There's a difference between additives that are meant to correct some underlying condition with a vehicle (like cleaners, etc.) and those that aren't. My comments are about the second group:
The EPA has specified test procedures (
Gas Saving and Aftermarket Retrofit Device Evaluation Program) which it recommends companies use if they want to sell fuel saving additives.
The vast, vast majority do not follow the prescribed test regimen. Those that have often show no change or such a small change as to be meaningless.
Here's how to evaluate additives. Look at their marketing:
- On road testing doesn't count. If that's the major marketing angle, walk away.
- Testimonials from "satisfied customers" don't count. Walk away.
- Money-back guarantees don't count. Are you equipped to scientifically evaluate their product? Didn't think so. They're hoping you'll view the guarantee is "proof" of effectiveness. It's not. Walk away.
Look for 3rd party, EPA-approved lab testing. (Guess what: you probably won't find any. That should make you wonder why.)
EDIT: as an aside, many companies that sell fuel pills, potions and other dubious stuff will often say that they're "EPA Registered" in their marketing. Doesn't mean squat as far as the effectiveness of their product is concerned. All it means is they've registered with the EPA as a company marketing a fuel saving product - that basic step is required by US law. Touting their "EPA Registeration" is meant to dupe the uninformed and is shady marketing, in my opinion.
Another edit: visit this site the next time you're tempted by an additive or otherwise dubious looking fuel saving device:
http://www.fuelsaving.info