The OEM calibrations are very conservative. They have to account for the guy that finds 85 octane and drives uphill to and from work, WOT the whole way

They have to make sure it can live through that. They rely heavily on the knock sensors as well. Spending time (days, weeks) with the knock and spark calibrations can lead to impressive results.

 

That would be .81 MPG
Not bad at all. Not sure how you calculated that net. 17.4 was your actual mpg based on your numbers. To get 18.13 MPG on that many gallons you would have had to have driven 18408.66 miles.

 

I had to subtract the negative impact of the towing from the pre-CTS average as I haven't had a chance to tow the boat with the CTS yet.

 

Ok, so you still gained .81 mpg with your custom tune. Not too shabby.

 

When I subtract the negative impact of towing, I have a pre-CTS MPG of 18.13.

Post custom tune it's 18.21.

Actual gain is .08.

 

OK, actual is based on miles traveled per gallons used. Your actual gain was .81 MPG. Your predicted towing adjusted gain was .08. I see your point though as you have not had a chance to tow the boat and do an actual comparison.