Drivetrain / Rear-end question

[cpnbnanamn]

So, I'm trying to understand why my truck doesn't have a lot of speed. I've got a '67 C10 SWB with the 250 cu in. inline 6, w/ 3ott. The rear-end looks like it has the stamping of HA, which by all research, should be a 3.73 ratio, right?
When I head down the road, I get it up to 3rd gear, and I can't get to any more than about 55-60 MPH without revving too high.
I'm pretty mechanically savvy, and have been doing a lot of work on the truck already, and understand the basic theory of trans and rear-end gearing, but I'm by far no expert. Am I geared too high, or too low? I'd like to get closer to highway speeds out of it. I could use all the education I can get.
Eventually, I'm wanting to remove the 6 and trans, and put in a 350 sbc, and a 5-spd manual, but I'm not ready to do that, yet.

[sangell]

The combination of 3.73 rear gear ratio, lack of overdrive gear in the trans, and low revving inline 6 is why your truck is happy at 55-60 mph. Once you put that 5spd in it with its overdrive you will like the 3.73 gear, it should be about perfect. If you change the 3.73 out for a higher ratio (lower number) to say a 3.08 you will use more throttle input and more clutch slippage to get moving from a stop but will do better on the highway. this would drop your RPMs roughly 20% however once you put the 5 spd in it the 3.08 will be too high.

[cpnbnanamn]

That's really helpful.. Guess it's time to start thinking about making that conversion to the 5-spd. I've got a 4:11 12-bolt main sitting in my garage that the guy gave to me when I bought the truck, and I was planning on having it rebuilt and use with the 350, and 5-spd.. Maybe I don't need to?

[LongBox]

That 55-60 sounds about right for a 6 cyl. I had the same problem with my 65 C10 back in the day, and I just learned to live life in the slow lane. At the same time, I had a buddy who had the same truck, but with a 4 speed (granny low style), and a higher back axle ratio. It could cruise a little higher, but it could not handle hills without constant shifting from 3rd to 4th, back to 3rd, then back to 4th....

Your 350 and 5 speed is the answer.

[Eddie H.]

If you have a 3.73 axle ratio and 30 inch tall tires, you should be turning about 2500 rpm at 60 mph and 2900 at 70 mph. It may sound like it is revving too high, but you're probably not gonna hurt the little 250 by just driving it that way until you can afford to go to a V8 with overdrive trans.

[SS Tim]

Run the numbers yourself, plug in three numbers and get the fourth for free.
http://www.summitracing.com/expertad...tio-calculator

Original published 67 250 hp/tq chart

[firedemon]

everyone is so used to modern cars they forget these were designed to run this way the power band on these engines is in a much higher rpm than newer cars . mine is the same as yours I run 2900 rpm at 65 just fine everyday 250 3 speed and 373 .

[TwoFiftyShifter]

I have 373s and a 3 on the column. My truck seems very happy at 70mph/3000r. It's a 350 V8. The inline sixes are happier about 500rpm lower than the V8s in my.experience.

[SS Tim]

Remember Bruce Crower writing that for day to day driving, the point at which the hp and tq curves crossed is RPM where you wanted to be. On the chart above that is about 2600RPM.