I got the 3.9 gallon (stretch tank to replace stock 3.3 gal) and I go to reserve almost same time my husband does with 3.3 gal stock tank. Of course I have more reserve than he does. In any case, I don't think people usually buy the stretch tank for the purpose of getting more miles between gas stops (to tell the truth I don't care how often I have to stop... I only do 60-80 miles between stops anyway).
You know I just noticed you said 125 miles. We can go 145 before switching to reserve, and then another about 30 miles after that on the reserve. When we went to CO the altitude caused bad gas mileage so it was about 135 when we went to reserve... never been down to 125 tho on a 3.3.