Todays fuel is hurting MPG also low sulfur [lubricant] Centane levels lower. I can really tell in my tractor plowing the fields new on road fuel I have to drop 1 gear to plow over off road 50 parts sulfur to <15 on road, and I can get even more power by mixing in some light crude oil. I had some in a can and mixed 3gal with 12 onroad in my tank and noticed a 3+ MPG increase and watched it drop each time I refueled until I was back to were I started in my 87 300D. If you can get auto matic transmission fluid un dyed it is a good additive and pump lube.
Speed, jack rabbit starts, and hard braking are big MPG killers but fun is driving watching everybody go by. Also if you don't regulary run an addittive try pulling you fuel in and return line and put it into a small jug with addittive such as Lucas injector cleaner [others work just what I have used with success]and runthrough it. If your system is dirty such as carbon it will help