champignon

Could you please cite the DOT requirement you reference? I do not think this is the case. Rather, cars are supposed to read either correctly or less than actual speed regardless of any rims or tires that may be installed on the car. There is no fixed number that I am aware of and many cars in fact read spot on to their speedometers when tested against any manner of things from onboard GPS, police radar, what have you.

I have owned 4 Subarus and every one of them was spot on. 2 of 2 recent VWs I have tested were spot on with the supplied tires. My current VW R (utility vehicle) reads 1 mph optimistic on its smaller diameter snow tires but spot on with the stock summer tires. One older BMW from 2000 (my Z3M Coupe) is spot on. 3 newer BMWs, which I currently own, all from 2011 to 2013 model years, read too high, which is intentional and can be coded out (as I have done myself with these three cars).

Tcc1999

This may be true for the U.S., I don't have any basis to think it is right or wrong. Rather, I think European standards are more proximally be the reason. This topic came up a bunch of years ago, perhaps as an offshoot of the true speed that the wing will automatically deploy. (I think the spedo reads 78 mph but it deploys at a true speed of 76 mph.). Anyway, it was a long time ago but the consensus, if my brain is working properly, was that there is a hefty fine in the EU (Germany) if the spedo reads slower than the actual speed. To ensure that this requirement is met spedos are intentionally set to read slightly faster than the actual speed. This seems to be supported by the fact that mileage is spot on while the spedo reads fast. So, to borrow a line from Edgy, "Your car knows how fast it is going", even if you are slightly in the dark.

Wayne Smith

Check this for its net affect ...

http://www.oecbi.com/blog/vehicle-sp...-really-going/

393 states plus or minus five in the US, but the Venn Diagram with the UK ends up with a positive error.

champignon

"The US federal regulation on speedometer accuracy reads: Each bus, truck, and truck-tractor must be equipped with a speedometer indicating vehicle speed in miles per hour and/or kilometers per hour. The speedometer must be accurate to within plus or minus 8 km/hr (5 mph) at a speed of 80 km/hr (50 mph)."

Not too relevant unless Porsche has expanded their line to manufacture buses, trucks, and truck-tractors :-)

Wayne Smith

From the link I shared ...

If we combine these two regulations, you end up with a car manufacturer standard of calibrating speedometers to show you are going faster than you really are, by about 3-4 mph. It's my guess they do it this way so they meet both US and UK regulations with one standard.

champignon

But the fact is that many non-German manufacturers sell cars in the USA that read mph correctly, at least on their stock tires. I have read and participated in many similar threads on brand forums for VWs, Subarus, BMWs, and now Porsche over the last few years. I have owned cars from all of these brands (except Porsche, so far) and measured the actual mph compared to that reported on the speedos and confirmed, at least for myself, that what other people were reporting was correct.

And that is that some manufacturers cars, probably most, are accurate on stock tires. Some manufacturers, mostly German, are not. BMW goes so far as to obviously code for this in their car's software/firmware, and it can be overridden by owners who purchase an OBD II gadget plus software to be used on an Android or IOS device; I have done this myself and I have personally observed the results on 3 BMWs. I have read pages and pages and pages of threads on BMW forums of similar people who have done exactly the same thing with exactly the same apparatus's and had exactly the same result as I have.

The original question was whether there was a problem with the individual's speedometer and why should he have to pay to fix it. The answer is that the coding on the car's software or firmware is to intentionally over read mph as the data comes in from the wheel sensors in the car. Porsche does this, I have read this many times. BMW does this, I have read and observed this personally many times.

Some other mfrs do NOT do this.

The rest is really irrelevant.

Wayne Smith

Over the last 50 years, from magnetic drive to digital to computer controlled, speedometer technology has changed. Growing up we calibrated our speedometers by manually holding speed and timing between mile posts. If they weren't accurate we figured it out and lived with it. No big deal. This was road rallying before the days of computers.

That being said, as I count back through my last 25 cars spanning Toyota, Ford, Chevy, Saab, Volvo, Triumph, Lotus, Jeep, AMC, Honda, Mazda, Rover, and a dozen plus motorcycles, I can't think of one that didn't indicate a speed higher than actual.

The first car I've had that was exact is Pam's GLA45. But as I said, she has over sized tires. The standard size tires still read high. And on long trips, if I believe mile markers and GPS, Pam's odometer reads low. Not by much ... about 1%, a mile every 100. Of course, my testing (distance measurement) may be error prone.

Of course YMMV. Manufacturers are allowed some variation. Computers on modern cars allow better accuracy. But it would seem that, given the European laws, setting them high would be the norm. Or at least not let the reading be low when the car is fitted with the largest factory optioned tires.

Back to the original concern, I suggest accepting the speedometer for what it is ... an approximation allowed for by the standards. Unless that is too annoying.

champignon

That's what I would do in most places, with a few exceptions. In my own state of Idaho, the speed limits are fairly strictly enforced. The state police and county sheriffs (those law enforcement people you encounter the most outside of the big city, Boise) will generally let you go 5 mph over, but don't count on 6. In California from what I have seen, the flow of traffic is at least 10 mph over the speed limit, whatever it is. In France they have radar cameras all over the place, and the word is 5 kph over is enough to trigger them.

But most places it is really a trivial issue unless your speedometer is set to read 6 or 7 mph over (some BMWs are), or you insist on habitually going at least 10 over the speed limit wherever you are.

Macster

My research into this with my Boxster is the odometer is unaffected by the optimistic speed reported by the speedo.

Shortly after I bought my 2002 Boxster I found my Boxster's speedo about 5mph optimistic at highway speeds. I took a GPS device and reset the trip odometer and drove 200 miles out across KS on i-70. After 200 miles the trip odometer and the GPS distance reading differed by just 0.2 of a mile. In 50K miles -- the then new Boxster's warranty -- this was an error of +/- 50 miles. After this I never really gave the error any thought and have adapted to it.

My 2003 Turbo's speedo reads 2mph optimistic. Other cars I have owned over the years since I bought the 2002 Boxster: VW Golf TDi, Pontiac GTO, Cayman S; have likewise had a speedo that read a bit optimistic but I can no longer remember the details of any of the cars other than recalling the error was just a couple of miles per hour optimistic.

My other car, a 2003 Turbo, its speedo reads 2mph optimistic and like the Boxster I have adapted to this and don't even think about it.

Petza914

Both my 997s are 2 MPH high. The active aero spoilers that are supposed to go up at 75 MPH go up at 77 MPH indicated on the speedo, and the speedo says 82 when waze says I'm doing 80. Haven't worried about it and now set my cruise to 81 instead of 79.

BIG smoke

Tread depth is also a factor. 10/32" vs 3/32"

or

Devil Boy

My speedo is about 5% off at any given speed. So when I'm indicating 80mph, my GPS says 76mph.

Surfer2001

I drove the Cab into work today, she has 20" HRE wheels, This one is spot on. lol, makes sense oversize OD spins slower. Anyway I'm over it now that I know the mileage is not affected. Coup gets 2MPH extra. Most CHP's allow 80 on freeways. Note to self set cruise control to 82

Lvt19672

Crester

Have you ever thought that the GPS on your phone you are using to monitor speed may not be completely accurate?