Speedometer Error

[jrblackman]

After reading this thread earlier today I installed GPS Essentials on my phone and compared the GPS reading with both cars. The Boxster is consistently 3mph over the GPS reading; while my Impreza is in the range of 1-2 mph over the GPS. So I guess I can set the cruise slightly higher when driving at police officer friendly speeds.

[986_inquiry]

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.

yeah GPS says mph is ~10% off all the time

I just thought it was a "Porsche" thing from the google searches I've done

[opus69]

I get similar results comparing GPS with 2002S. Speedo is consitently +3 mph fast.

[CoBeerToad]

Did the same on the way to work this morning. My speedo is jacked. There is no place to open it up, but I had to do 39-40 on the speedo to get a consistent 35 reading on my GPS. And had to do a solid 50 mph to get 45 on my GPS. No wonder I haven't been pulled since I bought this car 10 months ago.

Gonna' try and run I-94 around 10:00am (hopefully I won't have to dodge much traffic) and see if I'm off by a constant or a percentage.

P.S. Any fixes for this other than altering my tire size? I know back in the day, with my Chevys I could get a different plastic speedo gear to adjust for this, but I'm just beginning to become familar with the Porsches.

[dmairspotter]

With 18" Porsche wheels and Michelin Pilot Sports my 2003 S speedo is +2 mph at 70-80mph compared to gps.

[NoGaBiker]

By the way, some have mentioned the error as a percentage. Mine seems to be a fixed 3mph, whether I'm travelling 36 or 86mph.

And that's on OEM 18s with Pilot Sports or PZeros.

[Joel-Box-ster]

The easiest way to correct this is to switch to higher aspect ratio (series) tires, But I rather stick to stock size tires because your speed reads slower on radar gun.

[Dave928]

+4 MPH at all speeds. matched it to a roadside radar sign and a scan tool with live data.

[thstone]

My speedo reads +2-3mph.