: Speedometer accuracy



kmg
07-30-2011, 08:17 PM
I have not seen it discussed to date... Where does the speedometer display get it's data from? Is it calculated from the wheel turning rate, or is it being calculated by the GPS?

Reason I ask is I find it useful to know how much the speedometer readout is above or below true speed. One of my other cars shows I am generally going 5 mph faster than true speed, another car shows I am going about 1 mph faster than true. I know that GPS systems have incredible precision, so the Tom Tom GPS is for all practicable purposes, a true standard to be measured against.

So, I put my Tom Tom portable GPS in the Volt, and found out the Volt speedometer shows EXACTLY the same speed, DEAD ON, at 55, 60, 63, 70, etc., as the Tom Tom. When speeding up or slowing down, they both changed their identical speed readouts at exactly the same moment! That tells me the Volt speedometer is showing GPS derived data, like the Tom Tom does, or that GM did a terrific engineering job in measuring wheel speed. Anyone know for sure?

Ken

# 1228

stuart
07-30-2011, 08:21 PM
Can't wait to learn the answer to a really interesting question. Thanks
Stuart #2209

hodginator
07-30-2011, 09:06 PM
I imagine it is using wheel speed since GPS is not always available. They probably just did a very good job of making it accurate. But it would be interesting to know for sure.

eplantz
07-30-2011, 09:17 PM
I also compared it to a GPS app on my phone and found it to be dead accurate.

BlackVolt
07-30-2011, 09:55 PM
The Volt could use a mixture of both wheel speed sensors and GPS for the speedometer. I am not an engineer so I don't know the answer for sure. Will be interesting to know what the answer is. I do know the DIC does have a GPS compass but weather it measures the speed of the vehicle from the GPS is a different story. I have a gut feeling the Volt would use its speed sensors to calculate the speed of the vehicle and not GPS as there are limitations such as if you are going through a tunnel and the GPS goes out of range, it might take a moment for the vehicle to re-calculate the speed but by then you might of already been unlucky for a speed camera fine. I have a feeling GM has just engineered the Volt really well and used existing speed sensor technology.

Bob_Livonia
07-30-2011, 10:15 PM
No reason why a digital speedo shouldn't be dead-on accurate. GM speedo's have never used GPS. The wheel speed sensors used for ABS have multiple (at least 48) pulses per wheel revolution. I can't imagine someone's other car being 5 MPH off - but then again, when I read forum posts about the funny wheels and the snow tires people talk about putting on their Volts, a change in tire / wheel circumference would affect the speedo accuracy.

therfman
07-30-2011, 11:35 PM
It doesn't take that much. The difference between a new tire and a tire nearing end of life can affect the speed readout by a few percentage points. A typical new tire has 11/32" of tread, and it's generally recommended to change the tires at 3/32". That's a difference of 1/2 inch on the overall tire diameter.

Bob_Livonia
07-31-2011, 03:50 PM
It doesn't take that much. The difference between a new tire and a tire nearing end of life can affect the speed readout by a few percentage points. A typical new tire has 11/32" of tread, and it's generally recommended to change the tires at 3/32". That's a difference of 1/2 inch on the overall tire diameter.

A P215/55R17 tire, per the labeling system is nominally 26.32 inches in diameter, neglecting the flattening that occurs in all tires. 1/4 inch of wear would reduce the diameter by 1/2 inch, or 1.9%. 1.9% of 60 MPH would be 1.13 MPH (60 indicated would be 58.87 actual ) since the distance traveled per revolution (d) is proportional to diameter. 59 vs 60 is almost round-off error, even with a digital speedo - wouldn't cause a ticket, especially since you are erring on the side of going slower than indicated.

Frommologist
08-20-2011, 12:12 AM
All good points. Beyond tire wear there are a lot of tolerances that add up in measuring wheel rotation speed... may not very many with ABS break sensors. It would not be difficult to adjust a calibration constant via the GPS data every week or month. It seems the software and hardware engineering teams worked together very well on this project and may have added an occasional calibration.

banadictaustin
06-26-2012, 01:15 PM
I think speedometers must be adjusted to make sure the torque designed by the magnetic field perfectly shows the speed of the car. Calibration adjusts for these changes and is done by the manufacturer, which places up the speedometer equipment to match with the factory-installed band and pinion rate and wheel dimension.

supermachoman
06-27-2012, 03:11 PM
It's probably nothing fancy, just a set of magnets attached to a sensor on the output shaft of the transmission which sends its data through a wire to the CPU (at least I think that's how a modern speedo works)

Fulgerite
06-27-2012, 03:19 PM
I believe the Volt's speedometer is reading the wheel rotation sensors. (which are required to make ABS and traction control work.) Many cars (Like Toyota) deliberately make their speedometers read about 3 mph faster than the car is actually going because a few annoying people in the USA were successful at suing the auto manufacturer for speeding tickets. (So the chicken $hit lawyers make the engineers deliberately lie about your actual speed.) BMW does NOT lie on their speedometers. (I think a German engineer would be shot for making a speedometer that was not accurate.) Nissan and Toyota make their speedometers lie to avoid possible lawsuits from litigious Americans.

My Volt reads the actual speed. (As compared with my Garmin GPS.) I guess GM is not afraid of litigious Americans.

Frankly I think we should start a class action suit against Toyota for building cars with deliberately broken speedometers. ;-)

solar_dave
06-27-2012, 04:57 PM
We have those radar signs that show your speed around here, both the one @ 25mph and the one @ 45 mph exactly match the Volt speedo.