How does my ecu know how fast I'm going?
 

...And why does it care. I noticed that vehicle speed was displayed among other things on my code reader. Stock '97 with cable to speedo. From the speedo itself I guess? Why?

aliens

There is a wire that goes from your gauge cluster to your ECU.

Should I call someone? But they might be cool cats and want to hang.

But what if they harvest farts?

Thanks, That's what I thought. What difference does road speed make?

OUT man! They're out.

VSS into the ECU can be as simple as using it as an input for a slightly more advanced idle strategy. Or it's just an OBDII requirement.

most (all?) modern ecu's use vss for idle control. dunno how many years that goes back, but if you lose vss input on a modern car it throws a whole truckload of engine codes that seemingly have nothing to do with vss

VSS is useful for a lot of things in modern ECUs. Stability control, traction control, cruise control, airbag management, boost-by-gear (divide speed by RPM and you can figure out what gear it's in), all kinds of stuff relating to automatic transmission control, the list goes on.

As for your NA, yes, the speedo cluster converts the mechanical wire into an electrical signal and sends it to the ECU. Many of the above don't apply to an NA, but it's still useful in an OBD-2 ECU for debugging issues in the car -- VSS gets stored along with the other freeze frame data, which may help someone figure out what triggered the code.

--Ian

I honestly had no clue that a signal is converted and sent back to the ecu on an NA. I need to dig around tuner Studio or see if it's wired up. Boost by gear may be great since I'm losing traction in third.

I don't think the 1.6 cars have the wire, but you can run one.

there is a digital conversion in the cluster itself, that you can tap for an aftermarket ECU.

I'm not sure how nice it would play with your dual-table open loop flex-blended boost control though. Otherwise yes, BBG is useful on a monstrous Miata like yours

Thanks to all above! Don't quote me on this yet but it may also sense what gear I'm in. Will check. Also sounds like Big Brother is in my 20 year old Miata, too.