WOW. I missed that.

I have OFTEN said, "Anyone changing plugs on a 15 year old car and not also changing the plug wires is making a mistake".
Personally, twice, I changed plugs (on other vehicles) and made things WORSE because the act of wrestling the plug wires off and back on was enough to injure at least one of them.

I made a new rule: "If the engine needs plugs, it needs wires as well".

As to the odometer, the data that drives the odometer is the same data that drives the speedometer. This data comes from the ABS controller and I'd bet (don't really know) that the same mini-module in the cluster drives the speedometer and odometer. Since the speedo is correct, the only way the odometer would likely be off is if the time base for each function was different and the one for the odometer was off. But I doubt there is a separate time base for the two functions. As nutty as it all sounds, I'd bet he really IS getting 9mpg. But You'd think his plugs (and even exahust tips) would show it, as you have indicated.