I have always wondered why there has been a discrepancy between the speed on the speedo and calculated speed on my GPS - since this has happened on all but one truck I have driven AND my car I figured that there was an error in the calculations the GPS is doing - since it tends to read 5-7% slower than the speedo.
However on the M50 today there was a “radar” sign informing you just how much you are speeding through the roadworks (bad me 41mph) - which strangely enough matched the GPS reading, so I guess that is accurate after all ?!?! Is this some sort of conspiracy ■■
Any other GPS users noticed this ■■
I think car speedo’s have a 10% tolerance on speed, ie reading 55, should be between 50 and 60. Hence why speed camera’s only go off when a trigger level is reached, ie, in a 30 limit, camera goes off at 37 coz of 10% possible error in speedo plus a couple of mile an hour for luck. This relates to static cameras.
Some of the discrepancy can be due to worn tyres. If you had a truck from new it would have been calibrated on the new tyres. After maybe 2 years the tyres have worn down to nearly bald, which means you’ve lost some rubber. The tyre is now smaller than it was when new, it does more revolutions to the mile, this affects the gearing slightly.
Having said all that how even new trucks can be set way out, not sure why that is though.
It’s something to do with it being illegal for the manufacturers to allow the speedo to read under the actual speed of the vehicle. So they have to calibrate them to read any thing up to 10% higher.
The other point to remember, and this was pointed out elsewhere, is that when a truck tacho is calibrated on a rolling road, then it will be unladen and, particularly with an artic unit, once a trailer is connected, the tyres will be under a greater load and therefore the rolling circumference will be less.
Hence the speed shown will be higher than the actual speed because the tyres are being compressed to a greater degree.
Gazzareth:
I have always wondered why there has been a discrepancy between the speed on the speedo and calculated speed on my GPS - since this has happened on all but one truck I have driven AND my car I figured that there was an error in the calculations the GPS is doing - since it tends to read 5-7% slower than the speedo.
However on the M50 today there was a “radar” sign informing you just how much you are speeding through the roadworks (bad me 41mph) - which strangely enough matched the GPS reading, so I guess that is accurate after all ?!?! Is this some sort of conspiracy ■■
Any other GPS users noticed this ■■
GPS can’t account for elevation but apart from steep inclines/declines, it doesn’t affect it much. You’re triangulated from multimple points and so it’s very very accurate indeed.
car speedos aren’t calibrated and often have an inbuilt disrepancy to keep your speed down a few %.
truck speedos are only calibrated to the spec of the wagon on that day. by rights they should be re-done every time a new set of rubber hoops go on the rear axle(s) since a few mm over a circumference of a tyre, doing hundreds of RPM makes many metres per minute and hence miles per hour.
trust the GPS.
gps on a flat road at a constant speed is accurate 0.01mph