Incorrect speedo

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grimree

Active Member
Joined
Jul 4, 2015
Messages
162
Location
North Beds
Car
E Class Cabriolet
The speedo in my 2021 E class Cabriolet is indicating 2mph over the actual road speed. In an Audi A5 I had I could have corrected this via the OBD port, is that possible with the Merc and can only the Main Dealer do it?
The reason I ask is because I've just had a service carried out and asked for the fault to be rectified (Main Dealer) but nothing was done, if the correction can't be done then I'll shut up, if it can then the Service Manager can expect a phone call
 
99% of all road cars overread deliberately from the factory. All my cars have when compared to real GPS figures. Its because its illegal to sell a car in Europe with a speedo that underleads so they err on the side of caution. 2mph is neither here nor there....my Ford Ranger is over by about that at 30......its about 7 mph over at 80!
That's why fitting bigger tyres, contrary to popular belief, instead of making your car speedo under read just improves accuracy!!


EDIT....beaten too it!!!
 
Exactly that ! When I fitted 17 inch wheels to my S-type to replace the 16 inch the speedo read the exact same speed as the GPS.
 
The speedo in my 2021 E class Cabriolet is indicating 2mph over the actual road speed. In an Audi A5 I had I could have corrected this via the OBD port, is that possible with the Merc and can only the Main Dealer do it?
The reason I ask is because I've just had a service carried out and asked for the fault to be rectified (Main Dealer) but nothing was done, if the correction can't be done then I'll shut up, if it can then the Service Manager can expect a phone call
What are you comparing your Speedo Reading with? 🤔
 
Exactly that ! When I fitted 17 inch wheels to my S-type to replace the 16 inch the speedo read the exact same speed as the GPS.

It's the rolling radius (i.e. tyre circumference) that determines what speed is displayed. Bigger rims are fitted with lower profile tyres to keep the rolling radius the same (within a small tolerance).
 
The speedo in my 2021 E class Cabriolet is indicating 2mph over the actual road speed. In an Audi A5 I had I could have corrected this via the OBD port, is that possible with the Merc and can only the Main Dealer do it?
The reason I ask is because I've just had a service carried out and asked for the fault to be rectified (Main Dealer) but nothing was done, if the correction can't be done then I'll shut up, if it can then the Service Manager can expect a phone call

2 mph at 30, or 2 mph at 70?

The displayed speed is based on an assumed distance travelled for each wheel rotation (i.e. the circumference of the tyre). This reduces as a tyre wears (going from 8mm tread depth when new to 1.6mm legal limit means the diameter has dropped by 12.8mm), so it can't ever be 100% accurate. As mentioned, by law a speedometer isn't allowed to under-read at all (but it's allowed to over-read by 110% of the true speed + 6.25 mph!). So manufacturers always set them to over-read a little to take account of variations in tyre diameter, manufacturing tolerances, etc. and never show less than the actual speed.
 
I worked this out for my own tyres but most saloon cars will be similar:

1mm of tyre wear changes the rolling circumference by 0.31%

During the life of a tyre from new to the legal limit (6mm) the rolling circumference changes by 6 x 0.31 % =1.86%

So besides all the other reasons such as tyre pressure, the law insisting that a speedo can't under read and variability/tolerances between manufacturers, it's impossible for a car speedo to be accurate to better than 2% on tyre wear alone.

Taken all together an error of only 2% error is a good result.
 
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The speedo in my 2021 E class Cabriolet is indicating 2mph over the actual road speed. In an Audi A5 I had I could have corrected this via the OBD port, is that possible with the Merc and can only the Main Dealer do it?
The reason I ask is because I've just had a service carried out and asked for the fault to be rectified (Main Dealer) but nothing was done, if the correction can't be done then I'll shut up, if it can then the Service Manager can expect a phone call
This really is a non problem and no manufacturer can guarantee complete accuracy especially with so many variables such as tyre wear etc.
 

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