DON'T DO IT!!! You'll be giving the torque converter a really hard time! (I'm assuming the Jag has a torque converter and isn't one of these more modern "DSG-type" auto boxes, but I think it is a conventional one). If it is, the big mismatch between road speed and engine speed puts a big strain on it (hence why you're not supposed to tow them above about 20 MPH). Also, putting them in and out of gear increases the wear rate on the clutches inside the auto box (again, assuming a conventional auto). The advice for optimum gearbox life, is to leave it in drive all the time, even when idling at lights, because each time you go in and out of gear you engage and disengage these clutches. Aside from that, as has already been said, you won't save any fuel. Over-run fuel cut-offs have been pretty much standard now for over 20 years. When the throttle is closed and the revs are above idle, the engine management system stops the injectors from opening. As you speed gets down to idle, it will restore fuel in the last few hundred revs above idle so that the engine doesn't stall when you come to a stop. If you want to save fuel, the best bit of advice I ever got, was to think of the brake pedal as the one that uses the fuel. Absolutely counter-intuitive, but true!!! If you drive trying to avoid using the brakes, you will accelerate more gently, lift-off sooner when coming up to an obstruction, and maintain a lower overall speed. It really works!
