My Speedometer is 2 mps too fast
#16
There is a DOT requirement that the speedometer read 2 mph high.
Of course, if you buy larger wheels this can affect tire circumference and have an affect (Pam's GLA 45 with 20 inch wheels is spot on, but the same car with stock 19 inch wheels is 2 mph high ... MB isn't going to change speedometer drive gears to adjust for wheels and tires).
The odometer should be pretty well calibrated. Set your cruise control to 62, start a stop watch at an even mile, and drive for 10 minutes ... you will (probably) have gone 10 miles.
Of course, if you buy larger wheels this can affect tire circumference and have an affect (Pam's GLA 45 with 20 inch wheels is spot on, but the same car with stock 19 inch wheels is 2 mph high ... MB isn't going to change speedometer drive gears to adjust for wheels and tires).
The odometer should be pretty well calibrated. Set your cruise control to 62, start a stop watch at an even mile, and drive for 10 minutes ... you will (probably) have gone 10 miles.
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).
#17
Three Wheelin'
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.
#18
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.
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.
#19
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.
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.
Not too relevant unless Porsche has expanded their line to manufacture buses, trucks, and truck-tractors :-)
#20
"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 :-)
Not too relevant unless Porsche has expanded their line to manufacture buses, trucks, and truck-tractors :-)
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.
#21
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.
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.
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.
#22
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.
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.
#23
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.
#24
Race Director
First off, thanks everybody for the input.
I didn't know manufactures fudged these numbers. (Stress level and blood pressure) back to normal.
Secondly, Yes in my coup, I'm on stock 19" wheels. Tires - Front 235/35R/19, Rear 305/30R/19, pressures are set to factory specifications.
I used the GPS on my phone for the check, and then I used a Garmin NAV to cross check.
I'm okay with the MPH error, as long as the miles on my car are correct.
I didn't know manufactures fudged these numbers. (Stress level and blood pressure) back to normal.
Secondly, Yes in my coup, I'm on stock 19" wheels. Tires - Front 235/35R/19, Rear 305/30R/19, pressures are set to factory specifications.
I used the GPS on my phone for the check, and then I used a Garmin NAV to cross check.
I'm okay with the MPH error, as long as the miles on my car are correct.
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.
#25
RL Community Team
Rennlist Member
Rennlist Member
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.
#28
Racer
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Join Date: Nov 2004
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20" wheels
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
#30
Have you ever thought that the GPS on your phone you are using to monitor speed may not be completely accurate?