Swordfish99

Do you guys notice your speedometer is out a lot if you are driving at highway speeds? Mine is out about 7-8 km/hr when driving 110 km/hr. Mine were out with the original tires and wheels and I was hoping the new winter wheel package would fix it but it is out the same amount...

mikapen

Mine is pretty much the same reading as my radar detector and Waze. Mine's a GLE 53 with 21's.

How are you comparing speeds? What size tires? Do you show slower or faster?

Swordfish99

I also have 21 inch tires on mine. I have my radar detector (Redline 360) and portable Navigation device and both show the same however the car says I am going faster. The faster I go, the more it is out.

superswiss

That's normal. Speedometers don't actually measure the real speed of a car. They measure the rpm of the wheels and then convert that to speed based on the tires' circumference. This circumference constantly changes with the weight of the car, ambient temperature, tire pressure and tire wear, so to avoid speeding tickets, most speedometers read higher than your actual speed. It might be more accurate on a cold morning when the tire sidewalls are still stiff, than on a warm afternoon with the tires warmed up and the sidewalls being softer. Radar detectors and your phone apps use GPS to measure actual distance travelled in a given time, which is the real speed of the car, however, even radar detectors may intentionally read high to keep you from getting speeding tickets, especially as many newer radar detectors can factor in speed to decide when to be silent and when to alert.

https://www.roadandtrack.com/car-cul...ters-accurate/

Ron.s

Mine is like yours…so accurate I keep scratching my head. I have the new R7 Uniden Radar Detector and it is always exactly the same or within 1 mph when the RD needs to catch up. I have checked mine against the roadside Radar and had a nice policeman pull me over to confirm the accuracy at 10 over posted. No ticket just a warning👍

MBguy013

GregW / Oregon

Mine seems pretty accurate (21s).

Philamg

In the UK all speedometers read OVER the actual speed, it is illegal for them to read under, the over reading increases the faster you go.

js_cls

Pretty typical for German cars, my BMW’s are usually 3MPH over the actual speed.

TexAg91

Mine is dead on accurate when checked against GPS…all speeds.

Zeusmotorworks

21's (all seasons) dead nuts on per GPS

HotRodW

Our GLC and GLE are both spot on, unlike my BMW's which consistently indicated 79mph when cruising at 75. Our Audi and VW speedometers were accurate, too.

Lucky 777

Mine is very accurate

CMFUSA

Just checked mine. Dead on with 21 Pirelli Scorpions.

EddieKBG

I have 22" wheels and it's 5-6mph slower against both radar detectors 🥺

Robm974

Were you able to get your speedometer fixed? Mine is out a similar amount - very annoying- and the dealer said no sensor going off therefore nothing to fix.

TexAg91

Modern speedometers are computer based, sensors count the wheel revolutions per second and multiple it by the tire size (specifically the tire diameter) to calculate speed in MPH/KPH.

If your speedometer indicated speed is higher than your actual speed, it means the computer is using the wrong wheel size (diameter) for what’s actually on the vehicle (using a smaller tire size). The dealer should be able to check the tire parameter values being used in the vehicle’s software to verify they match the actual tire size on your GLE. I suspect they don’t match. Perhaps someone changed out the rim tire combo being used and didn’t update those values in the vehicle software?

Robm974

thanks for your response. I had the car (GLC 43 Coupe) into the MB dealership where they said there was nothing they could do. I plan to go back and ask them to pursue it further, seems very strange to me for a high end car to be out 5-7% on the speedometer.