Plug-in Prius conversions are great. Excellent fuel economy, not much in the way of emissions and more tech than a Cray XMP. Trouble is, you can't go very fast on electricity alone because Toyota limits the Prius to a top speed of 34 mph in all-electric mode.

Not any more. The guys at Plug In Conversions have developed software that'll let you do 70 mph in a plug-in Prius running on battery power alone. Install the kit with one of the company's pricey plug-in conversions - and you have to, because the software doesn't work with anyone else's conversion - and your fuel economy could hit as much as 170 mpg.

“What we’re essentially offering is all-electric performance for about 25 miles at highway speeds,” company founder and president Kim Adelman said. “The car is no longer limited to 34 miles per hour, all-electric. This also allows a Prius to perform much like the Chevy Volt, but for a much lower cost.”

Assuming you already own a Prius, of course. If you don't, then the Volt is probably cheaper.

Plug In Conversions calls the software "a breakthrough" because it is the first time a plug-in hybrid can run at highway speeds on the battery alone. These guys, located in Poway, Calif., are building on the fine Southern California tradition of hot-rodding cars by applying it to 21st century auto tech. Gotta love 'em for it.

The software is based on technology developed by Ewert Energy Systems. Other plug-in conversions might let you achieve 52 mph on electric power, but Ewert co-founder Andrew Ewert says they do so by using a "forced stealth mode" that requires the electric motor to run faster than is recommended. They also require drivers to stop and restart the car before the gasoline engine can engage, a move Ewert says can compromise safety and emissions.

"Our system allows the gas engine to turn on instantly, when needed, and allows us to initiate controlled engine warm-up at any time to meet strict emissions standards, thus eliminating the traditional problems of a cold start," he said.

If you're a hypermiler, or someone who simply wants to boost your fuel economy, Plug In Conversions has you covered there, too. Install the software kit with one of the company's plug-in conversion kits and you're looking at as much as 170 mpg - a figure the company says has been confirmed by tests (shown in the picture) at Argonne National Laboratory.

The conversion ain't cheap, though. The kit, now in its third generation, replaces the Toyota's 1.3 kilowatt-hour nickel metal hydride battery with a 6.1 kilowatt-hour pack. It costs $12,500.

So is a Prius decked out with all this tech cheaper than a Volt? That depends.

General Motors hasn't said what the Chevrolet Volt range-extended EV will cost when production begins late next year, but GM is widely believed to be aiming for $40,000. Toss in the $7,500 tax credit the car is expected to earn you and the bottom line is $32,500.

A new Prius, on the other hand, starts at $22,000. The Plug In Conversions software is expected to cost between $2,000 and $2,500 when it becomes available later this summer. Add in the plug-in conversion kit and your final price is at least $36,500. Adelman told Edmunds.com that most people who opt for a plug-in conversion don't start with a new car, so their cost is far lower.

UPDATE 4:15 p.m. Eastern June 11: A couple of points to address questions raised by commenters:

@ The Voice. We've written about the Raser H2. Twice, in fact. You can find a those stories here and here.

@thebike45: Good point about the battery warranty / lifespan; that was our mistake for omitting it. The battery is warrantied for three years and its expected lifespan is 10 years. The battery recharges in six hours using a 110-volt line. As for the plug-in MPG figure, "miles per gallon" is commonly used to state the fuel economy of plug-in hybrid vehicles, with the term specifically used by Argonne National Laboratory in this report titled "Test Procedures and Benchmarking Blended-Type and EV-Capable Plug-In Hybrid Vehicles." As another commenter noted, nowhere in the piece do we say "lithium ion" battery, though we do know the term applies to several types of battery chemistries.

Photos: Plug In Conversions