Rmp's and speed
Thread Starter
|
Cargo Master
Joined: Mar 2006
Posts: 2,789
Likes: 5
From: Sand GAp, KY
Rmp's and speed
I know I have read this before but can't find it. What is the formula for finding your actual speed by rpms? Right now I have 33x12.5x16.5"s, C6, and 3.55 gears my tach reads 2400 at 60mph and 2600 at 65. Since this truck is a frankenstien of parts I not sure of trans and tcase gearing (tcase is np208 out of an 84, trans is out of an 87-88). After we buy this house I will have a lot more interstate driving and would like to know how far off the speedo is. Those damn radar things are junk! We have 4-5 of them in northern kentucky on I75 north and south just south of the Ohio river and they all read different speeds when you go past them( I know it is my speed because of real late night runs when the highway is pretty empty).
Post Fiend
Joined: Nov 2003
Posts: 21,285
Likes: 15
From: Nutter Fort, WV
A C 6 does not lock the torque converter in high gear, so actual hard numbers are hard to do.
If the torque converter was locked, 2400 should be 66 MPH.
2600 should be 71.9 MPH.
Go out on the interstate and drive exactly 60 MPH.
It should take you exactly 60 seconds to drive from mile post to mile post.
If you are under 60 seconds, slow down a little till you drive one mile in 60 seconds, then note the speedo reading.
If the speedo shows say 54 MPH, then the speedo is off 10%.
So 30 MPH indicated would actually be 33 MPH.
70 MPH indicated would actually be 77 MPH.
If the torque converter locked, 2180 RPM should be 60 MPH.
No GPS required, just a watch that shows seconds and a road with mile posts every mile.
If the torque converter was locked, 2400 should be 66 MPH.
2600 should be 71.9 MPH.
Go out on the interstate and drive exactly 60 MPH.
It should take you exactly 60 seconds to drive from mile post to mile post.
If you are under 60 seconds, slow down a little till you drive one mile in 60 seconds, then note the speedo reading.
If the speedo shows say 54 MPH, then the speedo is off 10%.
So 30 MPH indicated would actually be 33 MPH.
70 MPH indicated would actually be 77 MPH.
If the torque converter locked, 2180 RPM should be 60 MPH.
No GPS required, just a watch that shows seconds and a road with mile posts every mile.
Thread Starter
|
Cargo Master
Joined: Mar 2006
Posts: 2,789
Likes: 5
From: Sand GAp, KY
Thanks Dave, if it is raining in the morning I will check it on the way to work (otherwise I will be on the bike lol). I do have a buddy with a gps, I will see if he will let me barrow it as well to verify my findings.
Methanoholic
Joined: Mar 2009
Posts: 6,442
Likes: 8
From: Maine (NorCal Native)
MPH = (RPM * TIREDIA) / (336.13552 * TransRatio * AuxRatio * RearRatio)
Transmission at 1:1
Aux Ratio at 1:1
66.372 = (2400 * 33) / (336.13552 * 1 * 1 * 3.55)
71.903 = (2600 * 33) / (336.13552 * 1 * 1 * 3.55)
-Enjoy
fh : )_~
Transmission at 1:1
Aux Ratio at 1:1
66.372 = (2400 * 33) / (336.13552 * 1 * 1 * 3.55)
71.903 = (2600 * 33) / (336.13552 * 1 * 1 * 3.55)
-Enjoy
fh : )_~
Senior User
Joined: Sep 2007
Posts: 317
Likes: 0
From: Snohomish, WA
Drive shaft RPM x final drive ratio = wheel RPM
Tire rotations per mile / wheel RPM = miles per minute
Miles per minute x 60 = Miles per hour
SO the combined formula looks like this:
MPH = 60 x {Tire rotations per mile / [(Engine RPM x transmission gear ratio) x final drive ratio] }
