This question came up in my mind, I'm sure a lot of you have the answers. Here goes, hopefully, the questions are clearly explained:

Comparing between different car makers (let's say Nissan and Toyota) using 15" and 18" factory rims, how do car makers adjust the speedo so we know when the speedo says we are traveling at 30 mph we are really traveling at 30 mph?

I'm 100% certain that the diameter of the rim + tire between 15" and 18" are not equal. So, if they are not equal, how are car manufacturers adjust it to be the same speed?

For consumers like us, we want our cars to be unique and look cool. So the custom wheel size are usually increased from either 18 to 20 or 18 to 19. Larger the rim size lower the profile for the tire. Otherwise, the speedo will either be slower or faster depdending on the combination is used. But as long as the total (tire + rim) diameter is equal to or almost the same as the original size that it came with the car manufacturer, the speedo will be accurate.

Just really curious... Any ideas?

 

You can also take it to the dealer and have it reprogrammed to have it match your new tire/rim setup.

- Don't argue with idiots. They will bring you down to their level and beat you with experience.=P-

 

Not too sure if I'm understanding your question but...

From what I understand, the car manufacturer calibrates the speedometer for the overall stock wheel size. So if it is 15" rim w/ X tire, the overall diameter is taken into account and the speedometer set / calibrated to that.

When you start to go aftermarket +1, +2, etc. then [as you noted] you need to plan what the new overall diameter is so that you either don't stray too far from what the speedometer was calibrated to or you at least know what your speedometer error is.

 

So there is a software program that controls the speedometer setup?
That means, if the total diameter is changed that greatly affects the speedometer's accuracy, then I can have dealer recalibrate the speedometer? Unless of course if I calculate the overall diamter to be the same as the original ones from the factory when I'm buying a custom wheels. Thanks for the info.

 

Prior to using ABS sensor as speed sensors, most makes just used a gear driven generator on the tranny tail shaft. A range of varying ratio plastic gears was available in 3 or 4% increments were fitted on the generator.

Inside the speedo assembly, an D/A convertor translated the frequency [driveshaft rpm] to a zero to 5 dc voltage which drove the speedometer [actually a zero to 5 volt meter].

Diff changes just required obtaining the correct plastic gear and swapping it on the speed sensor. Brand new to wornout tires creates a 2% change so 3-4% [step/increment selection]speedometer accuracy was deemed sufficient.

Currently the diff gear ratio is programmed into software via the Consult II.

 

Great detail info. Thanks Q45tech.