Quote:
Originally Posted by Qmulus
I guess this jackass shouldn't bring up the fact that the speedo reads high by design either.
It used to be that speedos got their signal off the output of the transmission, and gears were used to calibrate the speedo to tire size. Now the cluster gets a signal from the ABS and uses a calibration factor (like pulses per mile) to calculate speed.
In the end, it really doesn't matter how the Boxster calculates its speed. If you assume it is accurate now, which it really isn't going back to the first part of this post, all you have to do is calculate the percentage difference in circumference of the new rear tires compared to the originals, just like you would with any speedo.
Now if you want to actually fix the speedo so it reads right, you have to adjust the calibration factor in the cluster to match the tires being used. On some vehicles that can easily be done with diagnostics. Not quite as easy on these.
But what do I know; I'm just a jackass.  |
Of course I know the speedometer is not showing the actual speed, even from the factory, especially with German cars because European law (ECE-R39) says speedometers cannot show speeds less than the actual speed, and they must never show more than the 110 per cent of actual speed plus 4 km/h. So, under those rules, a car could be moving at 100 km/h, but the speedometer could legally display as high as 114 km/h.
Yes I know cars used to determine the speed from the transmission, some possibly still do. I have owned many GM vehicles that I could change out the gear attached to the cable that went from the transmission to the speedometer. I put a HiPo 327 into a Chevy S-10 that had a Muncie 4 speed and way lower profile tires and I changed the gears on that. Heck I eventually put in a full digital dash into that truck.
Right now I'm on my winter tires I have had my car's speed read by actual radar and the actual speed is off about 7% on the high side with these tires. So I know when I'm doing 38 MPH on the speedometer it's actually just over 35 MPH, when the speedometer reeds 65 MPH the actual speed is just over 60 MPH. I know I can drive at 70 MPH on the speedometer and be right at 65 MPH. So instead of messing with the radar readings again and knowing how the Boxster 986 determines it's speed, I can just calculate it out now that I know how my speedometer is actually reading per the circumference of the tire.
I'm buying a set of summer wheels and tires so I don't have to go get the tires changed out at the change of season. If it's the rear, tire it won't be a big change, but enough I should factor it in. My winter rear tires are 812 revolutions per mile. The summer wheel/tire setup 817 revolutions per mile, both front and rear. Unlike the factory setup that had the front tires rotating 38 more times per mile then the rear, which makes the car twitchy in the front.
Anyways, about the jackass part, yes your were being a jackass. After you got called out on it and realized it was true, you decided to redeem yourself and come back with some actual useful information. I'm old enough to know how people work, you're not pulling the wool over my eyes. BTW: I didn't say you're a jackass 24/7, I said you replied like a jackass, which is true.
Now I'm glad you responded like you did and got called out on it as I'm finally getting some useful information in this thread.