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1973 - 1979 F-100 & Larger F-Series Trucks Discuss the Dentsides Ford Truck

Highway RPM's

 
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Old 02-11-2019, 12:38 PM
Adam Woolridge
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Highway RPM's

I am getting ready to install a tach on my 79 with a 351m/C6 and was wondering what RPM's I should expect to see at 55-60mph. I'm not sure what rear I have as I cant find the tag. It's all original/stock setup that doesn't seem to mind cruising at 55 for 30 -45 min trips.
 
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Old 02-11-2019, 01:06 PM
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Without knowing the rear axle gears, ????
But maybe about 3000....??
 
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Old 02-11-2019, 01:12 PM
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I'm not positive, but I believe I have in the neighborhood of 3.50 gears and 255/70/15 tires. I am turning around 2800 rpms at 70 mph.
 
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Old 02-11-2019, 03:55 PM
dogdays
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Guess.

Originally Posted by Adam Woolridge View Post
I am getting ready to install a tach on my 79 with a 351m/C6 and was wondering what RPM's I should expect to see at 55-60mph. I'm not sure what rear I have as I cant find the tag. It's all original/stock setup that doesn't seem to mind cruising at 55 for 30 -45 min trips.
Ho ho ho. You have a sticker in the passenger side door opening that lists such things as transmission and rear end. The rear end is a code that tells you what it is. The C6 has a 1:1 Third gear.

I'm guessing your axle is 3.00 to one, that's what my '77 with a 400 has. 235-75 x 15s are 698 revs per mile, Should be about 2100rpm at 60. If your truck won't go 80 (2800 rpm) all day long there's something wrong with it.

R.

 
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Old 02-11-2019, 05:36 PM
Adam Woolridge
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Thanks guys, I just found my axle tag but itís almost completely hidden under muck. Still have to clean it off. I also just realized I forgot to add my main question. What is a ďsafeĒ rpm with this setup for extended trips? I have yet to venture past 45 min because I havenít figured out what my 60mph rpm is. Iíve been nervous about pushing it too hard since itís all original. Iíll go try and clean the tag to get my ratio. The passenger side sticker must have been removed at some point.

Originally Posted by dogdays View Post
Ho ho ho. You have a sticker in the passenger side door opening that lists such things as transmission and rear end. The rear end is a code that tells you what it is. The C6 has a 1:1 Third gear.

I'm guessing your axle is 3.00 to one, that's what my '77 with a 400 has. 235-75 x 15s are 698 revs per mile, Should be about 2100rpm at 60. If your truck won't go 80 (2800 rpm) all day long there's something wrong with it.

R.
 
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Old 02-11-2019, 06:28 PM
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Ok, scraped the tag and itís WDM CJ which is apparently a 2.75 rear and Iím running 275-75 R15 tires.
 
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Old 02-11-2019, 06:35 PM
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From this calculator, you should be able to do 70 mph at around 2100 RPM's, which won't put any strain at all on that 351.

https://spicerparts.com/calculators/...rpm-calculator
 
 


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