Speedometer Inaccurate?

[jadnashuanh]

Way back when (I'm dating myself), I seem to remember that Germany had a requirement that their speedos had to actually be accurate - long before the EU occurred. Typical German response, if true. I lived there for a bit over a year and came to appreciate their ethics and commitment to reality. You were responsible if you messed up, not the other guy for some obscure reason. Times change, at least some. The USA has way too many lawyers...

 

[Surge]

No speedo in any car is accurate, they all conform to the following UN document - http://www.unece.org/fileadmin/DAM/trans/main/wp29/wp29regs/r039r1e.pdf

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.

Not true. My lexus is dead on accurate.

 

[jadnashuanh]

Before the advent of the common market, German cars were required to be accurate. Today, they literally can't be and meet their requirements of the EU! Outside of that market, it's a crapshoot about how accurate things will be. Then, throw in some optional tires, and things could change for better or worse, and they do change as the tread wears down as well.

 

[boblucas]

The speedometer in my 2014, i3 consistently reads 2 mph higher than every speed indicator I've passed. To cruise at the speed limit, I'm always setting the cruise control to speeds like 27 or 37 around town and 67 on the freeway. The only downside is the speed control setting is displayed in red on the dash because I am setting it above the speed limit shown. My wife's X3, with the digital heads-up display, does not have this problem. It appears to be right on.

 

[jadnashuanh]

It might just be a difference on where the car was manufactured, and lack of attention to details. EU requirement is that the thing cannot read low, so to account for variations in tire and tread wear, I think they give things a slightly optimistic (high) readout.

Your X3 (at least I think it is) is made in the USA, and they may just have different software for the US market which would allow it to be exact.

Now, it could be made exact, and it could self-adjust to tire wear if they wanted to by utilizing the gps that is in every one of the i3's. You'd want to do a long-time average, but it wouldn't be all that hard to do. Same thing with the clock...it could be made to be extremely accurate (and is, in some markets, but not all).

 

[Cobra75007]

I would like to revive this post from 4+ years ago to see if there is any new information. I have a 2014 i three and the speed always shows 2 miles an hour faster than what a radar says, or what a GPS app says.

My main concern is not with the inaccuracy of the speed, but is with the inaccuracy of the mileage. So if my instrument cluster tells me that I have 20 miles left before my battery dies, does that mean I really Only have 18 miles? Or is it perhaps even a percentage? The speed seems to be 2 mph off at any speed, so that wouldn't really be a percentage of speed, but would potentially be a percentage of miles remaining.
So, if I'm looking to identify how many miles I have in my battery Before I need to recharge, then this number is inaccurate and i have problem with that.

What I have a bigger problem with is that if I show 34,000 miles, I may have only driven 29,000 miles. That potential he means resell value is a little lower, and I will hit the threshold for my warranty faster.

(I need a mathematician to help me out with this one - (I assume it is 2 miles an hour off and my average speed is 50 miles an hour, then it's 4%. Since my average speed is probably something more like 43 miles an hour, then a 2 mile an hour difference would really be closer to five or 6%. So if I have 34,000 miles then I am probably somewhere around 1900, or 2000 miles less on the odometer.


Any insight is appreciated
And especially…
Any ideas on how to adjust the speedometer.

 

[jadnashuanh]

Displayed MPH and the odometer reading don't have to be calculated in the same manner. IOW, the odometer is likely correct. Back when this was a totally mechanical thing with a gear in the transmission turning a cable, yes, they were tied together.

Most all interstates and some other major roadways typically have mileage markers on them. The next time you take a trip down one, zero the trip odometer when you pass one, then drive at least 10-miles or so to see what the odometer says in comparison to the mile markers. I-90 on the east from MA through NY has them every 0.2-miles or so. Some are closer, some further apart, but they're there if you look for them. Similar in other areas.

Note, though, that the allowable tread depth on your tires can produce about a 2% variation from between when they are new verses nearly worn out. The speedometer, I think, is calibrated so that you will never be going faster than it indicates, so needs a bit of being optimism.

I haven't bothered trying this, but if you do, let us know your results. I wouldn't compare it to a gps distance calculation, as you won't be certain of the update rate to account for any curves that the mile markers and your wheels will experience.

 

[Cobra75007]

Thanks. I'll try that. 2% diff from tier wear seems like a lot. I'll have to do some math on that one. I assume tire pressure could also effect.