I measured my speed today because it seems I am not going quite as fast as the car reads...At a steady reading of exactly 70 mph I am actually only going 63.1 mph.

I checked this at several speeds and consistently got a reading 7-9% wrong. I have stock tires and wheels with average wear.

My method? Drive exactly one mile and measure exact time for that mile. Divide that number into 3600 (seconds per hour). The resulting # is MPH.
So, it took me 57 seconds to drive one mile when my speedo said 70. 3600/57=63.15mph
Certainly unacceptable...any ideas/solutions?

 

How did you measure the mile?

 

Using a GPS device my car is off about 4-5 mph at 130.

 

Is the odometer off by that much as well? I know Honda/Acura had to settle a class action suit by extending warranty and lease overages by 5% because the speedo / odo combo read 5% fast.

 

When you get a speeding ticket, you cant blame Porsche speedo for it, that's why. .

 

I think this needs to be regulated, speedometers being off only leads to us having driven 30k miles when the odo says 35k, then you get antsy and sell and buy newer ones and manufacturers get over. There should at least be some type of calibration tool on the car IMO. I hate watching miles pack up on any of my cars and this only makes it worse.

 

According to the Pbox, my car is off 1.5mph up to 80 or so...never really noticed any faster than that...

 

Mine is the same as below with GPS, ODBII DashDyno, or Durametric.

 

Mine was off about 2% (high) with stock tires. I almost solved the problem by putting on taller tires.

 

What size tires are you running at the Mile, Mike?

 

AB Car These numbers are unusually high for a 996. Have you changed your tire sizes by any chance?

Alternatively, the air pressure that you have in them as well as speed have an effect on the speedo accuracy. At high speeds expect a tire expansion of about 3%.

Finally, a GPS datalogger measurement will give you better accuracy than your test given the human error on the timing front.

 

315/30-18 rear
235/45-18 front
Iam running Toyo R888s. They allow me to hook in second (now that Iam RWD) and are decent driving around town.

 

They are not. Our speedos over-read at about 5-7mph (tyre wear dependant) uniformly on our cars. I've checked with GPS with 9 satellites in orbit (less satellites in orbit and GPS isn’t all that accurate either) and that’s what I got on numerous 996TT's tested. On my 944TS speedo was dead on +/-1km/h on 911 not so.

 

measurement error...time, mile, air pressure, etc...everywhere.

 

Reviving this thread. I am experiencing a similar situation as ABCar. If its been an ongoing issue, I am just now noticing it, maybe a change lately has brought it to light. If I find that my tire pressures, are within spec, my tires have not gone through any additional wear, what else can I check? If none of that helps, is there anything that can be done or do I just have to live with it? Thanks!

Justin