Speedometer versus digital speed display

[dparm]

I'm struggling to understand this after coding -- why does the speedo gauge read a few MPH higher than the digital readout in the cluster? The anecdotes I see on here indicate that there is a certain fudge-factor built into the speedometer, and that through coding this is "corrected" and shown in the digital display.

So why does the speedo intentionally read high? Is the digital speedo showing the vehicle's true speed now?

[warp10]

the speedo is a few % points off most cars are like this. If you got the digital speedo coded in to show accurate speed then yes its the actual speed.

[dparm]

Originally Posted by warp10

the speedo is a few % points off most cars are like this. If you got the digital speedo coded in to show accurate speed then yes its the actual speed.

Right, but why is the analog speedo intentionally skewing the speed? Obviously it cannot be 100% accurate all the time, but if it is KNOWN to be wrong, why not correct it? Is it at least consistently inaccurate across the entire range?

[warp10]

I am not sure why BMW and other Manufacturers do this but it probably has something to keep speeds down. so if it says your going 80 but really 75 then you wont really speed as much. But your guess is as good as mine with why they do it?

[1sexyfd]

Originally Posted by dparm

Right, but why is the analog speedo intentionally skewing the speed? Obviously it cannot be 100% accurate all the time, but if it is KNOWN to be wrong, why not correct it? Is it at least consistently inaccurate across the entire range?

I'm pretty sure that you can code both to show a closer representation to actual speed.

As far as why they are high, I can only speculate for the US. I know European cars have a specific law that states they can never show a speed on the display that is lower than the actual speed. I think it is ECE-R39. (not entirely sure tho) I don't think the US or Canada have any specific laws in terms of what speed they show. Also, the speedo is calibrated for a certain overall wheel/tire size combo. So any variations in size (under/over inflation) or a completely different wheel size can mess with that reading a little.

I would think that automakers, lawmakers and insurance companies would much rather have a person going 60 thinking they are going 75. (Greatly exaggerated, of course.) Less tickets, and less risk of injury and damage to personal property. I guess on the flipside tho one could argue how much does 5 MPH (more realistic difference) actually change in all of that? Except a speeding ticket. Anything in the opposite direction has no benefit. So I think....

[Craigy]

Originally Posted by warp10

the speedo is a few % points off most cars are like this. If you got the digital speedo coded in to show accurate speed then yes its the actual speed.

We can code the speedo to be more accurate?