A Toyota Prius hybrid is a fairly frugal thing. As long as you don’t drive it like you stole it, you can keep trips to the pumps at bay for remarkable lengths of time.

This feat is achieved, of course, by virtue of an electric motor assisting the engine in the nose, and a modest stack of batteries at the rear.

For some years now, economy enthusiasts have been arguing that the Prius provides a perfect platform for more committed electrification. The battery in a standard Prius is only ever charged by the car itself, ultimately by burning petrol in the engine. So why not supplement that source with electricity from the mains? Fit a bigger battery, charge it overnight, and the electric motor can do much more to assist the engine, making the car even more of a stranger to petrol stations.

For a long time Toyota has kept this notion at arm’s length, not wishing to taint its well-selling hybrid with any hint of electric-car range anxiety. But finally it is relenting – at next week’s Frankfurt motor show it will display a plug-in hybrid version of the new Prius 3.

Alas we don’t have access to Toyota’s new toy, but we have just finished our own plug-in Prius road test. It’s the familiar mk2 machine, with a plug, a 9kW stack of lithium-ion-phosphate batteries and new software added by Amberjac Projects. Our ride was kindly provided by car-sharing club Zipcar, and the vehicle is available to members to hire by the hour, so if you live in reach of London and want to try it yourself, you can.

Behind the wheel, the plug-in Prius is mostly standard, which means mostly unusual by conventional car measures. There are no controls between the front seats, just an armrest full of cupholders and cubbies. And don’t confuse the “P” button on the dash with an electronic handbrake – it’s for putting the electronically controlled transmission into “park”. The handbrake is that US-style pedal lurking by your left foot.

Slot the keyfob into its recess, press the brake, hit the start button, release the parking brake, and move the transmission joystick momentarily into “D”. You’re now ready for the off – in absolute silence of course.

We said the cockpit is mostly standard. Just ahead of the steering wheel, Amberjac has added a small LCD instrument panel. It displays all sorts of information, none of which we could understand at a glance nor, indeed, after careful study. Three unlabelled buttons and three unlabelled warning lights completed the picture. Having searched in vain for an instruction manual for this addition, we decided to ignore it completely. If you know what it is and how it works, do let us know.

We did notice one other oddity during our test. The Prius has an “EV” button on the dash that normally allows one or two miles of electric-only motoring at speeds below 34mph and at very modest rates of acceleration. Even with EV mode selected, an unaltered Prius will fire up its engine and issue two little beeps if you ask for too much. Evidently Amberjac has messed with the Prius’s mind because pressing the EV button did nothing for us, and instead the car switched itself in and out of EV mode seemingly at random. This was another thing we decided to ignore.

What we did notice is that the battery– which stores about seven times more energy than the cells in the standard Prius – barely dropped during our 30-mile test, mostly lurching between traffic lights in London, but including about 12 miles on the M4.

The engine also seemed much more willing to switch off at speed than in a regular Prius. Keeping half an eye on the Prius’s central display, we noticed electric-only motion at steady speeds of up to 55mph on the flat. Above this speed, the motor alone is evidently incapable of beating wind and tyre resistance.

Another thing we noticed is that the Prius’s built-in economy software is no longer up to measuring consumption. Over 30 minutes of driving that included crawling through urban streets, a short stint at 40mph, plus a couple of motorway miles, the Prius consistently recorded 99.9mpg (see picture). One of the numbers on the incomprehensible LCD panel said 124mpg at the end of our test.

This is all very impressive, but clearly our short road test leaves a lot of hard questions unexplored. What kind of mpg will you actually measure at the pumps? How long will the batteries last? What will a Toyota garage say at service time? How much will your fuel bill drop and your electricity bill increase? And where will you find the £9,000 reportedly required to convert a standard Prius?

The uncomfortable fact is that a standard Prius will use about £10,000 worth of fuel at today’s prices over 10 years and 100,000 miles, so the extra economy of the Amberjac car cannot possibly pay for itself.

A plug-in Prius, built by Toyota at a modest premium over the standard model? Well, that would be a different kettle of fish altogether.