I've had my '12 CTT for a little over a month and have close to 4,000 miles. I typically drive 80 on the highway and have noticed on many occassions I barely keep up with traffic or am constantly getting passed. I have a portable Garmin GPS so I decided to compare it with the reading on the digital speedo. On the highway I found the car's speedo read exactly 4 mph faster than the Garmin - 85/81, 80/76, 75/71. On local roads between 25 and 35 mph the car's speedo was 3 mph off. Has anyone else suspected the same? Does anyone know if the odometer is tied to the speedometer? If so, the actual mileage would be less than indicated.

 

BMW is the same way. Tech told me it was for safety reasons, making you think you're going faster than you really are.

 

My understanding is that this is a result of European laws. They face stiff penalties over there if the manufacturer produces a car that under-represents the actual speed. So everyone is extra cautious and over-represents the speed instead.

Not sure whether this is also tied into the odometer but it's a good question.

 

Funny, my last car was a '10 750 and I always felt I was going faster than indicated on the HUD!

 

mine is 4 mph slower than what spedo is reading.

probable due to the different wheel/tire combos that come stock, not sure....

 

I have 21s also. Shouldn't matter as rolling diameter is same for all stock wheel sizes.

 

I have noticed that using my VBOX in my CTT, GT2 and my C2s. The cars are always anywhere from 2 to 4 mph slower then what the speedo said it was. That is assuming that the VBOX is more accurate then the speedo....which is what I assume.

 

As others have posted, your actual speed is usually less than what's indicated on your speedometer. I checked the speedometer in my '12 CTT against my Garmin, and at both an indicated 55 and 65, the Garmin indicated 3 mph less.