If our speedometers are 'off' then..

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If our speedometers are 'off' then..

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A while back, I changed my dashcam to show the GPS speed when moving. It was quite eye opening about how 'off' the speedometer was in the Avensis and the CT. I presume a lot of cars might be like this. Even new tyres of the original size didn't really 'fix' it.

It got me thinking... If the car isn't able to calculate the speed accurately, then is the mileage odometer technically also inaccurate? (showing lower miles than true)

And what about MPG calculations.. are they also understated as a result of inaccuracies?

I wonder if this can be corrected using some sort of MultiECUScan level tool (in the case of Fiat) or equivalent for other brands

I used to lock the Avensis on the speed limit (according to the digital screen) and wonder why 70% of people still overtook. Now, as I'm locked into the 'correct' GPS speed, I see some new cars / other cars (usually German to be fair) also bang on the 'correct' speed for the road. Quite interesting discovery to find that for most of the time when I thought I was going the right speed, I was unknowingly holding people up... and other slow coaches, are probably unknowingly doing the same
 
A while back, I changed my dashcam to show the GPS speed when moving. It was quite eye opening about how 'off' the speedometer was in the Avensis and the CT. I presume a lot of cars might be like this. Even new tyres of the original size didn't really 'fix' it.

It got me thinking... If the car isn't able to calculate the speed accurately, then is the mileage odometer technically also inaccurate? (showing lower miles than true)

And what about MPG calculations.. are they also understated as a result of inaccuracies?

I wonder if this can be corrected using some sort of MultiECUScan level tool (in the case of Fiat) or equivalent for other brands

I used to lock the Avensis on the speed limit (according to the digital screen) and wonder why 70% of people still overtook. Now, as I'm locked into the 'correct' GPS speed, I see some new cars / other cars (usually German to be fair) also bang on the 'correct' speed for the road. Quite interesting discovery to find that for most of the time when I thought I was going the right speed, I was unknowingly holding people up... and other slow coaches, are probably unknowingly doing the same
Until the recent addition of speedometers being GPS based nearly all cars overcompensated to allow for tolerances in tyre wear etc. Police cars used to have to be recalibrated before being used so that they read the correct speed. As a HGV driver I tend to notice this particularly in 50mph roadworks as HGV’s have to have there’s recalibrated every 2 years so are normally pretty accurate. Plenty of places can recalibrate speedometers if you’re that bothered but if you’re using a GPS alongside then is it worth the hassle? Agreed though about the MPG and mileage though, yes they are going to be marginally out
 
The odometer tends to be very accurate compared to the speedo.
Speedos are designed to read high, that way you cant blame the manufacturer for being caught speeding.
My dad always used to say the other reason is so that (where legal) your car would reach, or even exceed the cars published maximum speed 😀
 
I recently discovered that my seicento speedo reading of 70mph is only 60 on the GPS 😬
I'm finding that actual GPS 70mph on the Lexus requires 80mph on the actual speedometer! And actual 30mph, requires 35mph on the speedometer... When I first clocked on to this and started following the GPS reading, it made for some scary passing of speed check points but months later, no letters in the post. That's how I started to believe the GPS, though someone who knows more about the technology than me explained that GPS either works perfectly, or it doesn't work at all?
 
@SB1500 this wasn't you was it ?
https://www.bbc.co.uk/news/articles/cwywjjv4xz5o
I'm sorry, but I had to laugh at his name, given the context:
"A defence barrister told Dungannon Magistrates' Court that 55-year-old Moron was going to collect his mother from the airport."
That guy is adequately surname'd!

Not me, gosh, my MPG would be abysmal if I drove at that speed! as a boring Hybrid driver, that's now my whole thing!
 
And what about MPG calculations.. are they also understated as a result of inaccuracies?
I find mine is wildly optimistic!

My 169 Panda's computer thinks I'm getting over 60mpg. I wish!

Standard 155/80 R13 tyres, the computer always settles on this figure long term.

On long journeys I do brim tests and it routinely averages out around 50mpg.

But then I'm also relying on the computer for mileage, so who knows what it really is?!

I treat the computer as a very vague guide, I'd only pay it much attention if figures started deviating from normal.

Indicated mpg was mid-50s until I replaced my leaking thermostat, then it immediately jumped up to over 60.
 
I find mine is wildly optimistic!

My 169 Panda's computer thinks I'm getting over 60mpg. I wish!

Standard 155/80 R13 tyres, the computer always settles on this figure long term.

On long journeys I do brim tests and it routinely averages out around 50mpg.

But then I'm also relying on the computer for mileage, so who knows what it really is?!

I treat the computer as a very vague guide, I'd only pay it much attention if figures started deviating from normal.

Indicated mpg was mid-50s until I replaced my leaking thermostat, then it immediately jumped up to over 60.
My Panda 169 used to sit around 53-56 I think! Even though I was a broke student, I still wasn't putting too much money into it in fuel to have to do much further investigation. Never had a dashcam then either with GPS to show the true speed over ground.

I'd love to buy that car back if I could find it! Probably sitting rusting in a field somewhere
 
@SB1500 this wasn't you was it ?
https://www.bbc.co.uk/news/articles/cwywjjv4xz5o
I'm sorry, but I had to laugh at his name, given the context:
"A defence barrister told Dungannon Magistrates' Court that 55-year-old Moron was going to collect his mother from the airport."
My dad had a granada with a dodgy speedo. Turned out to tbe the speed encoder on the propshaft had failed in an odd way, reading about 10-15mph slow at speed, speedo said 75, he got done for 85mph. Cops didn't believe him, probably one of the many times a day they are told the driver wasn't speeding.
 
The odometer tends to be very accurate compared to the speedo.
Speedos are designed to read high, that way you cant blame the manufacturer for being caught speeding.
It's also the law, regulations below. Zero tolerance under, lots and lots of tolerance over:
  • No underreporting
    Speedometers must never report a vehicle's speed as less than it actually is
  • No overreporting
    Speedometers must never report a vehicle's speed as more than 110% of the actual speed plus 6.25 miles per hour

 
A while back, I changed my dashcam to show the GPS speed when moving. It was quite eye opening about how 'off' the speedometer was in the Avensis and the CT. I presume a lot of cars might be like this. Even new tyres of the original size didn't really 'fix' it.

It got me thinking... If the car isn't able to calculate the speed accurately, then is the mileage odometer technically also inaccurate? (showing lower miles than true)

And what about MPG calculations.. are they also understated as a result of inaccuracies?

I wonder if this can be corrected using some sort of MultiECUScan level tool (in the case of Fiat) or equivalent for other brands

I used to lock the Avensis on the speed limit (according to the digital screen) and wonder why 70% of people still overtook. Now, as I'm locked into the 'correct' GPS speed, I see some new cars / other cars (usually German to be fair) also bang on the 'correct' speed for the road. Quite interesting discovery to find that for most of the time when I thought I was going the right speed, I was unknowingly holding people up... and other slow coaches, are probably unknowingly doing the same
No they aint accurate either. (Miles). With a Panda you look at the substantial difference at 100K and wonder which is better and if either are actually representative. Even at lower milage you wonder of someone is having a laugh! People on here point out that the MPG from the computer isnt accurate. Both are a guide I suppose. It seems the speedo thing is common to all vehicles. Fear of litigation is behind that no doubt. In this reasonably high tech age its a bit lame saying they cannot get htis more accurate. I bet if you wrote a cheque to a garage for a car and said I ve paid £8700 when the cost was £10,000 they would complain.... "Well I thought Id under pay, near enough is good enough"?

Any car doing the actual speed limit will be using GPS / sat nav speedo, IUm sure Z but beware of this as sometimes you get loss of signal so you sail along for miles at a supposed constant speed. In an average speed monitoring area this could be expensive.
 
A while back, I changed my dashcam to show the GPS speed when moving. It was quite eye opening about how 'off' the speedometer was in the Avensis and the CT. I presume a lot of cars might be like this. Even new tyres of the original size didn't really 'fix' it.

It got me thinking... If the car isn't able to calculate the speed accurately, then is the mileage odometer technically also inaccurate? (showing lower miles than true)

And what about MPG calculations.. are they also understated as a result of inaccuracies?

I wonder if this can be corrected using some sort of MultiECUScan level tool (in the case of Fiat) or equivalent for other brands

I used to lock the Avensis on the speed limit (according to the digital screen) and wonder why 70% of people still overtook. Now, as I'm locked into the 'correct' GPS speed, I see some new cars / other cars (usually German to be fair) also bang on the 'correct' speed for the road. Quite interesting discovery to find that for most of the time when I thought I was going the right speed, I was unknowingly holding people up... and other slow coaches, are probably unknowingly doing the same
Speedos virtually Allways over read

Meaning it will show more mile then you have actually done not less
 
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