Back in July, our 2005 Ford Explorer was getting 20.5 miles to the gallon when we tested it without the Hydro 4000.

After we installed the device, the Explorer got 23.3 miles to the gallon -- an improvement of nearly 3 miles to the gallon.

The Hydro 4000 works by using the vehicle's electrical charging system to create hydrogen gas through electrolysis. The hydrogen gas travels through a hose to your engine and combines with gas from your gas tank to fire your cylinders.

"If you're burning the hydrogen, it's not gonna need as much fuel, so it will cut down the fuel consumption," explains ASE-certified mechanic Bill Rackley from Rick's Auten Road 66.

The Hydro 4000 cost us $1,200 dollars, and should be installed by a certified mechanic.

When the unit was first installed in July, it was placed behind the passenger-side rear tire because there was no room under the hood, which would have been the preferred location.

Because of the distance from the engine, the people who make the Hydro told us the mileage may not improve as much as it could.

So after our initial test, Rackley wracked his brain to find a way to shorten the distance to the engine. He found a spot behind the passenger side headlight, although it required removing the washer fluid reservoir.

The other factor limiting our success was the lack of mileage driven with the device attached. We had only driven about 400 miles on the Hydro when we tested it back in July. The manufacturer recommends that you drive at least a month or 1,000 miles in order to get maximum mileage improvement. It takes that long to get all the carbon out of the combustion chambers, which in turn should create a cleaner burning engine.

Over the last four months we ended up driving 5,000 miles with the vehicle.

During that time we did have some issues.

We blew about a half dozen fuses in the ammeter that controls the Hydro 4000. That's because you need to add four ounces of distilled water to the unit about every 100 miles or so. But if you overfill it or underfill it, the mineral levels can become unbalanced and cause a spike in amperage.

Unlike this Hydro system, new models allow you to better regulate the amperage and avoid blown fuses.

So with the Hydro 4000 mounted closer to the engine and thousands of miles under our belt, it was time, once again, to gauge the mileage improvement. Like in July, we drove the exact same 210-mile route, which took us through South Bend and Mishawaka, but mainly on the toll road. We drove at the exact speed limit.

After the test, I filled up the gas tank and prepared to do some calculations.

It turns out that we got 20.5 miles per gallon on our latest test. That's what the Explorer got before we installed the Hydro 4000 back in July.

This time, our mileage went down nearly three miles to the gallon.

Rackley checked all our connections and confirmed that the device appeared to be working.

Despite this surprising result, Rackley says he believes the Hydro 4000 and systems like it do work.

He thinks it may not work with certain vehicles, however.

"It could be a computer issue -- maybe this computer doesn't like the moisture going through the exhaust. The catalytic converter might be counteracting it by the time it gets back to that rear O2 sensor," he explains.

Rackley says he knows several drivers who have installed similar systems and have achieved significant results.

In fact, a television station in Florida tested the Hydro 4000 on a Dodge Durango and the mileage went from 9.5 miles to the gallon to just over 23 -- an improvement of about 14 miles to the gallon.

To view that station's report, click here.

The official Hydro 4000 website has testimonials from drivers who have seen significant increases in gas mileage as well. Their address is www.hydro4000.com.

To read our original report on the Hydro 4000, click here.

Finally, we would like to thank everyone at Rick's Auten Road 66 for helping with our road test.