For gasoline cars.  Magnets are placed on fuel, air, and coolant units.

Increases fuel economy and/or horsepower (contingent largely on how one drives).  Fuel economy may improve by up to 20%-40%.

Increased heating capacity, lower fuel consumption and reduced emissions, less environmental pollution.

Improve combustion efficiency, air/fuel ratio, burns higher proportion of hydrocarbons in fuel.

EXAMPLES

U.S. Postal Service Van (2.5L Grumman, fuel injected, automatic transmission.

The vehicle was tested under the supervision of the Northern California Diagnostic labs Inc according to the Code of Federal Regulations 40 CFR 600.113-88 protocol. After baseline measurements were taken for Hydrocarbon, Nitrogen, Carbon Monoxide and Carbon Dioxide emissions as well as fuel consumption at idle, a Magnetizer emission reduction system was installed on the vehicle and the following observations were made - a reduction in hydrocarbon 
emissions was observed of 15.9%, a reduction in Carbon Monoxide of 11.37% while fuel consumption decreased by 8.68%. The Postal test mpg achieved much lower than normal driving increases because of this stop & go type utility vehicle.

Brazil EPA

Tests show 20% gasoline fuel savings with substantial emissions reduction.
California

California emissions tests show 85% reduction of hydrocarbons, 73% reduction of nitrous monoxide, 84% reduction of carbon monoxide. Note – all emissions testing stations through the years have shown the approximate same results. These emissions are unburned fuel – any reduction is an indication of fuel savings.
Western US User

Went from 19 mpg to 22 mpg the first tank, then went to 32 mpg the second tank. 4 clylinder, Japanese car.