I just picked up a 02 GMC Seirra SLE z71 with 61K on it. completely mint with one owner and all service records! The guy claimed he can get 19 on the highway with it which I believe and I am not complaining about that. But I am wondering If I did one mod for increasing gas mileage when towing what would be my best bang for my buck? A tuner? I will attach a pic of the truck also! So glad to have a GM product again!