Hi (speedometer accuracy)

[Deleted member 16414]

Has anyone had the car tell you you're going faster than you are? I have been past the flashing lights that tell you your speed when passing them. Three of them all said I'm going 23 mph, but whinny car is telling me I'm doing 28. Has anyone got anything to say? What I can do about this?

 

[Bam Bam]

Has anyone have the car tell you you going faster than you are I been past the flashing lights that tell your speed being passed three of them all said I going 23 whinny car telling g me I doing 28 has anyone got anything to tell what I can do about this

I think Tesla Bjorn talks about GPS speed trackers (assume available on the phone) but haven't investigated.

Came here to say that the SE seems to be pretty accurate when I go past these speed signs near us. Maybe one mph out.

Cars have to errr on the side of over-reporting speed because otherwise you'd have an excuse if caught speeding.

 

[Deleted member 16414]

I was caught, and then I proved to them that I was doing 36 in 40 zone. But they didn't want to know. I called MG and they told me the software needs updating. So I called my MG dealer; he said software updating will cost £120 plus vat.

 

[webhiker]

Check your tire pressure and if you are on original wheels.
My GPS tells me that my Trophy EX is showing 1 km/h too much

 

[Deleted member 16414]

Ok I just had a wheel check but I will check them

 

[fnegroni]

I was caught and the I prove to them that I was doing 36 in 40 zone but they didn't want to no I call mg and told me the software needs updating so I call my mg dealer he said for software updating will cost 120 plus vat

Are you saying you were caught doing more than 40 mph in a 40 zone?
If so, is it because you assumed the speedometer in the mg4 would over read the speed like most do? Unfortunately it doesn’t. If you read 45 mph you are pretty much doing 45mph, not 40 or 41.

 

[siteguru]

Agreed - that's been my experience, and what I've taken from most others here who have commented regarding their speedometers. In my case the Speedo is pretty much bang on with a GPS speed tracker, all the way up to 70 mph.

 

[Anon70]

I find the MG speedo reads pretty close to actual when compared to roadside signs and phone GPS speeds and certainly closer to roadside signs than my wife's Mini electric which is always a few percent under (30mph in car shows as 27-28 mph on sign)

 

[dawson2099]

Me too - the MG4 has the most accurate speedo that I've ever had - it's bang on with detectors and my GPS speedo

 

[ReintjeWA]

Yeah can confirm mine is always bang on. All my older cars always had a 5 kph margin. I have to say I quite like it being so accurate.

 

[dsimpkins]

Mine also appears to be accurate, and the speed awareness signs near me overread to get motorists to slow down. Speedo says for example 28mph and sign says 30/31 mph. Haven't checked with GPS to confirm.

 

[Wizard of Oz]

Yeah can confirm mine is always bang on. All my older cars always had a 5 kph margin. I have to say I quite like it being so accurate.

Interesting. Australian Design Rules say it must not read less than the true speed. In practice, most cars usually have that few kilometres of margin.

The theory I've read online is to ensure that even with tyre wear and/or wheel size changes for different trim models, the reading will always be under the true speed.

I wonder how accurate your car will remain when the overall wheel diameter starts to "shrink" from tyre wear.

 

[siteguru]

As the tyres wear the actual speed will reduce vs displayed speed. (Assuming that displayed speed is based on wheel rotation speed - rpm). That's because for each rotation the distance travelled will be less, as the circumference will be less (because the diameter is less).

 

[Anon70]

Waiting for the first EV manufacturer to change the speedo to read in knots when they have a GT car designed for longer trips, because why keep things simple (like showing state of charge in % and not kWh but showing efficiency in miles per kWh or kWh per 100km). Same as removing all the easy to use buttons and dials and burying controls 3 menus deep behind tiny icons.

 

[fnegroni]

I think all cars produced in Europe in the last 2 years that comply with the ISLA directive will have accurate speedometers now.

I know Tesla overheads efficiency by some margin but not sure if it’s related to speedometer reading as well.

 

[bedsev]

mine is bang on too

 

[Ganymede]

Ours is pretty accurate - it might overread by 1mph but that's all.

 

[ReintjeWA]

Interesting. Australian Design Rules say it must not read less than the true speed. In practice, most cars usually have that few kilometres of margin.

The theory I've read online is to ensure that even with tyre wear and/or wheel size changes for different trim models, the reading will always be under the true speed.

I wonder how accurate your car will remain when the overall wheel diameter starts to "shrink" from tyre wear.

That’s an interesting point you raise. I would hope MG had the foresight to leave enough margin for tyre wear, but perhaps there is less margin in the speedometer to play with when getting different rims/tyres. Normally you want to stay within 2-3% of your original overall tyre diameter but perhaps for the MG4 this margin should be much less.

 

[decrep]

Waiting for the first EV manufacturer to change the speedo to read in knots when they have a GT car designed for longer trips, because why keep things simple (like showing state of charge in % and not kW but showing efficiency in miles per kWh or kw per 100km). Same as removing all the easy to use buttons and dials and burying controls 3 menus deep behind tiny icons.

When I check my cars speed I have to remember to change the GPS to km/h, it's too hard doing the knot conversion live.
And yes just eyeballing the 2 readouts, it's within 0.5 km/h.
And I know my GPS is accurate to 0.05km/h.

 

[Coulomb]

I think all cars produced in Europe in the last 2 years that comply with the ISLA directive will have accurate speedometers now.

I'm not so sure. They could still over-report speed, and then enforce that lower speed limit. I'll admit that cars that do this would quickly lose popularity compared to those that somehow get and keep the speedo accurate.

I would hope MG had the foresight to leave enough margin for tyre wear, but perhaps there is less margin in the speedometer to play with when getting different rims/tyres. Normally you want to stay within 2-3% of your original overall tyre diameter but perhaps for the MG4 this margin should be much less.

I think that MG must be relying on some sort of GPS calibration system to keep the speedometer accurate (perhaps over a many days or weeks timespan) to allow for tyre wear. But then I wonder what happens when you change tyres; does the car then over-report speed for the next week or several days? Maybe if it detects a sudden change in speedometer accuracy compared to GPS, it allows for a quicker change. Or maybe the fist time that conditions are ideal (flat road at steady speed), it allows an immediate speedometer calibration.

This could be tricky to get right; not one of MGs strengths, it seems to me. But maybe they import this technology from a third party like Bosch, who would have the resources to get this right, and a reputation to uphold. That is, Bosch probably actually care if they get it wrong.