RPM's at speed - Does this seem about right?
#1835443
05/27/15 06:16 PM
05/27/15 06:16 PM
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Joined: Jan 2003
Posts: 7,798 Lyons, CO
MileHighDart
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Lyons, CO
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So, for the first time in 10 years, I have a working speedometer. First time since I owned the car. Also just installed a Sun Supertach II.
When I installed the new speedo cable, I also installed a new 33 tooth speedometer pinion. Based on what people here told me, and after looking at some online charts.
So, now that everythings working, at 60mph, I'm turning 3350 rpm.
That's a 318/904, with 3:23 gears and a 25.6" (225/60/15) rear tire. At 70, I'm turning 3800 rpm.
These numbers sound about right? Seems to me the rpm is a little high for the speed, but I guess the gauges don't lie.
Last edited by MileHighDart; 05/27/15 07:06 PM.
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Re: RPM's at speed - Does this seem about right?
[Re: MileHighDart]
#1835445
05/27/15 06:18 PM
05/27/15 06:18 PM
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Joined: Apr 2010
Posts: 10,570 Sunny South Florida
Golden-Arm
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I Live Here
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Sunny South Florida
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with a known tire diameter/circumference, and a known gear ratio, and being in any selected gear, one could mathematically determine rpm's, per given speed.
"When Tyranny Becomes Law, Rebellion Becomes Duty"
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Re: RPM's at speed - Does this seem about right?
[Re: Golden-Arm]
#1835446
05/27/15 06:21 PM
05/27/15 06:21 PM
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Joined: Jan 2003
Posts: 7,798 Lyons, CO
MileHighDart
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sorry, my first post went live before I was done typing. More info there now.
Last edited by MileHighDart; 05/27/15 06:21 PM.
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Re: RPM's at speed - Does this seem about right?
[Re: MileHighDart]
#1835450
05/27/15 06:25 PM
05/27/15 06:25 PM
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Joined: Apr 2010
Posts: 10,570 Sunny South Florida
Golden-Arm
I Live Here
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I Live Here
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Sunny South Florida
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my 518 with 3:91 gives much lower numbers. have to calculate my final drive at tailshaft though. here's a very quick, "down & dirty" calculator, to help you out a bit. CALCULATOR
"When Tyranny Becomes Law, Rebellion Becomes Duty"
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Re: RPM's at speed - Does this seem about right?
[Re: MileHighDart]
#1835510
05/27/15 08:29 PM
05/27/15 08:29 PM
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Joined: Jan 2003
Posts: 25,748 Rio Linda, CA
John_Kunkel
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Too Many Posts
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Posts: 25,748
Rio Linda, CA
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A better speedo pinion accuracy tester is the odometer. It's possible for the odometer to be dead nuts and the speedo to be way off.
The INTERNET, the MISinformation superhighway
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Re: RPM's at speed - Does this seem about right?
[Re: DrCharles]
#1835514
05/27/15 08:30 PM
05/27/15 08:30 PM
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Joined: Jan 2003
Posts: 7,798 Lyons, CO
MileHighDart
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Boy am I dumb ! I got to thinking, this tach was on the car when I got it. And the car was a /6 back then. Turns out there's a little hole on the back of the tach, with a small switch in it, that you're suppose to use to switch it from 4cyl, 6cyl, or 8cyl. It was set on 6-cyl Didn't know it was there. When the mounting bracket is on the tach, it covers up this hole with the switch in it. Took it for another drive and sure enough, at 60mph on the phone app, the tach is sitting pretty at 2600 rpm.
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Re: RPM's at speed - Does this seem about right?
[Re: MileHighDart]
#1835527
05/27/15 08:43 PM
05/27/15 08:43 PM
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Joined: Sep 2007
Posts: 14,889 up yours
Supercuda
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About to go away
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up yours
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Can't beat a cheap fix and I be this never happens to you again, btdt.
They say there are no such thing as a stupid question. They say there is always the exception that proves the rule. Don't be the exception.
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Re: RPM's at speed - Does this seem about right?
[Re: DeMopuar]
#1837988
05/31/15 02:10 PM
05/31/15 02:10 PM
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Joined: Jan 2003
Posts: 5,789 Hamilton, Ontario Canada
Magnum
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Just bring a GPS deal in your car and see if it matches your speedo. ! Yes, we are working on old cars but we don't need to use old tools (charts). Take advantage of modern technology, it's cheap and easy to use. Similar to what Kunkel said. Odometer is better, simply because "speedometer is off by 5" is not an accurate observation. (unless the needle is clocked wrong). If yo were off by 5, it would be off by 10 at double the speed and it's would be correct at 0. Speedometers are off by a certain percent. Use a GPS to confirm then adjust up or down by the speedometer gear tooth count.
69 Super Bee, 93 Mustang LX, 04 Allure Super
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