Is there something magical in correcting an '08 Corvette speedometer calibration???

[JRHAWK9]

I sent my '08 Vette ECM (6 speed manual) to a very well known and respected tuner who has tried twice now to get my C6 speedometer to read 2.5% slower than what it's currently reading (stock calibration) to compensate for my 305/30/19 rear tires...doesn't seem too tough, right?

Well, I sent my ECM to this well known tuner to have their mail order tune done and have the speedometer calibrated. I gave him my tire size along with the other mods. Well, I get the ECM back, install it and take it for a ride. Low and behold I'm still off the IDENTICAL 2.5% I was before. I call them and he looks up my file and it shows that he did change it to my new tire size, but for whatever reason it didn't work. He told me to send it back to him and he will input it manually and that I have to just get him the speed I am actually going as well as the speed the speedometer thinks I am going. I do this and send the ECM back. He does this and sends it back to me once again. I just got back from taking it for a ride after the second try and it's -STILL- off the same exact amount it was before. Whatever is being changed is not doing a thing. What is the magic combination of check boxes and numbers in order to change a late model C6 speedometer calibration?

So, with my current wheels/tires installed, running the stock calibration = 2.5% fast, first attempt at changing the calibration = 2.5% fast, second attempt = 2.5% fast. If I were to put my stock wheels back on the speedometer would once again be accurate. Whatever is getting changed isn't affecting my speedometer reading.

All speeds were monitored using the same GPS in all three comparisons on the same stretch of FLAT road.

Any help would be greatly appreciated!
Paul