Speedo Calibration

[Martin4356111]

Afternoon all,

I just picked up my new Focus from the dealer on Tuesday. Yesterday I noticed that the analogue Speedo reads around 15 mph more than the digital one.

Having done a couple of rudimentary tests (following my mate at 30 mph), it's the analogue clock that out, definitely by around +15.

I'm going to take it in tomorrow but have one question. Would this mean that the miles being added to the mileage clock would also be wrong?

Thanks in advance.

[Allan L]

Martin4356111 said:

Would this mean that the miles being added to the mileage clock would also be wrong?

That's the easy one to check.
Motorways have numbered kilometre (and tenths thereof) posts so note the mileage reading (or zero the trip recorder) and the post number as you pass one of those posts and do the same as far along the motorway as you can. Compare the distance recorder reading with the motorway distance and there's your answer!

PS to save you looking it up 1 mile = 1.609344 km.
PPS I've just noticed you are in Scotland so perhaps not too near a motorway - however some of the old main roads had and still have milestones

Edited by Allan L on Friday 10th November 14:20

[carjerk]

I remember back in the 90's my grandad worked out that his Lada's speedo was calibrated a lot lower than the actual road speed. I swear Lada did it on purpose so the old gimmers that were buying their cars drove them faster than 20mph.

He took it back to the dealer and they promptly re-calibrated it with no fuss.

It will have affected the mileage on your odometer slightly.

[anonymous-user]

My Kawasaki runs about 10 to 12 %fast of a GPS speedo at 70 km, my place to the border in Thailand is 478km all on duel carriage way milestone, milometer reads a consistent 486, so not the 10%. I assume this is because the Milo is gear driven, while the speedo needle works of some form of induced current being generated by the spinning cable in a magnetic field. ( Not a expert on instruments but there is no direct gear drive from the needle to the speedo cable).

[Steve_D]

Berw said:

My Kawasaki runs about 10 to 12 %fast of a GPS speedo at 70 km, my place to the border in Thailand is 478km all on duel carriage way milestone, milometer reads a consistent 486, so not the 10%. I assume this is because the Milo is gear driven, while the speedo needle works of some form of induced current being generated by the spinning cable in a magnetic field. ( Not a expert on instruments but there is no direct gear drive from the needle to the speedo cable).

As you say all done with magnets. In simple terms a magnet on the drive spindle tries to pull the steel needle against a spring. one pass of the magnet does not have enough pull to move the needle against the spring. Pass it over 10, 50, 100 times there is enough pull to hold it up. Calibration would either be moving the magnet closer or adjusting the spring tension.

Steve

[anonymous-user]

The law requires the primary speedometer to read with +10% and -0%, ie it must never under read the cars true velocity.

On modern cars, the CAN bus speed data from the ABS will be pretty accurate (usually calibrated to be around +1% on full tread depth, and falling to around +3% on worn tyres), but the speedo itself is often calibrated to show up to approx +8%. Generally, the trip counter integrates the more accurate data value.