What is a low cost way to improve fuel mileage and performance?

“Our old 2006 Mazda 6, with 167,000 miles has been running terrible for months! Local shop told me it was bad injectors. I can’t think how many injector cleaner products I bought from the local auto parts store – tried them all, nothing changed. In fact, it only got worse over time. Emissions Check was due, I knew there was NO WAY it would pass. I called you guys and you said get some Cleanpower and FTC Decarbonizer in the gas tank, then take it for a drive. What a difference! Within 30 miles it was running better than it has in 10 years! So much smoother, crisper, and I think it’s quieter too. It’s nice to drive the car again. I took it to Emissions and it flew through. I’m really grateful for your help, these products really work! Amazing.”
Mike C, Springville, UT