Quote:
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Originally Posted by fatmike
I love my Boxster -- but I never know how fast I'm going...
This makes me wonder: Does anyone regulary use the tachometer to know your speed? I drive a 6 speed (its an '02 S) and use the stock 18" wheels. I know that there are some variables, but I think this method should be as accurate as any
...Any hints or tricks?
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Yes, it's fairly easy to do:
First, you need to calculate the circumference of the
rear tires
(important if you've changed tires sizes).
Assuming OEM Continental ContiSportContact 2
Tire data on the sidewall gives you:
Rim dia = 18 inches
Width = 265 mm
Tire aspect ratio = 35
Calculate the tire height -
tire aspect ratio x tire width/100
= 35 x 265 /100 = 92.75 mm
Calculate tire OD -
Rim dia x 25.4 + 2 x tire height
= 18 x 25.4 + 2 x 92.75
= 642.7 mm
Calculate tire circumference -
π x tire OD /1000
= 3.14159 x 642.7 mm /1000
= 2.019 m
Now you need to know the Gear ratios and Final Drive ratio. For the Boxster 6-speed, these are:
1st = 3.82
2nd = 2.20
3rd = 1.52
4th = 1.22
5th = 1.02
6th = 0.89
Final Drive = 3.44
Then, calculate wheel revolutions for a given RPM. For example, at 3700 RPM, in 5th gear:
wheel revs -
RPM / (Final Drive ratio x Gear ratio)
= 3700 / (3.44 x 1.02)
= 1054.49
Then calculate the speed (at the given RPM of 3700) -
tire circumference x wheel revs x 60/1000
= 2.019 x 1054.49 x 60/1000
= 127.74
KPH
Convert to MPH, from KPH -
KPH / 1.609344= MPH
= 127.74 / 1.609344
= 79.37 MPH.
Using these formulae, you can calculate the car's speed for any combination of Gear, RPM, Tire.