Need speed equation ASAP! court date
07-16-2005, 03:42 PM
#1
Need speed equation ASAP! court date i got shafted by the police,they told me i was going WELL over what i actually was going,and i was watching my speedo and tach so i know for sure.So i need the actual mathmatical equation to get speed in mph from tire height,gear ratio,what gear, and rpms.I see plenty of places on the web to calc it for you but i need the actual equation to bring to court to prove it.please help!!!
07-16-2005, 11:58 PM
#2
Originally Posted by ddnspider
i got shafted by the police,they told me i was going WELL over what i actually was going,and i was watching my speedo and tach so i know for sure.So i need the actual mathmatical equation to get speed in mph from tire height,gear ratio,what gear, and rpms.I see plenty of places on the web to calc it for you but i need the actual equation to bring to court to prove it.please help!!!
07-17-2005, 12:46 AM
#3
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I'm also a little confused over how this is going to help. There is no way to prove what gear you were in or what rpm your engine was at.
But I can tell you how to do it. Do you know what gear you were in? You take the engine rpm divide it by the gear ratio in the transmission gear (you can use search to find out the gear ratio). This will give you the speed of the driveshaft. Now divide by your rearend ratio. This gives you the tire rpm. Now, take the tire diameter in inches and multiply it by 3.14, this gives you the distance you go with each tire rotation. Now divide this number by 12 and this gives you the number of feet you travel with each rotation. Now take the RPM at the tire and multiply it by the number of feet you travel per revoltion. This will give you the number of feet you travel in a minute. Then take this number and divide by 5280 feet/mile and then multiply by 60 to get mph.
So let's go through an example. I will use 4th gear in a manual since I know it's 1 to 1 and a stock M6 has a 3.42 rear end ratio. Now if I was turning 3000 rpm and I had 25" diameter tires, this is what I'd do:
3000/1/3.42 = 877 tire rev/minute
25 x 3.14 = 78.5 inches/rev
78.5/12 = 6.542 feet/rev
6.542 x 877 rev/minute = 5737 feet/minute
5737/5280 x 60 = 65 mph
But I can tell you how to do it. Do you know what gear you were in? You take the engine rpm divide it by the gear ratio in the transmission gear (you can use search to find out the gear ratio). This will give you the speed of the driveshaft. Now divide by your rearend ratio. This gives you the tire rpm. Now, take the tire diameter in inches and multiply it by 3.14, this gives you the distance you go with each tire rotation. Now divide this number by 12 and this gives you the number of feet you travel with each rotation. Now take the RPM at the tire and multiply it by the number of feet you travel per revoltion. This will give you the number of feet you travel in a minute. Then take this number and divide by 5280 feet/mile and then multiply by 60 to get mph.
So let's go through an example. I will use 4th gear in a manual since I know it's 1 to 1 and a stock M6 has a 3.42 rear end ratio. Now if I was turning 3000 rpm and I had 25" diameter tires, this is what I'd do:
3000/1/3.42 = 877 tire rev/minute
25 x 3.14 = 78.5 inches/rev
78.5/12 = 6.542 feet/rev
6.542 x 877 rev/minute = 5737 feet/minute
5737/5280 x 60 = 65 mph
07-17-2005, 10:36 AM
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That's good, however, you may want to use a more accurate tire diameter. If they are 275/40-17's, then you:
(.4 x 2750) x 2 = 220 mm
220/25.4 = 8.66 inches
8.66 + 17 = 25.66
So you can see, you need to adjust my earlier calculations because I used a 25" tire diameter.
(.4 x 2750) x 2 = 220 mm
220/25.4 = 8.66 inches
8.66 + 17 = 25.66
So you can see, you need to adjust my earlier calculations because I used a 25" tire diameter.
