My Speedo is 100% Accurate !
Discussion
Hi Guys & Girls,
Just bought a Citroen C4 which has a digital speedometer.
I have compared the C4 Speedometer to GPS speed reading and the C4 speedo matched the GPS speed exactly or within 1mph at all speeds up to 90mph. (Not on a public road of course)
Cars with normal speedometers have never been this accurate, so is this Citroen C4 calibrated at the factory, and does anyone know of other cars that exactly match GPS speed up to 100mph ?
Just bought a Citroen C4 which has a digital speedometer.
I have compared the C4 Speedometer to GPS speed reading and the C4 speedo matched the GPS speed exactly or within 1mph at all speeds up to 90mph. (Not on a public road of course)
Cars with normal speedometers have never been this accurate, so is this Citroen C4 calibrated at the factory, and does anyone know of other cars that exactly match GPS speed up to 100mph ?
shuvitupya said:
Cars with normal speedometers have never been this accurate,
By "normal" you must be refering to old cars with cable driven speedo's. A lot of speedo's digital or not are electronic and are a lot more accurate than you think. "Scotty UK" has an S4 like mine, and at V max, he did 164mph, on his speedo, GPS and the official timing gear. There is a video of it on here somewhere.
Edit.. here is a capture from his 164mph run
Edited by FortJefferson on Wednesday 12th July 23:09
A car can't possibly know accurately how fast it's going, but only how fast its wheels are going round. Tyre pressures, load, tread depth, etc will all affect the conversion from wheel RPM -> car MPH. If the speedo is dead accurate, it won't always be at all speeds. The GPS is measuring exactly how fast its position is changing and is therefore always accurate.
SM
SM
shuvitupya said:
Hi Guys & Girls,
Just bought a Citroen C4 which has a digital speedometer.
I have compared the C4 Speedometer to GPS speed reading and the C4 speedo matched the GPS speed exactly or within 1mph at all speeds up to 90mph. (Not on a public road of course)
Cars with normal speedometers have never been this accurate, so is this Citroen C4 calibrated at the factory, and does anyone know of other cars that exactly match GPS speed up to 100mph ?
Just bought a Citroen C4 which has a digital speedometer.
I have compared the C4 Speedometer to GPS speed reading and the C4 speedo matched the GPS speed exactly or within 1mph at all speeds up to 90mph. (Not on a public road of course)
Cars with normal speedometers have never been this accurate, so is this Citroen C4 calibrated at the factory, and does anyone know of other cars that exactly match GPS speed up to 100mph ?
Couple of ideas here:
If it has SatNav, perhaps the speedo is calibrated against the GPS signal?
Secondly, thinking about it, speedos should be set so they are as close as possible when the tyres are new. As the tread wears the wheels will have to spin slightly faster to get the same speed however the speedo doesnt know this, all it knows is X revolutions per minute = Y miles per hour and thus overreads.
My S2 Elise had a very accurate speedo. Quite irritating following other cars with over-reading speedometers though!
However, the difference in radius between new tyres and old ones could be 6.4mm. On a rolling radius of, say, 300mm that's a 2% variation; if the speedo is not to underread at any point then it can only be perfectly accurate when the tyres are new. So with worn tyres, you're looking at a 2% overread.
However, the difference in radius between new tyres and old ones could be 6.4mm. On a rolling radius of, say, 300mm that's a 2% variation; if the speedo is not to underread at any point then it can only be perfectly accurate when the tyres are new. So with worn tyres, you're looking at a 2% overread.
SJobson said:
My S2 Elise had a very accurate speedo. Quite irritating following other cars with over-reading speedometers though!
However, the difference in radius between new tyres and old ones could be 6.4mm. On a rolling radius of, say, 300mm that's a 2% variation; if the speedo is not to underread at any point then it can only be perfectly accurate when the tyres are new. So with worn tyres, you're looking at a 2% overread.
However, the difference in radius between new tyres and old ones could be 6.4mm. On a rolling radius of, say, 300mm that's a 2% variation; if the speedo is not to underread at any point then it can only be perfectly accurate when the tyres are new. So with worn tyres, you're looking at a 2% overread.
Whilst the radius may differ by 2%, the margin of change when compared to the change in circumference is a lot smaller. Therefore a 2% change in speedo reading is an overestimate.
Edited by Lambochick on Thursday 13th July 18:00
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