Curious speedo question

So I noticed a small 5ish% discrepancy in my speed reading as follows:

Bought truck some years ago, only had cluster speedometer for reference

Upgraded head unit with one that connects to OBD port and has all kinds of fancy data displays, including vehicle speed. Happened to notice a while back that this speed that this number is about 4-5% lower than what the dash shows. Figure maybe there’s just a little error margin in the dash

Then recently, using Apple CarPlay, google maps now displays current gps speed along with speed limit.

Realized the GPS speed is dead-on-bawls in agreement with the dash speedometer, throughout all speed ranges

I don’t give half a turd about the discrepancy itself. I never reference the head unit speedometer anyway(though now I wonder if it may be throwing off my MPG readings..).

It just makes me curious, does this indicate my Speedo may have been adjusted for larger-than-stock tires at some point? I don’t know the procedure for this, if there is one. I bought it preowned from a Yota dealer with new stock highway tires, and then promptly sized up a little with the grabber A/Ts.

Or is this another case of the ECU displaying “estimated” table data instead of actual data, like the EGT readout?

 

The Meter receives the Vehicle Speed Signal from the Skid Control ECU (ABS module), uses it for speedometer and odometer functions and retransmits the same signal to the rest of the modules on the SPD circuit.

As you can see by the scope capture below the signals in and out are identical.




The only way to change the calibration to correct the speed for tires or gear changes is to modify the speed signal input to the Meter which would also change the signal output, several companies make such modules.

For more information on how the vehicle speed system operates see the post below (scroll down to P0500).
https://www.tacomaworld.com/threads/2nd-gen-information-diagnostics.784008/#post-28042157

 

Huh, that’s interesting. Seems obvious now that you mention it but I’ve never thought to check for a speed correction module. Either there may be one that the dealership missed, or maybe it’s just a happy accident that the meter happens to be a little off in the proper direction. The truck was largely clean and unmolested when I bought it. I’ll have to dig around eventually and see what I see

thanks for the input/info!