My Highway car a 97 Volvo 960 2.9L 24V use to get 29.25-29.75 mpg on the highway at 65 mph pre E10. Over the years the mileage slowly dropped into the high 26's which I was blaming on E10 fuel.
My buddy Wayne Gerdes last spring ran a specially built duel fuel V6 Charger for Shell. Two separate tanks each feeding its own bank of cylinders. Story is up on Youtube if you want more info.
Long story short after 5000 miles Shell tore the motor down to see how well the Shell V-power kept the engine clean vs the other premium fuel it was filled with.
Figured maybe this may be the problem with the Volvo, I usually fill with what ever premium was cheapest. Well?
I dumped a bottle of Techron in and filled it up with Shell V-Power and headed out on a 400 mile Sunday drive. I didn't see much change until about 200 miles out the car suddenly started to run much better. By the time I got home she was purring, all the roughness and hesitation was gone but I didn't see much increase if any in fuel mileage via the Scan Gauge in the low 27's.
After two and half tanks of Shell V-Power and the bottle of Techron I saw the light as they say. I made a Sunday drive to check out a car museum of about 500 miles, and found my fuel mileage had improved back up into the low 29's, 29.2 to be exact.
No more weasel pee gas for her, Top tier only if I can't find Shell V-power. Two plus miles per gallon or an 8%-10% increase in fuel mileage can't be ignored.
If your car isn't running smooth run a can of BG44K (the best, Dealer, Amazon, Ebay) or a bottle of Techron and a few tanks Top Tier fuel and see if you fuel mileage increases due to the cleaning effect.