The Ford Barn

Jim in S.C. · 06-06-2011, 07:07 PM

OK mathematicians, Can anyone tell me how many rpms a engine would be turning if you have a manual three speed transmission (no over drive)and you have a 4.44 differential and tires that are twenty eight inches in diameter and you are traveling forty miles per hour. Once again whats my rpms?
Wow, I have a headache trying to figure this out, HELP!

Max · 06-06-2011, 07:13 PM

2200 RPM would be 41.3 MPH

19Fordy · 06-06-2011, 07:23 PM

Use this.

http://www.westerndiff.com/rpm.html

oldford2 · 06-06-2011, 08:00 PM

I added a small tach below my dash and use a gps to check my speedometer accuracy and get true speed.

Flat Ernie · 06-06-2011, 08:25 PM

2141 RPM @ 40 mph

4dFord/SC · 06-06-2011, 08:54 PM

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There are three kinds of people in the world: those who are good with numbers and those who are not.

john hamilton · 06-06-2011, 08:57 PM

It's just steps, not hard if you pay attention:

(tire dia. in inches X 3.1416)/ 12" per foot = 28 X 3.1416/12 = 7.33' traveled per revolution

5280' per mile/feet traveled per revolution = 5280/7.33 = 720 revolutions per mile

revs of the tire per mile X gear ratio = 720 x 7.33 = 3197 engine revs per mile so at 60mph you would be wrapped out.

correct for 40mph which is less than a mile per minute by multiplying engine revs per mile X actual mph/60 = 3197 x 40/60 = 2132 engine rpms at 40 mph.

With your tires and gears you can apply this correction to any speed by putting your "actual mph" in the equation above.

john hamilton · 06-06-2011, 09:01 PM

By the way, my equations above agree with Flat Ernie and Max who gave you the answer. Note 3196 x 41.3/60 = 2200 rpm

Jim in S.C. · 06-07-2011, 09:03 PM

Thanks for all the answers. I put the calculation web sites in my favorites
Jim in S.C.

Jerry Parr WI · 06-08-2011, 08:05 AM

A minor correction if you want is to use the rolling circumference of the tire. Because the weight of the car flattens the tire at the road the car travels slightly less distance than the tire diameter would indicate. Lower air pressure or more weight equals less distance per revolution. Mark the tire and the road and measure one revolution.

06-06-2011, 07:07 PM	#1
Jim in S.C. Member Join Date: May 2010 Posts: 72	rpm math question OK mathematicians, Can anyone tell me how many rpms a engine would be turning if you have a manual three speed transmission (no over drive)and you have a 4.44 differential and tires that are twenty eight inches in diameter and you are traveling forty miles per hour. Once again whats my rpms? Wow, I have a headache trying to figure this out, HELP!

06-06-2011, 07:13 PM	#2
Max Junior Member Join Date: May 2010 Location: Perth, Western Australia Posts: 22	Re: rpm math question 2200 RPM would be 41.3 MPH

06-06-2011, 07:23 PM	#3
19Fordy Senior Member Join Date: May 2010 Location: Coral Springs FL Posts: 11,916	Re: rpm math question Use this. http://www.westerndiff.com/rpm.html

06-06-2011, 08:00 PM	#4
oldford2 Senior Member Join Date: May 2010 Location: pittsfield, MA Posts: 2,087	Re: rpm math question I added a small tach below my dash and use a gps to check my speedometer accuracy and get true speed.

06-06-2011, 08:25 PM	#5
Flat Ernie Senior Member Join Date: May 2010 Location: 74FL Posts: 323	Re: rpm math question 2141 RPM @ 40 mph __________________ Daddy always said, "If yer gonna be dumb, you gotta be tough" and I'm one tough sumbitch! T5 Tech

06-06-2011, 08:54 PM	#6
4dFord/SC Senior Member Join Date: May 2010 Location: Columbia, SC Posts: 1,579	Re: rpm math question Sponsored Links (Register now to hide all advertisements) There are three kinds of people in the world: those who are good with numbers and those who are not.

06-06-2011, 08:57 PM	#7
john hamilton Senior Member Join Date: May 2010 Location: Waxahachie, Texas Posts: 241	Re: rpm math question It's just steps, not hard if you pay attention: (tire dia. in inches X 3.1416)/ 12" per foot = 28 X 3.1416/12 = 7.33' traveled per revolution 5280' per mile/feet traveled per revolution = 5280/7.33 = 720 revolutions per mile revs of the tire per mile X gear ratio = 720 x 7.33 = 3197 engine revs per mile so at 60mph you would be wrapped out. correct for 40mph which is less than a mile per minute by multiplying engine revs per mile X actual mph/60 = 3197 x 40/60 = 2132 engine rpms at 40 mph. With your tires and gears you can apply this correction to any speed by putting your "actual mph" in the equation above.

06-06-2011, 09:01 PM	#8
john hamilton Senior Member Join Date: May 2010 Location: Waxahachie, Texas Posts: 241	Re: rpm math question By the way, my equations above agree with Flat Ernie and Max who gave you the answer. Note 3196 x 41.3/60 = 2200 rpm

06-07-2011, 09:03 PM	#9
Jim in S.C. Member Join Date: May 2010 Posts: 72	Re: rpm math question Thanks for all the answers. I put the calculation web sites in my favorites Jim in S.C.

06-08-2011, 08:05 AM	#10
Jerry Parr WI Senior Member Join Date: May 2010 Posts: 619	Re: rpm math question A minor correction if you want is to use the rolling circumference of the tire. Because the weight of the car flattens the tire at the road the car travels slightly less distance than the tire diameter would indicate. Lower air pressure or more weight equals less distance per revolution. Mark the tire and the road and measure one revolution.