My son and I have finished most of our mechanical repairs and upgrades on our 78 F150, 4wd, 300, 4spd NP435. We drove it over 300 miles to Louisiana and back for Thanksgiving and all is good. Except.... our top speed in 4th gear is about 55. It has 3.50 gears in the rear and I'm pretty sure 3.55 or 3.54 up front.

We have also added efi exhaust and a 4 bbl on an Offey DP. The pep is pretty good. I don't think a cam upgrade would do much to help with the 4th gear top speed.

I have searched the following to limited avail.

Adding overdrive to the NP 435
Swapping the trans
Re-gearing the axles

I can't find any ring and pinion gears for the front Dana 44 lower than what is stock. Am I searching for something that doesn't exist here?

Any other suggestions? Or should we just be happy that we have a mechanically solid truck that surprisingly got almost 13 mpg with mostly highway driving?

 

When you say top speed of 55, what is your engine RPM? I think of top speed as the truck simply will not go faster no matter how hard you push the skinny pedal, but many questions here been asked here where it was an unwillingness to push the truck any more RPM wise or it feels unsafe handling wise. Can you be more specific what you mean by top speed? One other quick thought- are you sure the speedo is accurate?

For reference, the one time I was able to drive my wife's 65 f100, it hit 55 no problem with the stock carb, intake, and exhaust- up hill on a winding road. It's not a NP435 but I think the gear ratio is similar as the rear end looks stock (haven't verified it yet). The 300 is known more as a torque motor rather than top speed, but I just can't imagine that it tops out legitimately at 55.

 

The speedo matches the gps speeds on our phones. Yes it will go a little faster, but it really winds up the motor. We don't have a tach, so I'm estimating RPM's at about 2400-2500 at 55.

Handling is fine at all speeds. The next time we have it on the highway we'll push it a little harder.

 

What tire size are you running?

With a 30 inch tire, you should only be turning 2186 RPM at 55 mph.

My '85 would bury the speed-o in .81 4th gear with 31-10.50x15s and 3.50 gears, given enough road. That would be close to 90. On the flat of course.

If you are tach'd out at 55 you are in low range in your transfer case or there is something else dreadfully amiss.

 

We'll also check to make sure the transfer case is actually going/staying in 2H. The lever seems solid, doesn't flop in or out in the cab.

Wheel/tire diameter is just under 31"

The truck came from my father in-law and he said it never really had much speed on the top end.

Thanks for the quick responses.

 

Continue checking things. If everything is right it should do more than 55 MPH. Check the rolling radius of the drive wheels and do some math with a speed calculator. Investing in an accurate tachometer would be better than guessing. Are you sure about the 3.50 ratio? A numerically lower one would likely help top speed but even 3.50 should do better than 55 MPH. Are the brakes dragging? Is the clutch slipping? Keep checking.......................something is not right.

 

That's a great feeling! Glad you had a chance to build a truck with your son. Sounds like a nice one too.

With a 30.5" tire your engine speed at 55 is 2100 rpm. At 65 it's 2500rpm. Not bad at all.
If you simply want the engine to run at a lower rpm then a taller gear is called for. Easy to test this theory with just the rear first. Then if you find you really like the one you choose, then re-do the front diff to match.
Saves you the money and hassle of doing two diffs if you find out you don't like your first choice.
Plenty of gear choices for Dana 44's and Ford 9" diffs.

Sounds great. With it pulling hard, maybe it's just the sound of the engine that makes you think it's straining. Fan noise, gear noise, tire noise, wind noise, all add up to making us think we're revving more than we think.
With my Bronco is was 100% the old exhaust. Once I changed to a different exhaust system (quieter) all of a sudden my perceived engine speed went way down!

They're out there. Or should be.
From 3.23 or so, all the way down to the high 2's there should be just about anything for the Dana 44.
Sure, "stock" the 3.50 was the highest/tallest/longest gearing available in the 4wd trucks, with 4.11 and even sometimes 4.56 being the lowest optional gearing. But Dana 44's were rear diffs long before they were fronts, and in passenger cars and 2wd trucks they came with much taller gearing (lower numerically) if I'm not mistaken.

Yep, get a tach! Even a temporary one you can hook up just to see what "sound" of the engine correlates to what rpm.
It's invaluable for tuning and for just knowing what's going on under the hood.
But I think you'll find that you're not over-revving the engine by any stretch.

Good luck!

Paul

 

Have you looked at carb linkage for full throttle? Double checked the secondaries? Ensured transmission fluid is clean and filled correctly? Verified the modulator operation on the transmission? What about ignition timing?

The issue seems to me is not at all a gearing issue, with the current gears you should have no problem hitting at least 85 mph

edit- just realized "4-speed"

How old is the clutch? Does the engine rev higher but speed stays the same?

 

It could be a perception problem. I'll check what has been mentioned. It just sounds like it is at a high rpm.

Thanks again for the replies.

 

Okay we finally got a tach installed and most of my concerns have gone away.

I probably should have titled this thread something different.

At about 70 mph the tach reads about 2500 which is pretty close to what it should be according to calculator that I used.

I think it was mainly the road, wind and engine noises. Plus the fact that we hadn't had it up to highway speeds before..

When we get a little more time we'll push it a little more.

Thanks to all who replied.

Kinda goes with the warnings on your chainsaws and weed eater's not to tune by ear.

 

Great news! Thanks for the follow up John. Appreciate knowing what was going on.
That 2500 should be right in the ballpark for what you'd like too I think. Do you think you'll find yourself wanting to spend more time at 65mph, 70mph, or maybe even more?
I imagine a 300-6 would chug down to 2,000 pretty well and still pull you through the wind at 65, but even just a few miles per hour more and you're pushing a lot of wind and that 300 is going to be huffing and puffing and want to rev up. So maybe you're right in the sweet gearing area after all.
And if you went too tall you'd be unhappy with the off-the-line and around town performance.

Very interested to hear what you think about it after spending some more time behind the wheel.

Thanks

Paul