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Transmissions and Rear Ends Three on the tree or four in the floor? Shift it all here.

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  #1  
Old September 19th, 2017, 04:07 AM
64GMCGerty 64GMCGerty is offline
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Default 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.
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  #2  
Old September 19th, 2017, 03:04 PM
BillT BillT is offline
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Default 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.
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  #3  
Old September 19th, 2017, 07:44 PM
AZKen AZKen is online now
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Default 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.

Last edited by AZKen; September 19th, 2017 at 07:58 PM.
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Old September 20th, 2017, 02:12 AM
bigblockv6 bigblockv6 is online now
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Default 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.
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Old September 20th, 2017, 05:30 PM
bpmcgee bpmcgee is offline
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Default 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.
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Old September 20th, 2017, 07:22 PM
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Default 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.
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Old September 20th, 2017, 10:11 PM
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Default 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.
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Old September 21st, 2017, 07:00 PM
chickentaco chickentaco is offline
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Default 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.
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Old September 26th, 2017, 07:16 PM
1965GMCK1502 1965GMCK1502 is online now
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Default Re: Topping out at 55 mph?

Quote:
Originally Posted by Ed Snyder View Post
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.
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.
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  #10  
Old September 26th, 2017, 07:26 PM
RichG RichG is offline
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Default Re: Topping out at 55 mph?

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