how accurate is the MPH reading on my SAT NAV?
Discussion
buzzer said:
I have just fitted a push bike speedo into my car, I was thinking of using the Sat Nav to calibrate it, rather than measuring wheel circumference and working it out.
How accurate is the sat nav given I would be at a steady speed?
How on earth does a push bike speedo work in a carr?How accurate is the sat nav given I would be at a steady speed?
If I ignore that part of your post, I think that sat nav speed displays are fairly accurate. I've noticed they always show a lower speed that the car speedo.
I think the only time that might not be true is if you're going up a hill, as the sats only measure horizontal time & distance. (I could be wrong!)
uk89camaro said:
How on earth does a push bike speedo work in a carr?
If I ignore that part of your post, I think that sat nav speed displays are fairly accurate. I've noticed they always show a lower speed that the car speedo.
I think the only time that might not be true is if you're going up a hill, as the sats only measure horizontal time & distance. (I could be wrong!)
Its easy... you use a wired type (the wireless ones dont work due to interference...) mount it on the dash and run a thin wire to a reed switch mounted where it can pick up rotation of a small magnet. On my caterham,that was a tiny magnet mounted in the cap head on one of the front hub bolts at the back of the disk. On my MX5 its from a magnet mounted on the rear drive shaft as per the picture. The reed switch in this case is one from an alarm system, its the same as the reed switch supplied with the bike computer, but a bit more robust. Lots of sports bikes fit these... they are much more accurate and have useful features like recording driving time, max speed, average speed, trip speed, miles that day and loads more depending on which type you get.If I ignore that part of your post, I think that sat nav speed displays are fairly accurate. I've noticed they always show a lower speed that the car speedo.
I think the only time that might not be true is if you're going up a hill, as the sats only measure horizontal time & distance. (I could be wrong!)
the pictures were taken before i tidied the wires and trimmed the cable tie!
uk89camaro said:
buzzer said:
I think the only time that might not be true is if you're going up a hill, as the sats only measure horizontal time & distance. (I could be wrong!)
A 1 in 3 hill would give you a difference of approx 6% between horizontal speed and actual speed, a 1 in 10 would be 0.6% buzzer said:
I have just fitted a push bike speedo into my car, I was thinking of using the Sat Nav to calibrate it, rather than measuring wheel circumference and working it out.
didn't realise pushbike speedo's had become that advanced, or indeed why you would worry about what speed you were doing, unless of course the bloody thing won't work unless you DO actually calibrate it.I have similar on the Westfield.
Your sat nav speed will be pretty much bang on if your holding a steady speed for several minutes its GPS speed very accurate as previous poster said his car speedo is usually showing a few miles an hour slower this is because they normally under read to keep you within the speed limit.
Have used our Garmin as a secondary check numerous times when assessing vehicle speedometers on official measured mile as used by traffic.
They were all on flat (ish) stretches of road and the Garmin always agreed closely with the speed as measured by calculation from chronometer timings, well within experimental error margins.
HTH
They were all on flat (ish) stretches of road and the Garmin always agreed closely with the speed as measured by calculation from chronometer timings, well within experimental error margins.
HTH
I've done something very similar (same installation, except it's a motorbike micro dash, rather than a pushbike unit. Has the advantage of backlight and 12v power rather than battery, but magnet/switch install is identical.) I set it appx using a tape measure on the tyre to get circumference. Then I went for a drive at steady speed and noted the indicated speed on GPS and on micro dash. I then used simple maths to work out by what % the micro dash was over/under reading and adjusted the circumference setting on the mirco dash by the appropriate factor and it's now bang on. So I get GPS level accuracy, but with no delay/lag. (I tired using a GPS based speedo before, but it was too slow to react to changes in speed. Hence the bike one being more suitable.)
So yeah, the GPS speed on your sat nav will be just fine for calibrating your bike speedo.
But if you want to go a stage further then you can head onto the motorway and look at the mile marker signs. You can time yourself between those, if you can drive at a steady speed, and use that as a benchmark too.
So yeah, the GPS speed on your sat nav will be just fine for calibrating your bike speedo.
But if you want to go a stage further then you can head onto the motorway and look at the mile marker signs. You can time yourself between those, if you can drive at a steady speed, and use that as a benchmark too.
Effortless said:
GPS is accurate +/- 1 mph steady state.
Cheers,
Effortless
Thanks, all calibrated now. Sad as it is, I also calibrated it by measurement... With the car on a flat car park I marked the tyre at the bottom with some chalk, and put a corresponding mark on the Tarmac. I then rolled the car forward until the wheel had rotated 15 times and the mark was at the bottom again. I the measured the distance between the two marks and devided it by 15 to get a. Rolling circumference.Cheers,
Effortless
I then used that to calibrate the computer! It now reads exactly the same as my sat nav! Interestingly, the speedo on my SLK55 AMG is spot on, something I have not seen before on a car...
Actually this came up on my "Speed Awareness Course" and so I can give you the official answer as supplied by "Kev" the comedy genius who ran the course.
"Sat Nav's are not at all accurate for speed measurement as 'the signal' has to go all the way up to the satellite and back again. Also, it will only ever tell you what speed you were doing 100yards ago so is no use whatsoever".
He also told us that stopping distances had not improved at all since the 1950's and the highway code was bang-on accurate even in a modern car.
As I paid £80 for this information it is obviously 100% correct.
"Sat Nav's are not at all accurate for speed measurement as 'the signal' has to go all the way up to the satellite and back again. Also, it will only ever tell you what speed you were doing 100yards ago so is no use whatsoever".
He also told us that stopping distances had not improved at all since the 1950's and the highway code was bang-on accurate even in a modern car.
As I paid £80 for this information it is obviously 100% correct.
I have a few Satnavs as well a proper RaceLogic AX22 GPS/accelerometer data-logger which uses a 5Hz GPS chip for extra accuracy.
Yes, provided it has enough signals a GPS will give a very accurate reading.
Most cars over-read on speedos by a small percentage. Notable ones for me:
Fastest: GPS 202mph vs speedo 206mph in Ruf GT2
Least accurate: My gallardo showed 190mph speedo at 177mph true.
Most accurate: Every car I've ever owned has been inaccurate apart from a huge V8 GMC Yukon 4x4 I hired for 2 weeks in the USA last month, which was completely accurate in the 50mph-90mph range. - Unexpected
Yes, provided it has enough signals a GPS will give a very accurate reading.
Most cars over-read on speedos by a small percentage. Notable ones for me:
Fastest: GPS 202mph vs speedo 206mph in Ruf GT2
Least accurate: My gallardo showed 190mph speedo at 177mph true.
Most accurate: Every car I've ever owned has been inaccurate apart from a huge V8 GMC Yukon 4x4 I hired for 2 weeks in the USA last month, which was completely accurate in the 50mph-90mph range. - Unexpected
snapdragon69 said:
Sat nav speed becomes less accurate on hills, as the distance to change elevation is not accounted for.
e.g. if you are going up a 45˚ slope at a 30mph road speed, the sat nav will only show 15mph.
Meanwhile in the real world....e.g. if you are going up a 45˚ slope at a 30mph road speed, the sat nav will only show 15mph.
You are driving up or down a 1 in 10 hill. You have a gps unit that is only displaying horizontal velocity either due to capability or only having lock on few satellites so cannot make an accurate calculation of altitude.
The indicated speed will only be 0.5% out, too low if climbing, too high if descending.
I wouldn't worry about that frankly.
snapdragon69 said:
e.g. if you are going up a 45˚ slope at a 30mph road speed, the sat nav will only show 15mph.
The horizontal speed when travelling up an incline of 45˚ can be found by speed * cosine(45˚) - which is speed * 1 divided by the square root of 2 (or approx .707) - so at 30 mph up incline of 45˚ is about 21mphIf you are going up a 1 in 3 (3 metres travelled along slope gains 1 metre in height) - Pythagoras says you will travel sqrt(3 * 3 - 1 * 1) along ground for every 3 meters travelled along slope = which equals about 2.82 metres - so going up a 1 in 3 (steepest on UK roads I believe) at 30mph your sat nav will tell you 28 or less than 7% error, at 1 in 10 (typically steep hill seen on roads) will give you a about 0.6% error
magnus911 said:
jackwibble said:
previous poster said his car speedo is usually showing a few miles an hour slower this is because they normally under read to keep you within the speed limit.
faster / over-read surely?Duncan
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