Speedometer Error

[blue2000s]

Originally Posted by schnellman

I know the topic has been talked about at great length here. And, I know there have been several solutions offered to find out your true speed versus what a Boxster speedometer will tell you. Don't forget about the HVAC diagnostics. Anyway, yesterday I saw a Sheriff's Deputy taking a snooze beside a local baseball diamond, so I decided to see if he believed in the "Protect and Serve" logo on the side of his cruiser. I asked him to clock me coming down a straight stretch of road. After a little grumbling he agreed. I held the Box (986) dead on 60 mph. When I circled back he said, "Looks accurate to me, I had you at 55 the whole time."
Just as I thought.

GPS is really accurate too. I'm about 5-8% off too. I'm also 5% off with both of my Subarus.

Every instrument of measurement has an amoint of inaccuracy, say +/- 5%. Automakers don't want their cars to ever read slow. Unintended speeding and all. So they set the meter to read low "nominally" so that one that reads really high will read no higher than true speed.

[Flavor 987S]

I trust my Garmin GPS's (I have 2) as being accurate. The Boxster (2006) shows me going 2 MPH faster than actual and the 1997 993 Turbo is dead-on. Funny how Porsche got it right +14 years ago!

This is all very common, BTW.

[DFW02S]

My speedo consistently shows 4mph faster than my GPS and pacing another car and matching speedos.

For Porsche, that little error in the 3 to 4% range saves them on warranty repairs, the miles go on just a wee bit faster.

I bet their accounting group knows exactly how much they save through this method.
Of course, it could be just tire variance, right?

[Highlow]

My car shows consistently 3mph over actual(radar) speed. This is for anything over 10 mph, I've never tried less.

[Flavor 987S]

Originally Posted by DFW02S

For Porsche, that little error in the 3 to 4% range saves them on warranty repairs, the miles go on just a wee bit faster.

I bet their accounting group knows exactly how much they save through this method.
Of course, it could be just tire variance, right?

Myth. Urban ledgend. The mileage on your DME is very accurate. And this is used for warranty claims. You should request a DME scan the next time your at the dealership or from someone who can read it.

[urban_legend]

Porsche speedometer an easy 5% fast. Audi dead nuts on.
Audi wins...oh wait... Audi check engine light flashing again ;(