I'm guessing it is the ECU but not sure.

What happened was, I was looking for a civic for my daughter. Found a clean looking 2004 reasonable price with 175k. That's where the problem started.

It was on a small dealer lot. I asked about mileage and was told about 175k. started car and display showed the same. Was about to buy when the dealer couldn't get the title number to work in the DMV system. It showed a newer title existed. I asked to see the title and it showed it had been traded at the original Honda dealership and reassigned through several dealers. The trade in odometer statement signed by the original owner was there and he had signed that it had a bit over 311k miles.

So how did the mileage get to the lower number, An ECU swap?

 

The odometer data is stored in the gauge assembly (i.e., cluster). There is a CPU built-in the gauge assembly.

 

if thats the case then it seems like that dealer was trying to pull some sort of scam. i never trust used car dealerships

 

You can swap the cluster to one with lower mileage, rather easily. I'd check the VIN for accuracy, and if the car has logged more miles then advertised... I'd walk away.