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#70920 01/01/2007 2:42 AM | Joined: Jul 2006 Posts: 163 Shop Shark | | Shop Shark Joined: Jul 2006 Posts: 163 | I searched but could not find an answer to this question. I have a 1951 1/2 ton truck and changed the ring and pinion gear with a 3:55 to 1 change. I have 15 inch tires on the truck with a 4 speed transmission. My question, how do I calculate the actual MPH with these tires and new gears. In other words, if my speedometer says 50, how fast am I actually going. :rolleyes: Any thoughts, or I guess I could follow someone and have them drive 50 to see what MPH my truck is traveling. Any help would be appreciated.  | | |
#70921 01/01/2007 2:59 AM | Joined: Aug 2001 Posts: 1,300 'Bolter | | 'Bolter Joined: Aug 2001 Posts: 1,300 | It would have had 4.1 gears before. I think if you look at oldgmctrucks.com there is a tire speed calculator that may help. | | |
#70922 01/01/2007 3:15 AM | Joined: Jun 2005 Posts: 145 Wrench Fetcher | | Wrench Fetcher Joined: Jun 2005 Posts: 145 | I have the 3.55 gears and my tires are 28" tall. I ran mile markers for 7 miles and only registered 6 on my odometer. This comes out to 1.16. If I am doing 50 mph by the speedo, I am actually doing 58 mph. 55 mph (indicated) is about 64 mph actual. This is very close using a stop watch.
I hope this helps.
Thruwurkin
Half Ton Will Travel
God bless America...One nation under God
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#70923 01/01/2007 4:45 AM | Joined: Oct 2005 Posts: 1,317 Shop Shark | | Shop Shark Joined: Oct 2005 Posts: 1,317 | Check out: http://www.csgnetwork.com/multirpmcalc.html I find the lower calculator more useful. If you scroll down and click on "automotive" there is a drop-down menu with a bunch of choices. Hope this helps. | | |
#70924 01/01/2007 6:13 AM | Joined: Jul 2006 Posts: 163 Shop Shark | | Shop Shark Joined: Jul 2006 Posts: 163 | Thanks to everyone who replied. I figure if I'm traveling 50, I'm actually going about 57 or 58. | | |
#70925 01/01/2007 4:31 PM | Joined: Jul 2006 Posts: 163 Shop Shark | | Shop Shark Joined: Jul 2006 Posts: 163 | I just thought of a more accurate way to find out the actual MPH. I will use my GPS, that gives a accurate MPH reading without having to travel very far. | | |
#70926 01/01/2007 9:50 PM | Joined: Oct 2002 Posts: 4,066 Bolter | | Bolter Joined: Oct 2002 Posts: 4,066 | or find a cop............worked for me 
Redryder pixMy HotrodA veteran - whether active duty, retired, national guard, or reserve - is someone who, at one point in his or her life, wrote a blank check made payable to The 'United States of America', for an amount of "up to and including my life."I am fighting cancer and I am winning the fight | Pain is part of life; misery is an option. | | |
#70927 01/03/2007 4:05 PM | Joined: Jan 2002 Posts: 2,845 'Bolter | | 'Bolter Joined: Jan 2002 Posts: 2,845 | To figure it out on a computer, you need the missing item, RPM. There are sites that help you figure this stuff but you need: gears in back, diameter of tires, speed & rpm.
Otherwise do it the most common way. Have someone follow you with cel phones turned on. If you don't trust their car, do it with another car.
When I'm doing this I have my wife or one of my kids follow me around my special loop. I end up knowing what speed or RPM I'm running in either the truck or Mustangs by a very small error %.
Craig My '50 Chevy 3100 5 window, '62-235cu, 3:55 rear My truck ....... Respect The Rust If I'm not working on my truck, '65 m00stang or VW camper, I'm fishing with the wife or smoking Salmon.
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