I have a 2002 Boxster S where the speedometer is WAY off....

- When the speedo says I'm going 25mph, I'm really going about 21mph.
- When the speedo says I'm going 50mph, I'm really going about 44 mph
- When the speedo says I'm going 75mph, I'm really going about 68mph

Verified with 3 different GPSs (iPhone GPS, Blackberry GPS, and a Garmin GPS).

I took it to the dealer today to get the oil changed and asked them to look at it. They verified the tires and wheels were correct (they are..) and said "Sorry, we see this problem quite a bit, but there's absolutely nothing we can do about it."

WTF?!

 

my 08 boxster is 3 mph slower...

 

My 07' Cayman was about 3-4mph off. I suspect my new '11 CS is off about the same. Common problem. Look at the bright side, it might save you a ticket down the road. Don't sweat the small stuff. Go out to your favorite twisty road, put the top down, drive the crap out of the car, and you'll completely forget about these little things

 

Speedos are almost never correct to GPS. Even if they are correct at new tread depth, they won't be by the time the tires are worn. 2-3 mph is pretty standard. Both my cars (F-150 and XC-90) have OE sized tires and the speedos run fast.

 

Small update on this....

The dealer says that the official Porsche policy on this is that the speedometer can be off by as much as 10%.

If you don't care about the speedometer being wrong... How does it feel to know the odometer might be 10% off on that next Porsche you buy? =)

 

sorry to hear.

my '07 CS is on the money.

yes, I've seen/got other cars that read 3-4 mph high... (to favor the manufacturers crash stats me thinks... ie keep people's speed down a little)... but it still pi$$es me off.

not sure if anything can be done about it. Don't know how the speedo is programmed??

but, fundamentally yes, I agree about the mileage thing you mention... but if I'm doing the math right, then the error is in your favor THIS time...

 

If that really brothers you, you can run slightly bigger tires to offset it.

 

That's not a bad idea.... When it's time for new tires, I'll put 19s on it... That'll probably get it a lot closer to reality.

 

http://en.wikipedia.org/wiki/Speedometer

car manufacturers do not establish speedometer/odometer accuracy requirements, government agencies do. wiki is a good place to start but you realy want to search through the CFR's and dept of transport requirements (if you're bored, or think this is a major issue)

 

I read that Pcars (or is it German cars in general) speedo always run 3 mph fast... it's a liability thing.

 

^ in many countries there are big penalties if the speedo reads too low, so most manufacturers purposely set them up to read higher. your best bet is just to get tires that are 1 aspect ratio larger in diameter next time around. Get factory wheels/tires off a 987 boxster and you should be set.

 

gents

there is nothing wrong with the OP's speedometer. it is operating within the specifications dictated by the EU regulations the car was built/certified too, which I've cut and pasted here (from the wiki link above)

International agreements
In many countries the legislated error in speedometer readings is ultimately governed by the United Nations Economic Commission for Europe (UNECE) Regulation 39[3] which covers those aspects of vehicle type approval which relate to speedometers. The main purpose of the UNECE regulations is to facilitate trade in motor vehicles by agreeing uniform type approval standards rather than requiring a vehicle model to undergo different approval processes in each country in which it is to be sold.

European Union member states must also grant type approval to vehicles meeting similar EU standards. The ones covering speedometers [4] [5][6] are similar to the UNECE regulation in that they specify that:

• The indicated speed must never be less than the actual speed, i.e. it should not be possible to inadvertently speed because of an incorrect speedometer reading.
• The indicated speed must not be more than 110 percent of the true speed plus 4 km/h at specified test speeds. For example, at 80 km/h, the indicated speed must be no more than 92 km/h.

The standards specify both the limits on accuracy and many of the details of how it should be measured during the approvals process, for example that the test measurements should be made (for most vehicles) at 40, 80 and 120 km/h, and at a particular ambient temperature. There are slight differences between the different standards, for example in the minimum accuracy of the equipment measuring the true speed of the vehicle.

The UNECE regulation relaxes the requirements for vehicles mass produced following type approval. At Conformity of Production Audits the upper limit on indicated speed is increased to 110 percent plus 6 km/h for cars, buses, trucks and similar vehicles, and 110 percent plus 8 km/h for two or three wheeled vehicles which have a maximum speed above 50 km/h (or a cylinder capacity, if powered by a heat engine, of more than 50 cc). European Union Directive 2000/7/EC, which relates to two and three wheeled vehicles, provides similar slightly relaxed limits in production.

21 x 1.1 + 3.7 = 26.8 the OP says his speedo indicates 25 so it complies
44 x 1.1 + 3.7 = 52.1 " 50 dido
68 x 1.1 + 3.7 = 78.5 " " " 75

 

Only bummer is, after 50,000 mi. I've also used up about 2200 miles that I really never put on. (unless odometer is separate from speedo)

 

The properly sized 19's are the same rolling radius as the 18's. Going up a size in width, from say 265/40-18 to 275/40-18 will make a minor difference. If you look at the tire specs for rolling radius or diameter, wider is taller. You can only go so far before you start to worry about clearance issues.