While some products on the market make ambitious claims, the bottom line for American drivers is, do any engine additives substantially increase gas mileage, for example by 10% or more?
The short answer is:
Engine additives may lubricate internal engine parts better and provide other operating benefits. However, there has yet to be a controlled study by an independent laboratory such as Consumer Reports that has documented a mass-marketed engine additive -- one that can produce a substantial increase in miles per gallon of 10% or more, across a fleet of vehicles, operating under typical driving conditions.
Listen to What the Market Is Saying
Look at it this way: If an engine additive increased gas mileage by 10%, 15%, or 20% or more -- and if it sold for $10 or even $30 per treatment -- you'd probably know about it. It would be hard to keep secret a product that dramatically decreases gasoline consumption, and, by extension, lowers your monthly gasoline bill. The fact the products out there are not universally used speaks to their inability to substantially increase a vehicle's gas mileage. In other words, if the additives were such big money-savers, everyone would use them, all the time.
Remember, the key term is substantially increase gas mileage -- by 10% or more. Some additives may offer other operational benefits, including prolonging your vehicle's engine life, But if you're buying an engine additive with the expectation it will substantially increase your gas mileage, don't bother: It will not happen.
One Qualified Exception
That said, there is one engine additive that is a qualified, nuanced exception. Tufoil, a lubricant made by Fluoramics, Inc. of Mahwah, N.J. Tufoil coats engine parts. The product can bring about easier engine starts, increased horsepower and acceleration, extended engine life, a cooler engine temperature when running, and -- in some vehicles -- a modest increase in gas mileage. Also, for new cars, Fluoramics recommends that you drive 5,000 to 10,000 miles before using Tufoil.
I still use an 8 oz. Tufoil treatment after every oil change in the vehicles I own today -- but any increased mileage has been minor in current vehicles, which have fewer miles on them than that Buick Century. However, the improved engine performance characteristics, including improved acceleration and a cooler engine temperature, continue.
Remember an Old Axiom
For the foreseeable future, the bottom line on engine additives and gas mileage is: if it sounds too good to be true, it probably is.
You're much more likely to get the highest gas mileage possible from your vehicle if you follow the manufacturer's maintenance schedule, keep the tires probably inflated, drive conservatively (don't drive at excessive speeds or accelerate fast unless needed), and eliminate seldom-used, weighty items from the cargo area/trunk.