How accurate is your speedo?

[Ziggity]

Mine seems to be consistently 3 mph under according to my phone gps speedo app.

 

[Unobtanium]

+-1mph based on every radar sign I've ever driven past.

 

[sillyxone]

I've always wanted to see if traffic cops can help me check the accuracy of mine by driving 5mph past the speed limit in their presence. Either the error range on mine is +-5, or the cops are not very helpful.

 

[dougal]

Mine seems to be consistently 3 mph under according to my phone gps speedo app.

Why do you believe the GPS and not the speedo?

 

[Ziggity]

Why do you believe the GPS and not the speedo?

interesting question. I always assumed that distance over time measurement with a GPS would be more accurate than a speedometer that has to deal with a multitude of tyre issues; wear, temperature, incorrect inflation, etc. A bit of googling has confirmed that GPS is more accurate.

Personally, i've just changed wheels and tyres and wanted to know. It does appear my speedo is accurate, Mazda have just added 3 to whatever speed I'm doing.

 

[craigo]

interesting question. I always assumed that distance over time measurement with a GPS would be more accurate than a speedometer that has to deal with a multitude of tyre issues; wear, temperature, incorrect inflation, etc. A bit of googling has confirmed that GPS is more accurate.

Personally, i've just changed wheels and tyres and wanted to know. It does appear my speedo is accurate, Mazda have just added 3 to whatever speed I'm doing.

GPS *could* be more accurate. However, there are many variables that go into the accuracy of a GPS - how many satellites are visible, signal strength, signal reflection problems, how the chip does its computation, the software you're using may be rounding up... if you're in a city it's going to be less accurate than the countryside. An antenna outside the car may be more accurate than your phone inside the car.

The actual answer probably is somewhere in the middle between what the speedo reports (it has its own variables), and what the phone says.

 

[Moonlighter]

GPS will be more accurate.

Manufacturers typically set speedos to read a touch conservatively - usually 2-3% - they dont want us to get speeding tickets and then blame the speedo for reading under when actual speed was over the limit.

 

[Unobtanium]

GPS will be more accurate.

Manufacturers typically set speedos to read a touch conservatively - usually 2-3% - they dont want us to get speeding tickets and then blame the speedo for reading under when actual speed was over the limit.

In the U.S., manufacturers voluntarily follow the standard set by the Society of Automotive Engineers, J1226. Manufacturers are afforded the latitude to aim for within plus-or-minus two percent of absolute accuracy or to introduce bias to read high on a sliding scale of from minus-one to plus-three percent at low speeds to zero to plus-four percent above 55 mph. And those percentages are not of actual speed but rather a percentage of the total speed range indicated on the dial. So the four-percent allowable range on an 85-mph speedometer is 3.4 mph, and the acceptable range on a 150-mph speedometer is 6.0 mph.


*Might explain why some vehicles that can barely break 100 have 180mph speedos, ROFL!

 

[piotrek91]

interesting question. I always assumed that distance over time measurement with a GPS would be more accurate than a speedometer that has to deal with a multitude of tyre issues; wear, temperature, incorrect inflation, etc. A bit of googling has confirmed that GPS is more accurate.

Personally, i've just changed wheels and tyres and wanted to know. It does appear my speedo is accurate, Mazda have just added 3 to whatever speed I'm doing.

You're in the UK, so you won't get many good answers here.
Your speedometer calibration is going to be different than US models.

In the US, the speedometer is allowed to be accurate, and in my CX-5 actually under reports speed by a little bit.
If I'm at a GPS 80MPH my speed will show 78-79ish.

 

[Moonlighter]

In the U.S., manufacturers voluntarily follow the standard set by the Society of Automotive Engineers, J1226. Manufacturers are afforded the latitude to aim for within plus-or-minus two percent of absolute accuracy or to introduce bias to read high on a sliding scale of from minus-one to plus-three percent at low speeds to zero to plus-four percent above 55 mph. And those percentages are not of actual speed but rather a percentage of the total speed range indicated on the dial. So the four-percent allowable range on an 85-mph speedometer is 3.4 mph, and the acceptable range on a 150-mph speedometer is 6.0 mph.


*Might explain why some vehicles that can barely break 100 have 180mph speedos, ROFL!

I did some more research. This is what I found.

"In Australia, From July 1 2006 a new standard began its phase in and by 1 July 2007 all new vehicles had to comply. The new standard requires that:

The speedo must not indicate a speed less than the vehicle’s true speed or a speed greater than the vehicle’s true speed by an amount more than 10 percent plus 4 km/h.

Odometer accuracy is no longer defined.

What this means:

For a vehicle travelling at a true speed of 100km/h, the speedo must read between 100km/h and 114km/h. The effect of this is that many drivers will find that at 100km/h they are driving up to 14km/h below the speed limit if they rely on the vehicle’s speedo.
The speedo must always read 'safe', meaning the vehicle must not travel faster than the speed indicated by the speedo.


This change was made to align Australian vehicle rules with those already in place in Europe.

It applies to all Australian motor vehicles except mopeds."

 

[Guest2019C20]

GPS *could* be more accurate. However, there are many variables that go into the accuracy of a GPS - how many satellites are visible, signal strength, signal reflection problems, how the chip does its computation, the software you're using may be rounding up... if you're in a city it's going to be less accurate than the countryside. An antenna outside the car may be more accurate than your phone inside the car.

The actual answer probably is somewhere in the middle between what the speedo reports (it has its own variables), and what the phone says.

Speed readings in consumer GPS units relies on *doppler* of the signals, -not- on position data. So it's *really* accurate.

(Doppler is the shift in frequency of a known source, due to traveling toward or away from the source).

 

[Guest2019C20]

My Garmin handheld shows my speeddo to be right on, with 225/60R18 Michelin X-ice 3 tires. These are 727 revolutions per mile. The stock A23's at 720 revs per mile would have me at 0.6 MPH slower than the speedo claimed.