to be workhorses....not the ghetto cruisers that you see today.

 

what are these 33's you speak of? tire size if I keep my 16" dually wheels....

 

My misunderstanding. I was thinking that you had a 4x4 not a two-wheel drive dually.

In that case, Gear Vendors makes an overdrive unit that is champion. When I bought my truck in October of 1985, my folks had just bought and '85 dually two-wheel drive with the 6.9 and T-19. Dave put an overdrive unit in it and was absolutely content. He pulled 35' camper around the country (he was a piping engineer on nuclear powerhouses) and never lacked for power.

The one that he had actually was a splitter. You could use it between gears on the shift so that you didn't have such a stretch between 1 and 2, 2 and 3, etc. and it also acted as an overdrive unit when you engaged it in fourth gear.

The tires that I was talking about are for a four wheel drive. They would be too thick to put on the rear end of a dually without some kind of spacer.

 

sorry, yes its a f350 dually.

I guess I will try the bigger tire first. That should lower my RPM's. Then I that does not work I will try the rear gear change.

 

Doing the research I gather the D5 axle code I have a 4:10 limited slip rear in this truck?

 

Yes, D5 = 4.10 LS.

I would not waste your time or money putting bigger tires on a Dually.

Your gains will be minute for the size of tire you'll be able to goto, sidewalls will be so tall that it will roll on the walls giving a loose feeling pulling the race car trailer.

Go for the gears, 3.55's would do that truck a great service and are fairly easy to find and not real expensive. Of course 3.08's would be better but are expensive. The Gear Vendor Overdrive is pretty big bux! in excess of 3k new. The used ones I've found were in the 22-2500 range.

Being that you have a 2WD you could put a ZF5 in it fairly easy, 2WD ZF's are not as hard to find or as expensive as 4WD ZF's and are a fairly easy swap... If you build/repair your race cars you are very capable of doing the swap.

If you do look for a ZF5 you have to get one from a Diesel 2WD... Period!

I basically have the identical truck, only mine has a F250 SuperCab on the F350 4x4 roller with 3.55's.

Personally I would go for the ZF5 and 3.55's...

-Enjoy
fh : )_~

 

thanks for all your help.

I am going to call a local rear shop here on Houston and see what the 3.55 swap would cost.

Again, thanks...