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I drive a manual Honda City with cruise control. I've noticed that if I press + three times, the speed increases by 5 km/h. Pressing - three times also decreases the speed by 5 km/h. This is very close to 3 miles per hour (4.828032 km/h). Six times is ±10 km/h, and so on, although it doesn't always sit exactly on multiples of 10 when I get to that speed.

When I get to 100 km/h the speed (as measured by a GPS app) is usually either 99 km/h or 101 km/h when I'm using cruise control. I.e. if it's 99 and I press + it becomes 101, 101 and - becomes 99. Of course the speed varies slightly for hills, etc. so it can sometimes go slightly above or below before correcting itself.

Does the cruise control actually adjust in miles even though the speedometer measures in km/h?

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The Honda City Owner's Manual states:

  • Each time you press the button, the vehicle speed is increased or decreased by
    about 1.6 km/h.

Cruise control instructions

There are exactly 1.609344 kilometres per mile, since the definition of the modern mile is based on metric. This is "about 1.6 km/h" as stated in the Owner's Manual. It is very likely that the cruise control is calibrated to increase and decrease in 1 mile per hour increments. Only an engineer with detailed knowledge of the cruise control system could answer whether 1.6 km is actually intended to be 1 mile or not.

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Usually the button press tells the ecu to adjust by X, which is a value added to the pulses received from the speed sensor (or the abs sensor rings).

This means that the number of pulses per mile or per kilometer will also be dependent on the diameter of the wheel / tire combination.

On another forum we discussed this and the opinion is that it matches fine for a particular size of wheel / tire (say 16” and a 55 profile but when a 17” wheel with a 45 profile is used which has a similar number of revolutions per mile or kilometer then the cruise control adjustment may not hit 100km/h exactly.

There are some sites that do tire comparisons and these can be useful (try this one). I compared a 16" tire to a 17" just out of interest and these were the results, note the number of revs per mile:

enter image description here

Do note that the gps speed is only an indication - I have noticed on bendy roads that the gps value “drifts” sometimes compared to the car...

Edit 2, UPDATE I have just checked the Jaguar X type workshop manual and the pertinent information is that the speed signal is from the abs (can bus via ems) and the increment is 2km/h or 1mph, which you will note is not an exact conversion. So see: enter image description here

  • @CJDennis so do I, but as my gps is mounted up by the rear view mirror it is easy to glance at as prat of checking mirrors etc... If that is all you take from my answer.... – Solar Mike Aug 24 at 6:34
  • @CJDennis so you did not read my answer... – Solar Mike Aug 24 at 6:45
  • @CJDennis now check how many pulses are used to represent the speed input to the ecu... then you can work out X... – Solar Mike Aug 24 at 6:56
  • I don't want to get into technical details. I just want to know if each click is calibrated to be a mile, even if the actual tyre diameter means it's not exact. If I teach you to count five apples by saying: "1.6 apples, 3.2 apples, 5 apples", you'd think that was weird. – CJ Dennis Aug 24 at 6:59
  • @CJDennis They are counting pulses, like 18 per revolution or 24 per revolution, not apples or oranges... That revolution count could be one or an average of the abs sensors or a sensor on the gearbox output shaft as that will be a lower frequency than the abs signal by a large factor due to the gear ratios. Frequency is measured in cycles per second, called Hertz abbreviated Hz. – Solar Mike Aug 24 at 7:05

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