A number of months back, I drove a 2013 Volkswagen CC and noticed, while driving towards an automatic traffic speed radar, that the indicated digital speedometer speed (shows up between the gauges) in the Vdub was 3 km/h over the radar’s reading.
I noticed similar occurrences with other cars over the years, but never thought much of it. I figured that not all speedometers are created or calibrated equally; therefore, there was nothing to it.
However, after the CC, I kept a watchful eye on indicated speed limits as I travelled past radars that post oncoming traffic speeds. The route I take from my in-laws to get home includes a short stretch of road that has a radar permanently set up.
I take this road on a weekly basis and I notice there is a clear trend with today’s cars.
Regardless of the car (or CUV, pickup, SUV or minivan) I drive past said radar, the posted speed is always lower than the indicated speed in the vehicle I’m driving.
The variations in speeds are nominal; the range is always between 3 and 4 km/h lower than indicated in the car.
I performed the same exercise with my 10-year-old Subaru WRX and noticed the same gap; less than a 5 km/h spread between the radar and my car.
Can anyone guess why our cars would show a speed that is actually higher than the one we are travelling? Safety? Probably, but it’s not only that.
So what does it mean? Not much in fact. I say we use this small speed discrepancy as a buffer, just in case... The real question is: What’s really going on with the odometer? Think about it.
The two are intimately related, and both the speedo and odo run off similar feeds. Let’s say that your speedo indicates 34 km/h when you’re actually going 30. Figure that 30 fits 3.333 times in 100, multiply the 4 km difference by 3.333 and you get 13.33. So, this means that 13.33 extra kilometers are tacked on every 100 km travelled?
No, it does not.
Readings for the speedo are purposefully off for a number of reasons, and they go a little something like this: As a tire wears out, its overall diameter changes, thus changing its speed of rotation. Tire pressures also have a comparable effect; same goes for when we switch from summer to winter tires. Occasionally, the exact overall diameter of the rotating tire and wheel combo are not the same as the OE size. As well, can you imagine the lawsuits if a manufacturer “underestimated” the travelled speed? The odometer, on the other hand, measures real distances.
Many of you who own a portable navigation system probably noticed a discrepancy between the posted speed on the unit and on the car’s speedometer. Now you know why there is a difference and, yes, the $300 is right but no, the $50,000 car is not wrong either; it’s purposely giving you the wrong reading.
I noticed similar occurrences with other cars over the years, but never thought much of it. I figured that not all speedometers are created or calibrated equally; therefore, there was nothing to it.
However, after the CC, I kept a watchful eye on indicated speed limits as I travelled past radars that post oncoming traffic speeds. The route I take from my in-laws to get home includes a short stretch of road that has a radar permanently set up.
I take this road on a weekly basis and I notice there is a clear trend with today’s cars.
Regardless of the car (or CUV, pickup, SUV or minivan) I drive past said radar, the posted speed is always lower than the indicated speed in the vehicle I’m driving.
The variations in speeds are nominal; the range is always between 3 and 4 km/h lower than indicated in the car.
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| Photo: Sébastien D'Amour |
I performed the same exercise with my 10-year-old Subaru WRX and noticed the same gap; less than a 5 km/h spread between the radar and my car.
Can anyone guess why our cars would show a speed that is actually higher than the one we are travelling? Safety? Probably, but it’s not only that.
So what does it mean? Not much in fact. I say we use this small speed discrepancy as a buffer, just in case... The real question is: What’s really going on with the odometer? Think about it.
The two are intimately related, and both the speedo and odo run off similar feeds. Let’s say that your speedo indicates 34 km/h when you’re actually going 30. Figure that 30 fits 3.333 times in 100, multiply the 4 km difference by 3.333 and you get 13.33. So, this means that 13.33 extra kilometers are tacked on every 100 km travelled?
No, it does not.
Readings for the speedo are purposefully off for a number of reasons, and they go a little something like this: As a tire wears out, its overall diameter changes, thus changing its speed of rotation. Tire pressures also have a comparable effect; same goes for when we switch from summer to winter tires. Occasionally, the exact overall diameter of the rotating tire and wheel combo are not the same as the OE size. As well, can you imagine the lawsuits if a manufacturer “underestimated” the travelled speed? The odometer, on the other hand, measures real distances.
Many of you who own a portable navigation system probably noticed a discrepancy between the posted speed on the unit and on the car’s speedometer. Now you know why there is a difference and, yes, the $300 is right but no, the $50,000 car is not wrong either; it’s purposely giving you the wrong reading.