I’ve been using my 83 base model in-line 6 a lot lately. I’m driving on the freeway with a total mileage of 460 once a week. My typical cruising speed is 70 mph, hitting 80 to 85 in spurts; usually down hill. The old rig doesn’t seem to mind it but I’m concerned I may be pushing it too hard. Is there a safe speed I should be adhering to, or can I abuse this old dog till the cows come home. I love this old truck and would like to keep it for as long as I can without any engine concerns. Original kms (being a Canadian model) is only 90,000 (55,900 miles), so it’s still a young pup with a patina-esk exterior.

 

Need a little more information on the truck.
What transmission dose it have?
What rear gear ratio dose it have?
What size tires dose it have?

All I can tell you is how my truck is set up and what my truck dose at speed.\
81 F100 flare side 300 / NP435, 2.75 gear, 235 / 75R / 15.
Speed limit here is also 70 MPH but you get run over at that speed.
I have put 400+ miles on in a week to / from work. I did not drive it over 70 MPH, most of the time it was @ 65 before I added the over drive.
MPH @ RPM
55 1800
60 2100
65 2250
70 2500
75 2700
80 2800
75 & 80 MPH done for testing for RPM

Now I have installed Advance Adapter Range Splitter (over drive) As I knew I was building the truck to drive.
MPH RPM
55 1400
60 1600
65 1700
70 1800
75 2000
80 2200
It felt better with the over drive @ 75 MPH but the MPG drops as you are pushing a lot of air out of the way.
Same with out over drive, above 65 MPH you are pushing a lot of air so I would try and stay that speed.

The other thing to know is in the USA the speed limit was 55 MPH through out the land to save gas.
Now that everything has over drive the speed limit max here is 70 and if you look at my reading in over drive at 70 MPH I get 1800 RPM same as no over drive at 55 MPH.

I think 70 MPH is ok at 2500 RPM but if you can go 65 MPH at 2250 RPM would be better for MPG less air pushing and wear on motor.
Dave ----
ps I have been faster that the 80 to show them new trucks this old truck could hang

 

Interesting info. I don’t have a tachometer so not sure what rpm’s are going on at speed. It’s a three on the tree. Don’t know the tranny or diff specs. Tire size is 215/75. The engine feels and sounds healthy at 65/70 mph. It’s only when I push her to 80/85 that the engine sounds a wee bit stressed. But that doesn’t happen often. BTW, speed limit here is 100/110 kms which is 60/70 mph. My gas mileage sucks. Single barrel Carter. But then, I do have a carb base to manifold vacuum leak that I have not yet addressed.

 

Ok 3 on the tree in 3rd is 1:1 just like my NP435 in 4th.

If you can take a picture of the cert sticker on the left door opening we can decode it for the rear axle ratio.
If no sticker, block wheels and take it out of gear. Get 1 rear wheel off the ground mark the wheel and the drive shaft.
Turn the wheel 1 full turn and count how many turns the drive shaft turns.
If the drive shaft turna a little less than 3 it could be a 2.75. If 3 turns then you have a 3.00
But the sticker is the best unless the axle has been swapped out.
Dave ----

 

My sticker has faded a long time ago and contains no information.

 

Try a photo and then zoom in. You might be able to see something left of your sticker details.

 

A 215-75x15 is about 28" in diameter.

If you have a 3.50 axle, a very common ratio, you would be at 3570 RPM at 85 MPH.

If you have a 3.25 axle, you would be at 3315 RPM at 85 MPH

But, there are a bunch of available ratios higher and lower.

What kind of axle do you have? Does it have a cover on the back, or is the back smooth with a bolt in front carrier?

 

Overdrive transmissions are a wonderful thing. Less engine rpms means less wear. Less fuel being used. And you can actually hear the radio instead of all the engine racket.

 

He dose not know the ratio
We dont know if a 150 or 250, dont think I would drive a 350 460 miles a week unless I needed the truck for the job.
But as a guess its a 150 base model (low GVWR) and being a 83 that dose not leave a lot of different ratios only 2.75 & 3.00
So he could use my non-OD RPM and be really close.

Believe it I got the same MPG with or with out over drive driving over the same road to / from work that was a shock as a figured it would have been a little better. But it is nice the lower RPM is less wear on the motor.
And because I can split the gears I can match the RPM & MPH better. There is 1 road the speed is 25 MPH thru town.
3rd gear to high and lugs the motor and 2nd to low high RPM so running 2nd over is just right.
The splitting of gears really shines when pulling the clubs duel axle enclosed trailer.
It is the best thing I have done to the truck. To bad Advance no longer sells the over drive unit. Gear Vendor is the only player for add on over drive now.
Thing is it will not fit the 3 on the tree transmission
Dave ----

 

With a little work a later model 5 speed will slide right into place. Clutch work and some driveshaft work.

 

Can we assume you have replaced all the fluids, even the brake fluid system flush ?

For a safe speed...there is a speed limit on most highways...
The tires will have a date code on the sidewall near the rim, they say, anything older than 6 or 7 years, they should be replaced.

 

Kill joy with all this safety stuff you'r no fun.
Maybe that is the only danger he faces?
Some of us are married and face danger ever day
Dave ----

No really please be careful

 

All fluids replaced including brakes (new master) Tires are good. Speed limits are boring.