How are RPM'S and MPH recorded by Nissan and others. Do they use the cam as a pickup for RPM's and what is the pickup for mph (distributor, fly-wheel, what?). Does changing rim size (17 vs 20") affect the speedometer or with modern computer technology is the system able to automatically adjust to the change and give the correct reading even with the rather dramatic change in rim & tire size.


Usually in newer vehicles (probably since about the early to mid 90's or a bit earlier), most vehicles use an electronic pickup somewhere in the final drive of the transmission to give a speed reading. Something like a Hall Effect sensor is used (usually) to give the computer a count, which is read over time and thus the computer can tell how fast the vehicle is going, which it then commands the speedometer to display so you can read it. It only know this count and then does its thing. If you change the rolling diameter of the tire, the computer doesn't know the difference, and thus your display is going to be screwed up. To fix the issue, you have to go into the computer and "tune" it to understand what the new "count" should be over time so it can then display it correctly. This can be done through a tuner shop or there are some handhelds on the market which can do it as well.

The RPMs are not used in any of these calculations. RPMs denote the engine speed and not the vehicle speed. Since most vehicles have gears in the transmission, any time the tranny changes up to the next gear, the speed of the engine goes down, but the vehicle speed usually goes up or stays the same. Due to this, it wouldn't make sense to try and utilize the engine speed to calculate. This also alleviates any issues with slippage in the transmission (especially an automatic) and gives a more accurate reading.

  • A bit more on fixing the issue of the incorrect speed with larger/smaller wheels. You can get 'speedo healers' which you wire in a program to fix the speed(calibrating it with a gps speedo). I believe they work by altering the signal between the ecu and pickup. – DamoC Jan 1 '18 at 23:05
  • SPEED is usually determined using ABS's wheels sensor. Different RIM size is not necessarily correlated with different wheel circumference. Foe an extreme example, MINI offers rims from 15 to 18 inch but the external tyre size is the same. Google the meaning on the marking on the tyres to understand :) – Caterpillaraoz Jan 13 '18 at 16:11
  • @Caterpillaraoz - Which cars are you talking about when you say "speed is usually determined using ABS's wheels sensor"? The only cars which I've dealt with which do are ones which have been modified by the owner to use the sensor at the wheel. References would be great! – Pᴀᴜʟsᴛᴇʀ2 Jan 13 '18 at 19:07
  • The very most of the car from 2000 onwards. The most recent also use GPS as calibration – Caterpillaraoz Jan 14 '18 at 16:21
  • @Caterpillaraoz - And I should believe you because you say this on the internet? – Pᴀᴜʟsᴛᴇʀ2 Jan 14 '18 at 18:27

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