This range increase is provided by the addition of two long range fuel tanks in the wings.
Range varies with weight and mixture leaning.
In addition to increasing the range, it allows your Mooney to run at higher power setting, thus achieving higher cruise speeds for long trips where it is most desirable, without lengthy fuel stops. The Long Range Upgrade allows the Mooney Airplanes to perform mission profiles rivaled only by more expensive aircraft such as some turboprops, business jets and airlines. The cost of these tanks can be recovered in about 15 long trips or less depending on how much you value your time. There is no other upgrade or modification that can improve the cruise performance by so much.