75MPH equals 72.1gps?
01-28-2008, 10:29 PM
#16
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To clarify my earlier answer:
Porsche may very well setup the speedometer to purposely read fast, but speedometers will rarely, if ever, read accurately anyway. When setting up a speedometer you have to know what the tire diameter is if you want to know the vehicle's speed. Rotation sensors and knowledge of what gearing is used can tell you how fast the wheels are rotating, but it won't give you the speed of the car alone. Vehicle speed=(RPM)(Tire Diameter)(pi)/(Gear Ratio) (this of course needs converted based on the units of the inputs you use.)
As an example lets say that Porsche setup the speedometer on a Carrera to be 100% accurate with brand new 295/30/19 tires that have a true rolling diameter of 26 inches. This would mean at 3000 engine rpm in 6th gear the car would be traveling at 76.7 mph and the speedometer would read 76.7. Now lets see what would happen after half of the tread is worn away. The tire radius on a PS2 would now be smaller by 5/32. At 3000 rpms in 6th gear the vehicle would be traveling 75.7 mph, but the speedometer would still read 76.7 The rotation sensor, wherever it is located, is still going to read the same, but the car will not travel as far for each revolution of the wheel because the circumference of the tire is now smaller.
Based on the above example you can see that even small changes to a tire's size can make significant changes to a speedometer's accuracy.
Porsche may very well setup the speedometer to purposely read fast, but speedometers will rarely, if ever, read accurately anyway. When setting up a speedometer you have to know what the tire diameter is if you want to know the vehicle's speed. Rotation sensors and knowledge of what gearing is used can tell you how fast the wheels are rotating, but it won't give you the speed of the car alone. Vehicle speed=(RPM)(Tire Diameter)(pi)/(Gear Ratio) (this of course needs converted based on the units of the inputs you use.)
As an example lets say that Porsche setup the speedometer on a Carrera to be 100% accurate with brand new 295/30/19 tires that have a true rolling diameter of 26 inches. This would mean at 3000 engine rpm in 6th gear the car would be traveling at 76.7 mph and the speedometer would read 76.7. Now lets see what would happen after half of the tread is worn away. The tire radius on a PS2 would now be smaller by 5/32. At 3000 rpms in 6th gear the vehicle would be traveling 75.7 mph, but the speedometer would still read 76.7 The rotation sensor, wherever it is located, is still going to read the same, but the car will not travel as far for each revolution of the wheel because the circumference of the tire is now smaller.
Based on the above example you can see that even small changes to a tire's size can make significant changes to a speedometer's accuracy.
01-29-2008, 03:58 PM
#17
I believe it has been mentioned before on this forum that Porsche 997 speedos read high. I have found the same on mine. That is one reason I am considering putting 305/30x19 on my new 11.5" Champion rims, as the slight increase in diameter/circumference from the stock 295/30x19 will help reduce the error. I agree that the Garmin is likely the best way to check. It is also handy to track top speed. I busted my son when he took a trip in my C55 to Vegas. When he returned I checked the max speed on the Garmin and busted him for doing 105 on the trip. Next time he'll wipe it clean before returning I'm sure. Fun while it lasted.
01-29-2008, 07:31 PM
#20
Drifting
It must be dependent on model of Garmin or resolution (number of satellites in view) at the time, because I've found my Garmin Nuvi 350 speeds to very closely match the speedo on my C55, which seems to be pretty accurate.
01-29-2008, 10:51 PM
#21
Race Car
01-30-2008, 11:59 AM
#23
speedo/odometer HIGH!
My first 996, a lease, was discovered to run high. Worse, so did the odometer. Translate that into allowable miles per year/term of the lease, and I got screwed out of 3 months of allowable miles over a 4 year term. When I approached Porsche about it, they said a 5% discrepancy was within limits for their cars. So, I guess you better negotiate the possible 5% discrepancy into the total cost of any future leases!
I just buy Porsches now!
I just buy Porsches now!
