6 cylinder top speed

[70gmcjimmy]

My stockish '55 GMC 230 I-6 DD cruises fine at 65 mph, accelerates well, has no power brakes or steering, SM420, beam front axle. It does not wander or shake and the drum brakes work just fine. I just need to seal up the cab better for noise and drafts.

DAC

[Jesse Z]

Isn't the California truck speed limit still 55?

And in urban CA traffic, a good cooling system for long periods in jam-ups is more important than top speed. The V8 will hurt you there, as it requires more cooling.

[nail pounder]

Originally Posted by 70gmcjimmy

My stockish '55 GMC 230 I-6 DD cruises fine at 65 mph, accelerates well, has no power brakes or steering, SM420, beam front axle. It does not wander or shake and the drum brakes work just fine. I just need to seal up the cab better for noise and drafts.

DAC

I'll hook up a tach this week and see what I'm spinning. Maybe it just sounds like it's rapping out. It will definitely go faster than 55, I just felt like I was pushing it too hard. I didn't have it floored, just cruising. 50 seemed like it's sweet spot, took very little throttle to hold 50. Most of my driving is county roads or state highway where speed limits are 45-55 but I'd like the ability to drive the interstate if I wanted to. Interstates are 65 or 70 depending on which one. 18wheeler or vehicles with a trailer, speed limit is 55. Urban driving, well I try to avoid it, but if I have to I take my '14 RAM w/AC & satellite radio. We live in a more rural area.

[Foot Stomper]

Originally Posted by nail pounder

Manual trans or auto? What size rims? Original?

Stock 3 speed.
16" stock rims.

Dad put the diffy in... he ran 15 inch rims and car tires which ran the truck nicely at 65 mph and 2100 rmp.

I'm restoring it and have the 16's and the Cokers ready.

There's a number of gear ratio/tire size/rpm calculator on line.

The 235 has plenty of torque to get it rolling so not at all concerned about too tall of tire.

The 16's with a tire about 31 inches did come stock if ordered... another reason I'm going this way.

[wolffcub]

This is mine in 5th with a 323 rear and 28" tall tires. Will accelerate slowly in 5th but holds 65 fine at about 1800-1900 rpm. When in 4th I'm a 2500 rpm to do 65 mph and then I have lots of passing power in reserve. 2500rpm is still nothing for these engines. The fastest i have gone in it is about 87 due to the road being slightly uneven from all the heavy rigs driving on it. If i find a better road one day i will stretch it's legs again.

https://www.youtube.com/watch?v=1zSVDqrEyb0

[nail pounder]

Originally Posted by wolffcub

This is mine in 5th with a 323 rear and 28" tall tires. Will accelerate slowly in 5th but holds 65 fine at about 1800-1900 rpm. When in 4th I'm a 2500 rpm to do 65 mph and then I have lots of passing power in reserve. 2500rpm is still nothing for these engines. The fastest i have gone in it is about 87 due to the road being slightly uneven from all the heavy rigs driving on it. If i find a better road one day i will stretch it's legs again.

https://www.youtube.com/watch?v=1zSVDqrEyb0

That looks good there. I guess I better start doing some research.

[nail pounder]

Thinking of checking into 3speed overdrive trans also. A little more $ but cool. Anyone have feedback?

[yossarian19]

3 speed overdrive? Sounds weird, and weird = expensive, usually. I'd look for a 4 speed and shorten the first section of driveshaft, work up a kickdown cable & shifter, and enjoy.

[Janky59]

Thumbs up on the 3 speed w/OD. I have had one in mine for about a year, makes highway speeds comfortable (60-70 mph) with the 6 cyl. and stock rear (3.90). There is a good thread on them here: http://www.67-72chevytrucks.com/vboa...d.php?t=313886

[nail pounder]

Yossarian, 4spd auto would still be 1:1 in 4th I believe or maybe I'm wrong. The 3spd w/od is .73ish? It would be cool as far as originality goes and functional. T5 might be less expensive with similar results (and synchro) first, just not a 3 on the tree anymore. Thanks for the link janky, lots of good info there. I'll be on the lookout for the trans with synchro.

[yossarian19]

Nah, you're off by a gear. In a 4 speed auto, third gear is 1:1 and 4th is 0.7 : 1
I'm still wondering why 70gmcjimmy's truck drives OK at 65 and mine feels like a death trap. Makes me wonder if I have a bent wheel or tires aren't balanced right because the shakes really do line up with the magic speed of 60 mph (for whatever reason, 55-65 is a sweet spot for the frequencies that tire issues generate and it's where you're most likely to feel it)