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Old 09-21-2016, 06:58 PM   #1
cw253748
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Default rpm vs. mph

Hi guys. I have a 31 Tudor with I'm guessing a 3.78 rear end. I have a tachometer on the car and I rev at 2500 rpm's at 45 mph. Is this correct? I doubled checked the speedometer with a gps and it is dead on. There are 19 inch tires on and fresh rebuild engine. No over drive.
Thanks for your input.
Chris
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Old 09-21-2016, 07:25 PM   #2
ryanheacox
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Default Re: rpm vs. mph

Seems high. 45mph is just shy of 2000rpm with everything stock and a 3.78 rear end. That would even be high with a 4.11 rear. I put the numbers in to a calculator and 2500rpm would be 58mph with 3.78's and 54 with 4.11's. Maybe the tach needs to be calibrated?
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Old 09-21-2016, 07:35 PM   #3
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Default Re: rpm vs. mph

I was thinking it was high also. I figure around 2,200 @ 50mph. 3.78, 5.50R19.
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Old 09-21-2016, 08:26 PM   #4
Tom Wesenberg
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Default Re: rpm vs. mph

Measure from the road to the center of the hubcap. This is your rolling radius, which will be less than the radius on the rest of the tire. Double this measurement for the rolling diameter, then multiply by pi for the distance the car travels with one tire revolution. Now divide that number into 5280 to get how many revs the tire needs to make to go one mile. now multiply that number by the rear end ratio 3.78 and you will know how many revs the engine needs to make to go one mile. So, if you are going 60 MPH, that's the RPM the engine would be turning. At 45 MPH the engine would be turning 3/4 of that number.

I'll measure my 28 and do some calculating.

I just measured my 28 and have 14.5" from the floor to the center of the axle, so 29" diameter X 3.1417 = 91.11" per tire roll. Divide that into inches per mile (63,360) = 695.423 tire turns per mile X 3.70 rear end ratio = 2573 engine revs per mile. So at 60 MPH which is a mile a minute, the engine would be turning 2573 RPM, and at 45 MPH it would be 2573 X 3/4 = 1930 RPM.

With a 3.27 rear end the engine would be turning 2270 RPM at 60 MPH, and at 45 MPH the engine would be turning 1702.5 RPM.

Last edited by Tom Wesenberg; 09-21-2016 at 09:10 PM.
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Old 09-21-2016, 09:23 PM   #5
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Default Re: rpm vs. mph

Thanks Tom. I will measure my tire tomorrow and check my results.
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Old 09-21-2016, 09:28 PM   #6
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Default Re: rpm vs. mph

Wow that is some hard math!
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Old 09-21-2016, 09:47 PM   #7
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Default Re: rpm vs. mph

gee wiz Tom now we have to even do math too? You are a tough guy
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Old 09-21-2016, 09:55 PM   #8
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Default Re: rpm vs. mph

What is the reason you have a tachometer? Wayne
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Old 09-21-2016, 10:11 PM   #9
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Default Re: rpm vs. mph

calculating mph/rpm's is math. Stopping distance is physics. Mixing paint is chemistry. Shucks guys I thot model A's were 'posed to be fun. ken
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Old 09-21-2016, 10:18 PM   #10
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Default Re: rpm vs. mph

I like how Tom has thought through the math on this. But for us lazy guys, here is an on line calculator to do the grunt work for us...

http://bgsoflex.com/rpmmph.html


With it, you can compute the RPM to attain a given MPH or the MPH resulting from a given RPM.
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Old 09-22-2016, 10:23 AM   #11
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Default Re: rpm vs. mph

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Originally Posted by TinCup View Post
Wow that is some hard math!
Dog here,
Iffin' you measure the inches around the tire, convert 5,280 feet, into inches, DIVIDE the LITTLE number into the BIG number & it'll tell you how may turns it takes to run a MILE BUT, the centrifiugal force causes the tire to "grow", SO, the calculations are "out the window"--BACK to the "drawing" board OH CRAP, the battery in my CALCULATOR jist "DIED"
Buster T.
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Old 09-22-2016, 10:37 AM   #12
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Default Re: rpm vs. mph

Some tachs have a wire or two you have to clip to set them up for 4, 6 or 8 cylinders. Someone may have not configured yours for a 4 cylinder.
Good Luck
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Old 09-22-2016, 11:37 AM   #13
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Default Re: rpm vs. mph

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Originally Posted by Tom Wesenberg View Post
Measure from the road to the center of the hubcap. This is your rolling radius, which will be less than the radius on the rest of the tire. Double this measurement for the rolling diameter, then multiply by pi for the distance the car travels with one tire revolution. Now divide that number into 5280 to get how many revs the tire needs to make to go one mile. now multiply that number by the rear end ratio 3.78 and you will know how many revs the engine needs to make to go one mile. So, if you are going 60 MPH, that's the RPM the engine would be turning. At 45 MPH the engine would be turning 3/4 of that number.

I'll measure my 28 and do some calculating.

I just measured my 28 and have 14.5" from the floor to the center of the axle, so 29" diameter X 3.1417 = 91.11" per tire roll. Divide that into inches per mile (63,360) = 695.423 tire turns per mile X 3.70 rear end ratio = 2573 engine revs per mile. So at 60 MPH which is a mile a minute, the engine would be turning 2573 RPM, and at 45 MPH it would be 2573 X 3/4 = 1930 RPM.

With a 3.27 rear end the engine would be turning 2270 RPM at 60 MPH, and at 45 MPH the engine would be turning 1702.5 RPM.
Geezz Tom...you made my head hurt......but you'r figures are correct....lol

I carried this too every track I raced at(engine man) to figure for gear changes since 1973,...it works and it's accurate. Ya!...I still use it to this day!
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Old 09-22-2016, 05:26 PM   #14
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Default Re: rpm vs. mph

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Originally Posted by d.j. moordigian View Post
Geezz Tom...you made my head hurt......but you'r figures are correct....lol

I carried this too every track I raced at(engine man) to figure for gear changes since 1973,...it works and it's accurate. Ya!...I still use it to this day!

VERY cool !
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Old 09-22-2016, 06:01 PM   #15
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Default Re: rpm vs. mph

Quote:
Originally Posted by cw253748 View Post
Hi guys. I have a 31 Tudor with I'm guessing a 3.78 rear end. I have a tachometer on the car and I rev at 2500 rpm's at 45 mph. Is this correct? I doubled checked the speedometer with a gps and it is dead on. There are 19 inch tires on and fresh rebuild engine. No over drive.
Thanks for your input.
Chris
No that is not correct. RPM is correct for "revolutions per minute". RPM's is....well i don't know what that would be. Just sayin'. One of my goofy little pet peeves.
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Old 09-22-2016, 06:23 PM   #16
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No that is not correct. RPM is correct for "revolutions per minute". RPM's is....well i don't know what that would be. Just sayin'. One of my goofy little pet peeves.
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Old 09-22-2016, 06:39 PM   #17
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Default Re: rpm vs. mph

If you have 19" rims and Firestone 450/500 tires you will get 14,4" from center of hub to floor. Any other tires on 19 in wheels will be so close it will be negligible. Buy the way , Tom W. is correct in everything he said about the post. It is not complicated at all. But if you prefer, There is a very easy constant that will get you within 1 mph. That is to take your speed in mph and multiply it by 44 and you will have it. I was not able to calculate for 21" because I had not 21" wheels. Now I know since Tom has measured for us. Use his method and you will not go wrong. It is perfect.
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Old 09-22-2016, 08:17 PM   #18
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Default Re: rpm vs. mph

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Originally Posted by BILL WILLIAMSON View Post
Dog here,
Iffin' you measure the inches around the tire, convert 5,280 feet, into inches, DIVIDE the LITTLE number into the BIG number & it'll tell you how may turns it takes to run a MILE BUT, the centrifiugal force causes the tire to "grow", SO, the calculations are "out the window"--BACK to the "drawing" board OH CRAP, the battery in my CALCULATOR jist "DIED"
Buster T.
That's not all Bill. The tire patch that is on the ground compresses a bit, and the part NOT on the ground stretches. Very complicated formula. You need the get the specs from the tire manufacturer, then plug it all into a formula as long as your arm....and don't forget about slippage of the driven tire... OUCH! Yeah, makes my head hurt!
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Old 09-22-2016, 09:10 PM   #19
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Default Re: rpm vs. mph

I just did Tom's math. I have 14.25 to center of hub cap with 19 inch tires. I get 2675 at 60 mph and 2006 at 45 mph with a 3.78 rear end
If I substitute a 4.11 rear end in the equation I get 2908 at 60 mph and 2181 at 45 mph. So either I have a 4.11 rear end or the tach is off. It is a Westach from Snyders.
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Old 09-22-2016, 09:24 PM   #20
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