I know the topic of the speedometer being off 4mph or more is well covered, but mine seems like it's off at least 10 high. How are you guys cross ref on speed to get an accurate reading? I know these are fast cars, but I'm second guessing myself all the time when driving.

 

Use a GPS system or app to see your actual speed and then compare to your speedo. Mine reads 3 mph high and I'm on stock wheel and tire sizes. Did you upsize?

 

My speedometer reads high by ~2-3 MPH; I have timed my speed with a stopwatch many times using Interstate mile markers. OEM wheels, tires, and pressures.

 

Mine is off 4mph.....did a search for 4mph and found nothing about it????

 

Car has 19" that were on it when I bought it. Figured there would be some deviation with non stock. You suggested apps which one in particular?

 

Never checked but will now=curious.

 

My speedo shows faster than my Garmin and the deviation increases as my speed increases. Figured it was due to my 19" wheels. I was assuming it would be more accurate with stock wheels.

 

GT2 on factory size 315/30-18...2-3kph(1-2mph) optimistic last I checked.

 

Mine is 9 kph over at 120 kph (5.4 mph fast at 75 mph) on stock rims and tyres.

 

Downloaded a speedometer app off of iPhone store, here's my findings:

The app seems to work well enough. I tried it in my wife's car and it was spot on so tried it with the porsche and found that cars speedometer is 4-5 mph off.

these cars just make going fast feel slow because they do it so effortlessly.

 

Part not being stock tires!

 

So no recourse exists to calibrate the speedo? Pity, I would love to do that.

 

At least when you get pulled over for speeding, your ticket will be less. Download an app on your phone for GPS, and then measure the speed.

 

my speedometer reads 2 mph high. In my 964 its 0.

 

My speedometer reads 3-4+ mph when using GPS radar and I am on stock oem twist.