What speed is correct?
Discussion
Hi guys,
I was wondering, what would the correct speed be of my car?
For example; Lets say on the speedo I'm doing 70mph, but on the GPS speedo apps on my phone, it'll say I'm doing about 73-75mph, soo, which one
?
As its kinda annoying having to use the speedo app on the phone constantly, as I don't want to risk a speeding fine, when the car says I'm doing 70 but I'm actually doing 75mph.
Thanks
And sos if the post isn't clear, half awake now
I was wondering, what would the correct speed be of my car?
For example; Lets say on the speedo I'm doing 70mph, but on the GPS speedo apps on my phone, it'll say I'm doing about 73-75mph, soo, which one
?As its kinda annoying having to use the speedo app on the phone constantly, as I don't want to risk a speeding fine, when the car says I'm doing 70 but I'm actually doing 75mph.
Thanks
And sos if the post isn't clear, half awake now
GPS is generally more accurate. Most cars over read slightly on their speedos, modern ones too. If your speedo is under reading it would suggest you might be running the incorrect wheel size or tyre profile for your car? I found this to be true when i put 17" wheels on my Peugeot 206.
Our T4 had 18" wheels fitted by a previous owner, and the speedo is bang on in agreement with GPS, so I'm guessing it under-read by about 10% on its original 16" ones, unless the tyre profile was bigger. It's very handy for trundling through the miles of 50mph on the M1 without having to glance at the GPS all the time.
So, would I get a ticket for going about at speedo 70 (gps 75) on like the motorway cameras, like for example the ones on the M1 near the luton airport area? Or the m25 speed cameras, I know before they had a leeway of the maximum speed you could do? But not sure if thats the case now.
Chuck the GPS out of the window and just look at your speedo. All that is happening is that you have two devices measuring the same thing and giving you different readings.
We use GPS at work with RTK differential correction. There is probably 10 grand per machine in hardware and software plus the £500/yr/machine subscription for the RTK signal. This gives sub centimeter accuracy. Most of the time. It does go wrong. It does f
k about. It is brilliant when it works. I wouldn't trust it much further than I could throw it.
The only variables on a calibrated speedo are tyre wear (bugger all on a car) and inflation both of which will affect the rolling radius. You'll always get a reading rather than when all the planets align like with GPS. I doubt a cheap app on a phone is going to be that good.
We use GPS at work with RTK differential correction. There is probably 10 grand per machine in hardware and software plus the £500/yr/machine subscription for the RTK signal. This gives sub centimeter accuracy. Most of the time. It does go wrong. It does f
k about. It is brilliant when it works. I wouldn't trust it much further than I could throw it. The only variables on a calibrated speedo are tyre wear (bugger all on a car) and inflation both of which will affect the rolling radius. You'll always get a reading rather than when all the planets align like with GPS. I doubt a cheap app on a phone is going to be that good.
P4Pistonhead said:
Hi guys,
I was wondering, what would the correct speed be of my car?
For example; Lets say on the speedo I'm doing 70mph, but on the GPS speedo apps on my phone, it'll say I'm doing about 73-75mph, soo, which one?
As its kinda annoying having to use the speedo app on the phone constantly, as I don't want to risk a speeding fine, when the car says I'm doing 70 but I'm actually doing 75mph.
Thanks
And sos if the post isn't clear, half awake now
Your Speedo is only allowed to read the correct speed or up to 10% more than your true speed. I was wondering, what would the correct speed be of my car?
For example; Lets say on the speedo I'm doing 70mph, but on the GPS speedo apps on my phone, it'll say I'm doing about 73-75mph, soo, which one
?As its kinda annoying having to use the speedo app on the phone constantly, as I don't want to risk a speeding fine, when the car says I'm doing 70 but I'm actually doing 75mph.
Thanks
And sos if the post isn't clear, half awake now
You need to sort your wheels out or fix the Speedo.
As long as you are holding a steady speed your GPS device will probably be accurate to +/-1mph. As soon as you change speed it will need to catch up and that depends on the device you have. Unless you have paid a lot of cash for an industrial unit it won't be adequate. A phone won't be sufficient.
Willy Nilly said:
Chuck the GPS out of the window and just look at your speedo. All that is happening is that you have two devices measuring the same thing and giving you different readings.
We use GPS at work with RTK differential correction. There is probably 10 grand per machine in hardware and software plus the £500/yr/machine subscription for the RTK signal. This gives sub centimeter accuracy. Most of the time. It does go wrong. It does fk about. It is brilliant when it works. I wouldn't trust it much further than I could throw it.
The only variables on a calibrated speedo are tyre wear (bugger all on a car) and inflation both of which will affect the rolling radius. You'll always get a reading rather than when all the planets align like with GPS. I doubt a cheap app on a phone is going to be that good.
What RTK system do you use out of interest?We use GPS at work with RTK differential correction. There is probably 10 grand per machine in hardware and software plus the £500/yr/machine subscription for the RTK signal. This gives sub centimeter accuracy. Most of the time. It does go wrong. It does f
k about. It is brilliant when it works. I wouldn't trust it much further than I could throw it. The only variables on a calibrated speedo are tyre wear (bugger all on a car) and inflation both of which will affect the rolling radius. You'll always get a reading rather than when all the planets align like with GPS. I doubt a cheap app on a phone is going to be that good.
Willy Nilly said:
The only variables on a calibrated speedo are tyre wear (bugger all on a car) and inflation both of which will affect the rolling radius. You'll always get a reading rather than when all the planets align like with GPS.
Err, except for when as other have pointed out wheels whacked on that may be different sizes than when the car was sold. Wheel size or tyre profile. Which affect the rolling radius as you say. GPS is the cheapest way to get a 98% accurate (guesstimation) comparison to your cars speedo on a good constant run.
OP has already said car doesn't have its original wheels.
It's also surprisingly common to find cars with slightly different width and /or profile tyres to the originals, usually due to cost or availability. If you're not the original owner, it's often hard to be sure what the correct size is.
It's also surprisingly common to find cars with slightly different width and /or profile tyres to the originals, usually due to cost or availability. If you're not the original owner, it's often hard to be sure what the correct size is.
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