Topping out at 55 mph?
 

Hello all,
First time poster here. I recently was given a 1964 GMC with the 305 V6 BB. It has 87,000 original miles and mostly used on a farm as a work truck. Initially I planned on fixing it up but switching out the motor and trans with a little newer 350 SBC. But I have grown to love the 305 so I am going to just keep it all stock (mostly) My question is, I am only able to top out at like 55 mph without really cruising in high RPMs. (tach doesn't work so not sure exactly). I would like to think these motors are capable of higher speeds so is it a transmission issue or a gear ration issue? I am not looking to race or anything, just be able to cruise at highway speeds when needed. Thanks and glad there is a forum for these cool old GMC's.

Re: Topping out at 55 mph?
 

I could bring my '66 305 1/2 Ton to about 70, but I agree, I never liked to and I wouldn't want to try to cruise at that speed. I believe I could cruise pretty comfortably at about 60.

The best way to achieve better cruising would be to find an old Overdrive (if you have a 3-Speed) or change the Rear to a Higher Gear, as far as staying original.

That said I'm not even sure Overdrive was offered in our year trucks, but the mid 50's Chevy cars had one that could be installed in your truck with some simple modifications, again if you have a 3-Speed. If you have a 4-Speed, it would be Rear Gear only and I believe they may be a little hard to find.

Re: Topping out at 55 mph?
 

The 305 is a low RPM motor. To get speed at the wheels, as Bill says, you will need some gearing changes: OD, rear end ratio, tires.........or "Stay Alive, Drive 55." None of the required changes are quick and easy and none are cheap. If you had leaf springs in rear, would be easy. Keeping the 305 has it's good and not so good sometimes. Built to haul in an era when 55 was fast.

Re: Topping out at 55 mph?
 

The best way to achieve better cruising would be to find an old Overdrive (if you have a 3-Speed) or change the Rear to a Higher Gear, as far as staying original.

That said I'm not even sure Overdrive was offered in our year trucks, but the mid 50's Chevy cars had one that could be installed in your truck with some simple modifications, again if you have a 3-Speed. If you have a 4-Speed, it would be Rear Gear only and I believe they may be a little hard to find.[/QUOTE]

Chevrolet did offer nan overdrive in their 60-66 trucks but GMC didn't till 1967. If you can find one of these setup and are running a three speed that may be the way to go. I myself would prefer converting to a modern 5 speed overdrive.

Re: Topping out at 55 mph?
 

Gerty,

Do you know what rear-end gears you have? I have 3.07s in mine, which was a factory option.

Using an RPM calculator, 3.07s:

29" tires (235/75x15s) will give you an RPM of 2490 at 70 MPH.
30" tires (295/65) it would drop the RPM to 2399 at 70 MPH
31" tire, (which some say will fit) would drop that to 2329 at 70 MPH.

Red line for the 305 is in the 3600 RPM range, so to me the answer is "Run stock 29" tires, and get a 3.07 rear end."

B

P.S. I assumed that you had a manual with a 1:1 top gear -- you didn't mention manual/auto.

Re: Topping out at 55 mph?
 

You ID your truck as a 1000 which is a half ton. If you indeed have a half ton, the available rear end ratios were 3.08, 3.54, and 3.92. If you top out at 55 MPG, you may have the 3.92 ratio. Available ratios for 3/4 ton trucks were 4.10 and 4.56.

Fitting the tallest tires you can will help. Changing your rear end ratio will help even more. Installing an overdrive would most likely be more work and more money.

Re: Topping out at 55 mph?
 

Welcome GMCgerty! Would love to see some photos of your truck. Also get that Tach fixed so you can see what rpm's you're really turning.

Re: Topping out at 55 mph?
 

Old trucks arent made to go real fast. Steering, crash survivability, brakes, all like that. Probably bettr upgrade those too while your in there.

Re: Topping out at 55 mph?
 

Continuing to try and educate myself, and so very thankful for this board!
My current set-up is 1965 305v6 with SM420, K1500 3/4 ton, Stromberg, Dana 60 rear (not sure if 4.1 or 4.56)
33" tires
I'm topping out at 50-52 mph and feel like I'm pushing pretty hard.
Can anyone provide info on max RPM?
Did the calculations and looks like with a 4.1, I'd be at about 2300 rpm for 53 mph....can that be right? 4.56 would get me over 2500 rpm for similar speed. Not looking to cruise up there, just trying to learn.
Maybe next mod should be a tach!
Thanks in advance.

Re: Topping out at 55 mph?
 

FWIW, your math is correct 2500rpm with 4.10 gears and 33" tires yields 55mph

Re: Topping out at 55 mph?
 

Thanks Rich, imagine that's close to max rpm on this engine?

Re: Topping out at 55 mph?
 

305s had horsepower ratings at either 3600 or 3800 RPM so there is still some room.

Re: Topping out at 55 mph?
 

I have my grandfather’s ‘63 GMC, long bed step-side, 3/4t, 4x4, winch (front), PTO, K1502E, w/ a 511 front/rear end. It tops out at about 60mph. I too have looked at all the options in an effort to attain higher hwy speeds and concluded that w/ all that would have to be done to achieve that, it’s simply not worth it, b/c #1 The expense and cascading effect on other components of the drivetrain, etc. and #2 there’s nothing like original equipment. I concluded I’d keep everything factory original, w/ exception of the split-rim wheels. Good luck. I look fwd to following your decision and course of action.

Re: Topping out at 55 mph?
 

Thanks for the details Robert! I am most definitely staying original. Two weeks in with this 305 and I'm very pleased with how it runs. May look into power steering options, but other than that just some cosmetic changes in the works.

Re: Topping out at 55 mph?
 

I have to agree with BigBlockV6, swapping in a newer 5-speed or 6-speed overdrive trans would provide best highway cruising MPG and at lower rpm and best lower gears for starting out heavy loads.

When I ordered out my 1987 pickup I had a choice of two manual 4-speeds, NP-435 and a Mazda over-drive trans. 2nd gear starts even when towing were the rule. MPG suffered if trying to cruise over 65 mph. I had 3.55 gears, and 31 inch tall tires, 4th gear was direct drive.

Finally when I ordered out my '96 I had a good 5-speed manual overdrive available. Same 3.55 gears, 31 to 32 inch tires, but the .77:1 overdrive lets me easily cruise at 75 mph. I 've even pulled brush in 4wd Hi in low gear, and was able to spin my tires at idle on blacktop. So I have the best of both ends of the operating range. Took me a while to find a truck with the manual 6-speed, but my dealer finally got one in. One Saturday morning Son and I and the salesman took it for a ride. Same close ratio My 5-speed is, just smaller splits between gears. But 1st and 5th gear ratios the exact same as my 5-speed. I just had to shift one more time to get to my cruising gear. With my 5-speed and 3.55 gears I'm good towing to 20,000#, if I occasionally towed heavier I'd either go 4.10 or 3.73 gears or the 6-speed. If I towed heavy a lot, I'd go 3.73's and the 6-speed.

I think you will find your running your 305 V-6 around 2400-2500 rpm, which is about where it's torque peak is at, and it's horsepower peak is 3600-3800, I've run everything from 3-on-the-tree to 13 speed Road-Rangers and 5+4 twin stick transmissions, and everything in between. More torque and a wider engine rpm range is always better, but more closer spaced gears makes up for a shortage of horsepower in lots of situations. Like my little commuter car, 218 hp, flat torque curve from 1700 to 5000 rpm, and manual 6-speed. Most times I skip a gear or two when not in a hurry.

I missed my calling decades ago, I wanted to be a fleet buyer for a major trucking company. Ended up buying parts for manufacturing for 30+ years. I wanted to match transmissions with engines to provide drivers the best performance with the best mpg and durability for the company.

Re: Topping out at 55 mph?
 

Please let me know what you discover regarding power steering. That’s intriguing. A guy down in Killeen told me an older corvette pwr steering module would work...odd I know.


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Re: Topping out at 55 mph?
 

I’m staying original and manual steering on mine....but I too would love to hear about power steering options for the 4x4 versions.