I installed a Garmin GPS on my Smart and the speed reading (mph) on the GPS is consistently 3 miles slower than the speedometer on the car, regardless of the speed. E.g.
if GPS shows 37, speedometer shows 40
if GPS show 27, the speedometer will show 30
if GPS show 57, the speedometer will show 60
My tires/rims are original, tire pressures are set to recommended values (29 front, 36 rear).
I tended to believe the GPS more than the speedometer.
Is it normal? When the GPS is used on my other car (Honda Accord 2013), their readings are matching just fine.
Can it be adjusted, calibrated somehow?
May be I should over-inflate the tires a little?
Car companies calibrate their speedometers to read faster than actual speed to keep from being sued by owner if they get a speeding ticket. This is very true in Europe where the company can be sued for this. I owned a 2006 Suzuki wagon and I was doing 70 mph by speedometer but trucker stated he was doing 65 mph alongside me. Had Suzuki replace speedometer that read correct under warranty.
Got a cheap OBDII reader with bluetooth that can display the OBDII view of car speed on my phone.
Among the speed readings of these 3 devices (Speedometer, OBDII and GPS),
GPS and OBDII are very close to each other but the Speedometer reading is consistently 3 mph more than the other 2.
Draw you own conclusion...