I just re-geared my 2015 2500 from stock 3.42 to 3.73. The difference is noticeable, but not significant. The place that installed them said I had to take my truck to a dealer to have the ECM flashed and updated to accommodate the new gears. That made no sense to me as the gears are obviously mechanical not electronic or electrical. So I made it to 3 dealers. 2 stated that they had no idea *** I was talking about. The third said it would cost approximately $135 to flash the ECM, but was unable to confirm that it would change settings for the new gears.
Does anyone have an idea?

 

They probably mean that you need to update the speedo settings for the new gears. I the old day, you changed the speedo gear in the tail of the transmission. They had different color spoeedo gears to go with different ratios. Now it's a ECM setting.

 

It is as 123 said to reprogram the ecm for the speedo. You should be off about 9% with that gear change. I think it could also affect the shift points if an auto trans. A simple reflash/update isn't going to change it they need to go in and change the parameter.