Mathematics test
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Joined: Nov 2014
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From: wisconsin
Mathematics test
on the track (road america, etc.) in my M7 2015 Z06/Z07 i am run rear conti slicks with an overall diameter of 27.9". the oem rear cup 2's have an overall diameter of 26.6".
the front slicks have the same overall diameter as the cup 2's of 25.7".
i believe my speedometer reads low on the track. is that true? what is my actual speed at speedometer indicated 50, 100 or 150mph?
how does my vette calculate indicated speed? wheel rotation? trans gear and engine rpm? some other way?
...or do the slicks only change my final gear ratio.
thanks for any help!
the front slicks have the same overall diameter as the cup 2's of 25.7".
i believe my speedometer reads low on the track. is that true? what is my actual speed at speedometer indicated 50, 100 or 150mph?
how does my vette calculate indicated speed? wheel rotation? trans gear and engine rpm? some other way?
...or do the slicks only change my final gear ratio.
thanks for any help!
Tech Contributor
Joined: Oct 1999
Posts: 41,058
Likes: 9,820
From: Charlotte, NC (formerly Endicott, NY)
Sort of the same thing. Multiply the indicated speed by 1.048872. I got the number by dividing 27.9 by 26.6 which is the ratio of the two tire diameters.
At an indicated speed of 100 mph you would be actually be running at 105. At an indicated speed of 150 you would be right around 158.
I don't know how much tire growth adds to speed but there could be differences in how each tire grows in diameter as it spins. The growth won't be much but it will be there.
The speedo uses electrical pulses from the Vehicle Speed Sensor in the diff to determine speed. The signal is first sent to the ECM and then to the speedo. I am not sure where the actual calculation takes place in the ECM or speedo module but basically it counts pulses to determine shaft rpm and then knowing the gear ratio and tire diameter can calculate speed. If tire growth is a factor they can account for that in the calculation.
Bill
At an indicated speed of 100 mph you would be actually be running at 105. At an indicated speed of 150 you would be right around 158.
I don't know how much tire growth adds to speed but there could be differences in how each tire grows in diameter as it spins. The growth won't be much but it will be there.
The speedo uses electrical pulses from the Vehicle Speed Sensor in the diff to determine speed. The signal is first sent to the ECM and then to the speedo. I am not sure where the actual calculation takes place in the ECM or speedo module but basically it counts pulses to determine shaft rpm and then knowing the gear ratio and tire diameter can calculate speed. If tire growth is a factor they can account for that in the calculation.
Bill
Last edited by Bill Dearborn; Aug 5, 2016 at 06:56 PM.
Le Mans Master
Joined: Jun 2008
Posts: 6,716
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St. Jude Donor '15
on the track (road america, etc.) in my M7 2015 Z06/Z07 i am run rear conti slicks with an overall diameter of 27.9". the oem rear cup 2's have an overall diameter of 26.6".
the front slicks have the same overall diameter as the cup 2's of 25.7".
i believe my speedometer reads low on the track. is that true? what is my actual speed at speedometer indicated 50, 100 or 150mph?
how does my vette calculate indicated speed? wheel rotation? trans gear and engine rpm? some other way?
...or do the slicks only change my final gear ratio.
thanks for any help!
the front slicks have the same overall diameter as the cup 2's of 25.7".
i believe my speedometer reads low on the track. is that true? what is my actual speed at speedometer indicated 50, 100 or 150mph?
how does my vette calculate indicated speed? wheel rotation? trans gear and engine rpm? some other way?
...or do the slicks only change my final gear ratio.
thanks for any help!
