I have seen a Merc quite close (60 GPS /61 speedo), and a BF Ford Falcon XR6 I drove dead on (60km/h GPS and 60km/h on the speedo, 100 GPS/101 speedo).Īs a result, if you monitor and have checked your OBDII speed you can use that in situations where the GPS won't work. The manufacturer will program in a higher speed into the speedo for the reasons XT-Sub outlined above, at least in Oz. the car *knows* reasonably accurately what my speed is. I've been looking for that info.įrom my Forester Diesel fitted with standard rims and tyres:: Some more reading: How accurate is your speedo?
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This means that if your vehicle's actual speed is 100kmh, the displayed speed is permitted to be anywhere between 100kmh and 114kmh.”Īustralian Design Rules (ADR) pdf file where part of the above is taken from: 1803.pdf
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(Displayed speed minus true speed must be greater than or equal to 0, and less than or equal to 4kmh plus 10% of true speed). This formula means that the vehicle's actual (true) speed must not be greater than the displayed speed. above, there shall be the following relationship between the speed displayed (V1 ) and the true speed (V2). At the test speeds specified in paragraph 5.2.5. The speed indicated shall not be less than the true speed of the vehicle. “The current ADR (Australian Design Rules) prohibits any under-reading:
The following relates to vehicles in Australia (local & imported), but I think there would be something similar for other Countries: The error in the speedometer is intentional.Ĭlick to expand.Don’t the design Engineers calibrate the speedo to make certain they fall within the various countries' vehicle design rules for compliance? I can almost guarantee that the odometer will be *dead on*. The odometer and speedometer do get their data from the same sensor.
This accomplishes a few things (1) it makes the car seem more smooth and predictable - less sensitive - at "speed", (2) it reduces the instance of speeding tickets since when you're running 120 kph on the giggle-gauge, the real speed is only 112 kph, which is below the magical 115 kph threshold for the "3 points" ticket, I don't know what it is like in NB, but here in Ontario, cops typically won't even bother with you unless you're 115+ kph in a 100 zone., (3) it reduces the chances of fatality in a crash, the slower you go, the less chance you are going to die in a crash at that speed. Car makers will *intentionally* make the reading higher than normal to try to keep your real speed down. Legislation required that the speedo be accurate to within a set percentage, which I think (don't quote me on this) is around 10%. I'd rather just find a rabbit on the highway and follow them at a discrete distance than worry about 3 mph.Ĭlick to expand.A reading of 7-8% higher than reality should be considered "normal". Looks like I'm speeding on the gauge (62 or 63), but I'm really only doing 60. For the others I used my GPS to get the rough out of spec measurement at 60mph and then I just compensate my speed accordingly (usually 3mph). Too much work fiddling with the gauge and not breaking it, and swapping around tires is expensive. This was back when I cared about it - 15 years ago.
On the '95 I changed the tires and wheels giving me a longer rolling circumference of the tire and I was able to carefully remove and reposition the needle on the gauge and get the dial to read the actual speed from 10 to 80. Been awhile since I've seen that, but it stuck in my head.
I recall reading a website awhile back where they looked at speedo error by manufacturer, and the American cars tended to be dead on, the EU cars 1mph slow, and the Japanese cars 3mph slow. The '08 Outback is about 2 mph slow, but it has Michelin Primacy tires on it, which have a slight longer rolling circumference than the Bridgestones that came on it - which may play into your thoughts on it being a percentage.ģmph difference seems to be common on all Japanese cars. It seems to be a pretty fixed amount, though I'm certain that percentage fits into this picture somewhere, as different tire brands of the same size have different rolling circumferences.įor everything other than my '08 Outback, this is about 3 mph slower than shown. It doesn't seem like a percentage though. One of the things I found to be common with my Subarus, past and current (1986 GL, 1995 Legacy, 1999 Forester, 2006 Forester, and 2008 Outback) is that the speedo always reads faster than your actual speed with the stock tires.
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