Speedometer Inaccurate?

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tiburonh

Well-known member
Joined
Jan 1, 2014
Messages
206
Location
Tiburon, CA
I've noticed while driving my i3 that every time I pass by one of those automated devices that flashes out your speed at you in an effort to get you to slow down, the automated sign shows I am doing two miles per hour less than my car's speedometer shows. The first time it happened I didn't think much of it, but it keeps happening, in a variety of locations passing by a variety of speed detection signs.

So, am I safe in assuming my speedometer is actually off? Can I drive what my car tells me is two miles per hour faster than the posted limit without any worry of getting a ticket? Actually, the better question is, where I would normally try to limit myself to 10mph over the posted limit, can I actually do 12mph over? :)

Are others having this issue? If so, can it be fixed?
 
Car makers never want the speedometer to read below actual speed. There are laws in some areas about that. So they err on the side of reading high.

Think of it as your margin of error.
 
tiburonh said:
Are others having this issue? If so, can it be fixed?

Every BMW I have owned (at least 6 of them) have had this quirk. I think the anomaly would be if I had a BMW with an accurate speedo! :roll:
 
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.
 
jackt said:
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.

This! I was trying to find it for 5 minutes and then gave up.

I don't mean this comment as negative/mean, it still surprises me that people aren't aware of this kind of stuff.
 
elptex said:
jackt said:
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.

This! I was trying to find it for 5 minutes and then gave up.

I don't mean this comment as negative/mean, it still surprises me that people aren't aware of this kind of stuff.

And why in the world would people be aware that the UN is involved in regulating speedometers? What the heck does that have to do with their mission of promoting peace and health?

In any event, it would appear that the United States has not signed on to this particular agreement so perhaps we Yanks can be forgiven for being so bloody ignorant about it. :)
 
tiburonh said:
In any event, it would appear that the United States has not signed on to this particular agreement so perhaps we Yanks can be forgiven for being so bloody ignorant about it. :)

In any case, a lot of you yanks are pretty quick to go to court, and I can imagine there would be a lot of court actions if you started getting speeding tickets because of under-reading speedos. A slightly over reading speedo is a simple manufacturer defence, no need to agree to a UN decree to see the logic of it.
 
Blue20 said:
Which wheel do you have ? 19 or 20? Sometime different wheels will impact the accuracy.
I wonder if there is a software setting to adjust for different size tires that we or a BMW tech can access? Unless the 19's and 20's have the same rolling diameter, which is possible.
 
The 19" wheel/tire vs the 20" wheel/tire is for all practical purposes, exactly the same diameter (within about 0.04") when new.

It's not hard to figure out rolling diameter: (aspect ratio * width *2)/25.4 + wheel diameter. Remember that the aspect ratio is a percentage, so a 70-series would be 0.70 in the above equation. The 25.4 is the conversion ratio to get from millimeters to inches.

There's only one rolling diameter a vehicle is designed for, otherwise it will change the speedometer/odometer and the gear ratio. Sometimes, you have room to go wider, but not always. In the case of the i3, wider means more drag, and less distance per charge.

A taller, narrow tire can have the same footprint and therefore grip as a smaller, wider tire...the i3 has about the same footprint as a mini with their performance tire option.
 
The signs over read - there are several threads on laser slippage effect as the beam traverses the contours of the vehicle. Try driving at the same indicated speed (under the limit obviously!) with another speed sign to see if there is a difference.
 
ecoangel said:
The signs over read - there are several threads on laser slippage effect as the beam traverses the contours of the vehicle. Try driving at the same indicated speed (under the limit obviously!) with another speed sign to see if there is a difference.

That's the thing of it -- I've gotten the same result going past at least six different signs. And I do NOT have this problem in my wife's x3.
 
The signs over read because they are set to over read.......we have two in our town which are moved around at random and depending upon if its a 30mph or 40mph limit the read out is set a few mph over to set a mental "wall" for the passing motorist, sometimes it works! :roll:
 
Boxbrownie said:
The signs over read because they are set to over read.......we have two in our town which are moved around at random and depending upon if its a 30mph or 40mph limit the read out is set a few mph over to set a mental "wall" for the passing motorist, sometimes it works! :roll:
I very carefully calibrated the speedometer on my bicycle (careful distance measurement of 10 turns of the wheel, converted to mm/1 rotation entered into speedometer, then verifying a known 10 mile path measured 10.00 within 0.01 miles on the odo), and found that the radar speed posting signs measured my speed exactly right. The sign would move from, for example, 17 to 18 MPH coincident with my speedometer going from 17.9 to 18.0 MPH. Worked for speeds between 15 and 22 MPH. The hard part was finding access to the sign when no other cars were on the road, as the sign always reads the highest speed vehicle, and that never seemed to be my bike unless I was alone on the road. Through 6 years of daily bike commuting, I got to test this many dozens of times.
 
This is common among nearly all new cars sold in the U.S. They tend to read 1-2mph faster than actual speed. It is a design feature to reduce liability. The higher than actual speed reading also helps in the event the customer changes to a different model tire which can have a slightly different diameter than the O.E. tire despite being the same "size".

Over time the error can become even higher as tires wear down and their rolling diameter shrinks.

If you want to check your error simply download a good GPS app on your smartphone. I've observed them to be nearly as accurate as a $3,000 Trimble GPS unit in most cases.

FWIW the Prius reports 1.5-2mph too fast and my Volt is about 1mph too fast.
 
My i3 speedometer is "accurate" just like my chronometer: it is inaccurate, but always by the same amount..... Just like navigation, you simply allow for it as long as it is consistent. Both by GPS, and by cautionary speed radar that shows you your speed to shame you into slowing down, my car is always showing 2mph above actual speed.
 
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