Speedometer Error
 

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.

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.

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?

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

From what I've read (with VW at least), the odometers are correct while the speed usually reads a bit fast. Probably the same with Porsche, I believe there would be issues if the odometers were inaccurate.

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.

GPS says...
 

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

That's what I read too. (see below) I'm curious though. What method would they use? What method could they use to keep the odometer separate from the speedometer?

Speedometer Miscalibrated

Diagnostic: Speedometer reading shows higher than your actual speed.

Cause: Engineering problem in design of Boxster.

Details: Porsche claims that the tolerance range for the speedometer is 10% over your actual speed to 1% slower than your actual speed. There are several ways to have this checked:
•Radar gun
•Dyno
•Mile or kilometer markers on a freeway
•GPS (Global Positioning System)

To use the mile or kilometer markers on the freeway, they must be accurately spaced. You will need a stopwatch. Measure your time driving past several markers at a constant speed (use the cruise control if you have it), then use this formula: (3600 � number of markers) � number of seconds = actual speed.

One owner ran his June 1997 Stuttgart built Boxster on a dyno. It showed the speedometer to be within one mph of accuracy at 60 mph, so not all cars have this deficiency. This is important to know because you cannot presume to have five mph slack without checking your speedometer.

Time in Shop: None

Repair Instructions: None

Notes: Even if the speedometer were calibrated correctly at the factory, there are a number of factors which can contribute to the speedometer reporting inaccurate speed:
•Tire tread
•Tire pressure
•Tire size

The odometer uses a separate technique to measure distance, so it does not have this problem.

At highway speeds, I'm running 2.5-3 mph slower than indicated. Bugs the hell out of me that with all the technology available, they can't get a correct speed reading.

Mine, 2001 S, reads +3 mph over the actual at all speeds and confirmed by GPS and Radar.

It helps me avoid speeding tickets so I can overshoot a bit....oooops did 95 on the highway on-ramp getting in front of a truck this morning....thrilling!

But there is other issue too
Wheel size
Not many. Of us running stocks, right?
If You run stock tho, speedo is calibrated to the height of brand new tire
When miles go on tire is getting smaller so it's got to spin faster to be at the same speed
I don't know how much does it affect the speedo, but in some .0... It should

Happy speeding!!!

I was just looking this up because I passed a radar sign that showed I was doing 36 mph and I had the car holding at 40 mph. Tires are new-ish and original sizes. I'm going to double check on the way home today with a GPS app. The 4 mph difference just seems to be way to much for that low of a speed.

But about the odometer... What other method could they use to measure miles besides using the speedometer which I imagine is driven from the transmission?

According to Magellan GPS my speedometer reads 2MPH over the actual speed which is slower in real world. Changing into bigger diameter tires can also affect your speed reading. :(

Every vehicle that I have owned and checked out with GPS has shown speedometers that were optimistic - BMW X3, VW Passat and my 986 Boxster. If a manufacturer has to allow for a margin of error, it stirkes me as obvious why they would prefer them to overstate rather than understate the speed - just think of the potential liability issues if the owners of a particular car were all liable to receive (and in many cases were receiving) speeding tickets because their speedometers read 55 when you were actually doing 60!

Brad

My Corolla is rock-solid on the speedo. I have an app for my Android called "GPS Essentials", lets you display everything GPS - altitude, average speed, max speed, min, instant, bla-bla-bla. It consistenly shows exactly what my dashboard says. On my 986, I am off 4 mph at 70, I've never checked if it is off by a percentage or an absolute number. Either way, it seems pretty piss-poor for an expensive car to be so wrong. I lock the cruise at 75 on a trip to keep the car at 71, which I figure is about where law enforcement starts to perk up its ears.

I don't believe GPS' are accurate. If they are, then that means most cars are exactly -2mph off.

My GPS shows 2mph faster than every single car I have used it in. Always 2mph faster whether its in a Honda, Mazda, Nissan, my Porsche, or a Chevy Tahoe.

I get the same result with my fathers GPS and my GF's GPS. Always 2mph faster reading no matter what the car. I tried it with Garmin and Tom Tom and my dad's Android phone.

Actually, especially when averaged over any sort of distance on cruise control, GPS is incredibly accurate. I sail and, while my GPS/chartplotters have WAAS (wide area augmentation system) which improves their accuracy to about 6 feet, nevertheless even without WAAS they are extremely accurate. Consider even a two minute run on cruise at 60 mph: your GPS should be accurate within 24 feet at the worst (12 feet on either side) over 2 statute miles - a margin of error of less than .25% (and likely much less). If your speedometers are always 2 MPH optimisitic (and I have seen variance greater than that on my BMW X3), it is likely because it is programmed into the car for reasons that have already been discussed.

Brad

This. I sell Farm GPS systems, and even the cheap non-differential GPS are more accurate the the radar systems we used to sell. Typically, within .1 mph

My new Jeep GC has a big Garmin screen and it displays speed. That and the speedometer are dead nuts the same. It's like the speedo is taking its reading from the GPS.

Of course, like everyone, my 986S is +3 at most speeds.