Now that I have this old beast back on the road, I feel it is geared entirely too low for my applications. 1970 F-250 camper special with strong rebuilt 390 and C-6 turns 2900RPM for 60-62 MPH cruise. Main use is to haul around 1700lb camper and occassionally pull a 20' boat that weighs right at 2000lbs. Have investigated adding an overdrive, but the new ones are cost prohibitive considering the value of the truck and how infrequently I use the rig. Truck has the Dana 60 with 4:10 gears. I think my best bet is to change ring and pinion to something a little taller, but don't know whether to go with 3.73, or 3.55. Of course, the 3.55 would yield the best level highway cruise performance, but I don't want to get too far toward the lower end of the torque curve since performance in pulling hills might suffer.

Would welcome any thoughts from those with experience in this area.

Anybody know of a good source for torque curve for the 390?

Also, any after market add-ons for stiffening up the steering on the old twin I-beams. Have new steering gear box and no slop in any of the tie rods or king pins, but don't care for the over-steer characteristics of the rig.

 

I have a 72 f-250 `Camper Special` 3.73 rear end..
I wish I had the 4.10 you have....
I don`t haul any real loads, show quality truck, But there have been times i wish i was in the Upper RPM range even for Up Hill passing.ie: LA. Grapevine trips.
If i did haul a camper and boat i would be looking for a 4.10 gear change in a heart beat.

Trailering speed limits in calif is 55 and would still not be over 3,000 with the 4.10 gear.
with engine temps at or near 190 lugging it down at 2700 or less will not do any good.


>Now that I have this old beast
>back on the road, I feel
>it is geared entirely too low
>for my applications. 1970 F-250
>camper special with strong rebuilt 390
>and C-6 turns 2900RPM for 60-62
>MPH cruise. Main use
>is to haul around 1700lb camper
>and occassionally pull a 20' boat
>that weighs right at 2000lbs.
>Have investigated adding an overdrive, but
>the new ones are cost prohibitive
>considering the value of the truck
>and how infrequently I use the
>rig. Truck has the Dana
>60 with 4:10 gears. I
>think my best bet is to
>change ring and pinion to something
>a little taller, but don't know
>whether to go with 3.73, or
>3.55. Of course, the
>3.55 would yield the best level
>highway cruise performance, but I don't
>want to get too far toward
>the lower end of the torque
>curve since performance in pulling
>hills might suffer.
>Would welcome any thoughts from those with
>experience in this area.
>Anybody know of a good source for
>torque curve for the 390?
>Also, any after market add-ons for stiffening
>up the steering on the old
>twin I-beams. Have new steering
>gear box and no slop in
>any of the tie rods or
>king pins, but don't care for
>the over-steer characteristics of the rig.
>
>

 

John H.

Thanks for the imput. Probably saved me time and money obtaining something I didn't really need or want.

Thanks again

 

I have replaced my entire front end to include:
Steering rod pins.
Tie rods.
Radial arm bushings w/ rubber. ( made a big diff)
'C' bushings with Polyurethane.
Track arm bushings w/ poly.
It still has slop, though it tightened up some.
The only thing left is the steering box - but I have a feeling this is just the way these old trucks are.

Good luck,
Shawn

 

Doug, I have a 66 F100, new 352 & C6. I bored the engine 30 over and shaved the heads 30, I changed the 3.25 rear to a 2.75 and there is plenty of power. Because of some power stearing problems I have not had it on a trip to check the mpg, but I like the 2.75 better than the 3.25

John

 

According to Edelbrock, a stock 390 has it torque peaking (375 ft-lbs) at 2500 rpm so now at 60 mph you are on the downside of power as well as fuel economy; at 2900 rpm you are down to 355 ft-lbs. The more rpms, the worse it gets. Your 390 will do its best between 2000 rpm (365 ft-lbs) and the peak rpm. In order to have power to pull that load over the hills, you should be running 2300 rpm at 60 mph. Backing in the info you supplied to the equation, this calcs to a 3.25:1 axle ratio rather than a 3.55:1. A 3.55 would get you 2500 rpm at 60 mph but when you hit the gas, you would get less torque just like you are now.

 

Doug, I went from 15" rims to 16" rims on my truck. Now it acts like my 4th gear is in 5th, if you know what I mean... Hope this helps. Jack

 

given a standard tire of 235.85.16, a 245.75.16 is smaller (more gear), and a 255.85.16 is less gear. cheaper to change tires than rearends, and easier to change back and forth.

 

Yes, you can change the final drive ratio with a tire change but not in this instance. Doug needs to drop 600 rpm at 60 mph and by the numbers he provided, he is currently running 30 inch diameter tires. Your solution would require him to run 38 inch tires. I'll bet a gear change will be cheaper than those tires. And how about a matching pair for the front? Then those expensive tires will wear out whereas the rear axle components will outlast many sets of pricey tires. A rough rule of thumb is that for every 3 inches increase in tire diameter, you can loose about 200 rpm at 60 mph. But always do the math first before making any change.

 

Doug, I have a '70 F100 with a bone stock and very tired (150,000 miles and 30 years) 390 with a two barrel carb. My F100 has a C6 and 3.50 gears and 31X10.50X15 rears. I have a 3000 pound pontoon/trailer combo and I don't have any trouble passing or climbing hills. Stopping is another story! [img]/dcforum/Images/happy.gif[/img] My cruise rpm is about 2600 rpm.