M3 Speedo accuracy

[RX BG69]

The speedo on the M3 appears to be the most accurate I have had on my cars - it’s consistently within 1 or 2 mph across the speed range (well at least up to 80mph); measured using GPS and road site speed readers.

I consistently drove previous cars at +10% of speed limit in zones with cameras - have to remind myself not to push it too high in this car. Often find on motorways with cameras that others are doing nearer 5mph below.

Anyone else noticed this or is it inconsistent across cars?

 

[Gatsojon]

It’s an interesting observation and means I can’t rely on that pessimism now.

 

[Dangerous Fish]

It's a long running myth that cars are made to over read by 10%. In the days of cable-driven speedometers, which weren't particularly accurate, the car makers erred on the side of caution and they would exaggerate your actual speed.

Now we have GPS and wheel speed sensors, which means speed is usually reported within a few percent of actual.

As for speed limits, the majority of drivers on the roads are over-cautious so slow to under the posted limit when they pass a camera when actually they'll be safe going a few mph over the limit. On most motorways you can pass cameras at 10% + 2 and nothing will happen to you.

 

[NorfolkMustard]

GPS speed will have Taycan-like refresh frequency, about once a second. Don’t rely on it, especially when going around a bend

 

[Jason71]

It's a long running myth that cars are made to over read by 10%. In the days of cable-driven speedometers, which weren't particularly accurate, the car makers erred on the side of caution and they would exaggerate your actual speed.

Now we have GPS and wheel speed sensors, which means speed is usually reported within a few percent of actual.

As for speed limits, the majority of drivers on the roads are over-cautious so slow to under the posted limit when they pass a camera when actually they'll be safe going a few mph over the limit. On most motorways you can pass cameras at 10% + 2 and nothing will happen to you.

true but its illegal for a speedo to under read but legal to over read by upto 10% so I think the manufacturers still err on the side of caution and make them over read a bit.

 

[Dangerous Fish]

true but its illegal for a speedo to under read but legal to over read by upto 10% so I think the manufacturers still err on the side of caution and make them over read a bit.

Yes, but only 1-2% as opposed to 10%+ in the good 'ole days.

Model 3 uses wheel speed sensors not GPS to display speed on screen.
GPS modules are working at 5-10Hz in consumer goods these days so refresh rate is much better than 1Hz.

 

[MrBadger]

I checked on first drive and it was spot on. So I am now forever warning that 74 does not really mean 70. 70 means 70.

Of course, tyre wear will make a difference over time and I suspect, so will wheel type setting.

 

[MaDProFF]

I found that my previous cars over the last few years have been 2 MPH across the range, I agree with OP M3 is actually the most accurate to date. spot on to 1 MPH out.

 

[Rob R]

The speedo on the M3 appears to be the most accurate I have had on my cars - it’s consistently within 1 or 2 mph across the speed range (well at least up to 80mph); measured using GPS and road site speed readers.

I consistently drove previous cars at +10% of speed limit in zones with cameras - have to remind myself not to push it too high in this car. Often find on motorways with cameras that others are doing nearer 5mph below.

Anyone else noticed this or is it inconsistent across cars?

I have not done a full test, but as far as I can see 30 mph in the car is 29 mph in reality. As measured by roadside speed indicators, while driving with cruise control set. I am assuming therefore that 72 or so in the car is about 70 on the road. Someday soon I will get out my old Satnav and do some tests.

 

[interbear]

I agree with the OP. My previous cars were always 3-4 mph out based on comparing Speedo with roadside indicators, the Model 3 is always exactly the same as the roadside indicators.

 

[Adopado]

I agree with the OP. My previous cars were always 3-4 mph out based on comparing Speedo with roadside indicators, the Model 3 is always exactly the same as the roadside indicators.

I can only say that you must drive past some particularly well calibrated roadside indicators! I seem to drive past speed indictors that consistently give higher or lower figures in relation to the car reading ... which one is right?

 

[AndrewGR]

My M3 over-reads by 1 mph at all speeds I’ve checked. My last car - a Qashqai, read 76 mph at 70. Not quite 10% but very nearly. It had standard wheels so no excuse. It was the most inaccurate of any car I’ve had since GPS made it easy to check. I checked the odometer accuracy - that was within 1%.

 

[MrBadger]

I wondered how much lifetime wear of tyre would make to indicated speed

tl;dr - not much it would appear

A new 235/45R18 has approx 67cm diameter, so approx 210cm circumference when new
A worn tyre will be approx 1cm less diameter, so 66cm diameter, 207cm circumference

So 1-1/2% difference in speedo reading.

indicated speeds, rounded to nearest integer - assume 'new' indicated speed is also actual.
new worn
30 30
40 41
50 51
60 61
70 71

So lifetime wear of tyre makes negligible difference to a wheel driven speedo - personally I thought it might be a fraction more.

 

[Mark_T]

It is true for the Model S as well.

I noticed it soon after getting it almost 2 years ago, All my other recent cars had been cautions by about 2-14 mph, but the S was bang on accurate.

 

[Mr Miserable]

I wondered how much lifetime wear of tyre would make to indicated speed

tl;dr - not much it would appear

A new 235/45R18 has approx 67cm diameter, so approx 210cm circumference when new
A worn tyre will be approx 1cm less diameter, so 66cm diameter, 207cm circumference

So 1-1/2% difference in speedo reading.

indicated speeds, rounded to nearest integer - assume 'new' indicated speed is also actual.
new worn
30 30
40 41
50 51
60 61
70 71

So lifetime wear of tyre makes negligible difference to a wheel driven speedo - personally I thought it might be a fraction more.

How interesting.

 

[phillcom3]

The speedo on the M3 appears to be the most accurate I have had on my cars - it’s consistently within 1 or 2 mph across the speed range (well at least up to 80mph); measured using GPS and road site speed readers.

I consistently drove previous cars at +10% of speed limit in zones with cameras - have to remind myself not to push it too high in this car. Often find on motorways with cameras that others are doing nearer 5mph below.

Anyone else noticed this or is it inconsistent across cars?

I compared it to waze it read like for like

 

[EV_Hopeful]

Might be a US thing. Using my GPS as a baseline, I have always found that UK cars have always over-reported the speed by a few percent whereas when I'm in the US the speed is usually pretty much spot on. I know that the M3 RHD is built for the UK market, but it's US heritage might be showing?