3 speed RPM question - The 1947 - Present Chevrolet & GMC Truck Message Board Network

03BlkZ · 12-21-2021, 08:47 AM

Hello, I finished my resto on my 70 C10 about 10 months ago and now I have a question about RPM. My truck didn't have a factory tach and I didn't add one during the resto but now I wish I would have. My truck is a 350 with 3 on the tree and 3.73 gears. I am running a 255/70/15 tire. When going 60 to 70 mph it sounds like I am running a pretty high RPM. Anyone here have a tach with the same setup as me that can tell me what RPMs you are running?
Thanks

geezer#99 · 12-21-2021, 09:47 AM

At 70 it’ll be spinning about 3000 rpm.
Use this calculator.

http://www.advanced-ev.com/Calculators/TireSize/

**kwmech** · 12-21-2021, 12:16 PM

What do you have for a fan?

**Accelo** · 12-21-2021, 01:27 PM

Good point on the fan. A none OEM fan can add a lot of noise.
As we all have become accustomed to modern cars the typical motor RPM of vehicles the old cars and trucks, without overdrive transmissions, seem noisy because they are. If you had installed a modern automatic with overdrive installed the motor rpm would be closer to 2100 rpm. The early motor speeds seem ridiculous now days. You could change the differential or look for a stock three speed with overdrive for your truck. Frankly I am a little surprised there weren't more of these ordered back when. I did add an automatic overdrive transmission to a 1971 3/4T 396 truck I had. Increased the highway millage from 10 to 13mpg. a 33% improvement with no other changes. It quieted down the interior of the truck immensely.

BTW you would have to have a 2:61 rear end ratio to get the motor rpm down to 2100rpm with the transmission you have. It would be impossible to get moving on a hill. Yes, the overdrive is one of the best upgrades you can do in our old vehicles to make drive better and quieter.

03BlkZ · 12-21-2021, 06:09 PM

Quote:

Originally Posted by kwmech

What do you have for a fan?

I have the stock clutchless fan.

**kwmech** · 12-21-2021, 06:33 PM

Hard blade or flex?

03BlkZ · 12-21-2021, 09:25 PM

Quote:

Originally Posted by kwmech

Hard blade or flex?

I assume hard.

**special-K** · 12-21-2021, 09:34 PM

Why no clutch? They are meant to be and worth having, too. You don't even need a fan most of the time. Also, you know the revs have nothing to do with having a 3spd, right. It's all the same, 1:1 final drive unless you have overdrive

**70STOVEBOLT** · 12-21-2021, 10:28 PM

I have a 70 with a 402 and TH400 with 3.73 gears, also running 255/70-15s and my tach reads about 2500-2600 at 60 mph. The engine doesn’t matter much, the RPM should be the same as your 350, given the same final drive ratio and tire size.

**jocko** · 12-21-2021, 10:39 PM

Geezer and 70Stovebolt are spot on.
Exact numbers by the math for your transmission final drive (1:1), rear end (3.73), and tire dia (29.1" for 255/70/15) combo:

50 mph=2153 RPM
60 mph=2584 RPM
70 mph=3015 RPM

03BlkZ · 12-21-2021, 11:38 PM

Thank you all. It's just hard to get used too the higher RPM. I am used to rolling 80 at about 1800 rpm in my Z06.

geezer#99 · 12-22-2021, 12:41 AM

You want to try 4200 rpm for 240 miles in a 56 gmc with an L-88 427.
That’ll get you buzzed for awhile!

AJC · 12-22-2021, 11:58 PM

Here is an approximate engine RPM depending on speed.

I may have got the 3 speed gear ratios wrong; I can update it if I did.

12-21-2021, 08:47 AM	#1
03BlkZ Registered User Join Date: Nov 2015 Location: Hewitt, Tx Posts: 386	3 speed RPM question Hello, I finished my resto on my 70 C10 about 10 months ago and now I have a question about RPM. My truck didn't have a factory tach and I didn't add one during the resto but now I wish I would have. My truck is a 350 with 3 on the tree and 3.73 gears. I am running a 255/70/15 tire. When going 60 to 70 mph it sounds like I am running a pretty high RPM. Anyone here have a tach with the same setup as me that can tell me what RPMs you are running? Thanks

12-21-2021, 09:47 AM	#2
geezer#99 Registered User Join Date: Aug 2009 Location: Bowser Posts: 13,736	Re: 3 speed RPM question At 70 it’ll be spinning about 3000 rpm. Use this calculator. http://www.advanced-ev.com/Calculators/TireSize/

12-21-2021, 12:16 PM	#3
kwmech Senior Member Join Date: Mar 2003 Location: Colfax-California Posts: 8,650	Re: 3 speed RPM question What do you have for a fan?

12-21-2021, 01:27 PM	#4
Accelo Senior Member Join Date: Mar 2015 Location: washington Posts: 2,301	Re: 3 speed RPM question Good point on the fan. A none OEM fan can add a lot of noise. As we all have become accustomed to modern cars the typical motor RPM of vehicles the old cars and trucks, without overdrive transmissions, seem noisy because they are. If you had installed a modern automatic with overdrive installed the motor rpm would be closer to 2100 rpm. The early motor speeds seem ridiculous now days. You could change the differential or look for a stock three speed with overdrive for your truck. Frankly I am a little surprised there weren't more of these ordered back when. I did add an automatic overdrive transmission to a 1971 3/4T 396 truck I had. Increased the highway millage from 10 to 13mpg. a 33% improvement with no other changes. It quieted down the interior of the truck immensely. BTW you would have to have a 2:61 rear end ratio to get the motor rpm down to 2100rpm with the transmission you have. It would be impossible to get moving on a hill. Yes, the overdrive is one of the best upgrades you can do in our old vehicles to make drive better and quieter.

12-21-2021, 06:33 PM	#6
kwmech Senior Member Join Date: Mar 2003 Location: Colfax-California Posts: 8,650	Re: 3 speed RPM question Hard blade or flex?

12-21-2021, 09:34 PM	#8
special-K Special Order Join Date: Nov 2004 Location: Mt Airy, MD Posts: 85,851	Re: 3 speed RPM question Why no clutch? They are meant to be and worth having, too. You don't even need a fan most of the time. Also, you know the revs have nothing to do with having a 3spd, right. It's all the same, 1:1 final drive unless you have overdrive __________________ "BUILDING A BETTER WAY TO SERVE THE USA"......67/72......"The New Breed" GMC '67 C1500 Wideside Super Custom SWB: 327/M22/3.42 posi.........."The '67" (project) GMC '72 K2500 Wideside Sierra Custom Camper: 350/TH350/4.10 Power-Lok..."The '72" (rolling) Tim "Don't call me a redneck. I'm a rough cut country gentleman" R.I.P. ~ East Side Low Life ~ El Jay ~ 72BLUZ ~ Fasteddie69 ~ Ron586 ~ 67ChevyRedneck ~ Grumpy Old Man ~

12-21-2021, 10:28 PM	#9
70STOVEBOLT Senior Member Join Date: Jun 2005 Location: Winona Lake, IN Posts: 6,424	Re: 3 speed RPM question I have a 70 with a 402 and TH400 with 3.73 gears, also running 255/70-15s and my tach reads about 2500-2600 at 60 mph. The engine doesn’t matter much, the RPM should be the same as your 350, given the same final drive ratio and tire size. __________________ 70 C/10 SWB 402/TH400/3.73 "The Needy Beast" 200,000 Mile Club Disc Brake Club Owner installed options: Front Sway Bar Power Steering Power Brakes Cigar Lighter Courtesy Lights Deluxe Side Markers Wiper Delay Sliding Rear Window Power Windows Power Locks Sniper EFI 2015 Silverado 1500 LS 4.3/6L80/3.23 lowered 2" front & rear Last edited by 70STOVEBOLT; 12-22-2021 at 09:17 AM.

12-21-2021, 10:39 PM	#10
jocko Senior Member Join Date: Oct 2009 Location: Godley, TX Posts: 17,975	Re: 3 speed RPM question Geezer and 70Stovebolt are spot on. Exact numbers by the math for your transmission final drive (1:1), rear end (3.73), and tire dia (29.1" for 255/70/15) combo: 50 mph=2153 RPM 60 mph=2584 RPM 70 mph=3015 RPM

12-21-2021, 11:38 PM	#11
03BlkZ Registered User Join Date: Nov 2015 Location: Hewitt, Tx Posts: 386	Re: 3 speed RPM question Thank you all. It's just hard to get used too the higher RPM. I am used to rolling 80 at about 1800 rpm in my Z06.

12-22-2021, 12:41 AM	#12
geezer#99 Registered User Join Date: Aug 2009 Location: Bowser Posts: 13,736	Re: 3 speed RPM question You want to try 4200 rpm for 240 miles in a 56 gmc with an L-88 427. That’ll get you buzzed for awhile!

12-22-2021, 11:58 PM	#13
AJC Registered User Join Date: Jan 2017 Location: Le Sueur MN Posts: 209	Re: 3 speed RPM question Here is an approximate engine RPM depending on speed. I may have got the 3 speed gear ratios wrong; I can update it if I did. __________________ AJC Last edited by AJC; 12-23-2021 at 04:05 AM. Reason: Fixing photo link



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