Long term test: 50’000km in a Toyota Prius II

50’000km in a Toyota Prius mkII: real world conclusions on the poster child of hybrid cars. 

Toyota celebrated in December 2012 the sale of its 5 millionth hybrid car, out of which 3 million Prius. An undeniable commercial success, a strategic and technological pillar for the world’s number one car maker. An excellent opportunity to reflect on our experience at the wheel of our Prius mkII after 5 years and 50’000 km of hybridization.

48’903 km, 5.44 L/100km, no problem to report. This could be the summary of this long term test.

Gas mileage is obviously of central interest with a car which symbolizes, for good or bad reasons, environmentally responsible mobility and conscience. Our measured average of 5.44 L/100km could appear unconvincing in comparison with small turbodiesel engines, but it is important to place it in the context of a broad spectrum of commutes, some short, some longer and faster, and mostly on hilly terrain. This is not a best case average from a freeway cruiser, but a realistic value obtained from conditions which would be rather unfavorable to a conventional automobile and where hybrid techonology makes a strong case for itself.

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First and foremost, energy recovery in downhill segments. According to google maps, the road from our home to the nearest main road is a 90m altitude drop over a distance of 1140m. This is sufficient to fully recharge the NiMH battery pack and travel the entire village in electric mode, as long as the engine is deemed warm enough by the control unit to authorize it. It is however not necessary to drive through a mountainous landscape to benefit from energy recovery. Every time you coast to a stop or lift the throttle, as long as you anticipated it, is an opportunity to harvest electrons and charge up. In addition, whenever conditions allow, the Hybrid Synergy Drive system  naturally uses excess torque from the thermal engine to drive the generator and feed the batteries.

The blue curve of consigned tank refill volumes could be interpreted as the sign of a high variability in gas mileage, but it actually reflects the difficulty to precisely measure the amount of fuel required to fill the car. The Prius has a soft tank housed within a rigid enclosure, this makes it difficult to refill precisely to the same full level, furthermore as one is strongly advised not to top off beyond the second click of the gas pump, for two reasons. First, there have been reports of ruptured tank bladders in case of over-filling. Second, an excess ration of unleaded fuel has the same impact as poorly managed binge drinking: excess liquid is promptly returned through the inlet. Tanks are therefore sometimes  incomplete, which requires catch up the next time around and translates into the appearance of excess gas consumption. The on board computer turns out to be a more accurate and reliable indicator of gas mileage for any given tank,as show with the green curve. Its average is 5.24 L/100km, a 3.7% favorable error compared to the measured 5.44 L/100km. Incidentally, this on board computer 5.24 average is identical to the measurement made on the first 10’000km this car covered on the roads and freeways of California, in rather different climate, altitude and terrain conditions from Switzerland (see previous article).

By smoothing the computer average with a polynomial approximation, a seasonal trend emerges, with minimas during summer months and peaks during winter months. Two possible explanations: battery efficiency and tires. The fact that summer 2012 was driven with old winter tires (Michelin Pilot Alpin in 195/55/16 size instead of the Good Year Integrity summer tires in 185/65/15) can be a significant contributor. Tire width, rubber hardness, the weight and inertia of 16 inch alloy rims, and the higher operating pressure of summer tires (3.0 bars front, 2.9 rear) all influence fuel consumption.

Beyond this seasonal swing, there is no sign of an overall adverse trend in gas mileage which could be indicative of a loss of battery capacity. Toyota engineers seem to have been hard at work to hone the management of the NiMH cells, limiting their use to half of the full theoretical charge, and scheduling periodic full discharge cycle in the firmware. The online gas mileage databases provide coherent data with 47.5 US mpg (4.95 L/100km) on GreenHybrid  or 5.2 L/100km on Spritmonitor in Germany.

One of the most irritating shortcomings of the Prius II is the driving position. The seat is not adjustable in height, and designed for short people. My 182cm find themselves seating far too high, and the steering column angle adjustment does not bring a satisfactory solution. Headroom remains generous, but I have had to resolve myself to drive with the feeling that i am holding the steering wheel between my knees. The front seats also lack lateral support, but it is a minor gripe as nothing – or so little – entices aggressive driving and fast cornering. Rear passengers are very well treated with ample, luxury-sedan-league legroom.

The ergonomy of controls is well design, most important functions are available through clusters of buttons on the steering wheel itself, and the rest can be accessed easily through the resistive touch screen, with clear menu buttons on either sides. We do not use the voice command system, finger access to the settings is faster and, somehow, less distracting. The vacuum fluorescent display cluster is simple but efficient, but I stil regret the bizarre absence of a real dual trip computer with average speed computation. The gas mileage and energy recovery histograms are a useless gadget, the possibility to draw correlations between average speed and mileage would be far more informative in a car where fuel consumption is a key selling point, if not an obsession. I regret at times the absence of a rev counter, although the continuously variable transmission makes it impossible to influence it otherwise than through your right foot. I would just be entertained to know at times how fast or slow the small 1.5L four cylinder is revving.

The interior of the Prius II is finished with average quality materials, a mix of structured surfaces, slightly rubberized but difficult to clean, and textured surfaces with tend to scratch more easily. Fittings are accetable for the category, and I am pleased to report that no squeaks or rattles have developped, even on deformed tarmac. Assembly and structural integrity are good and well designed. The faux-leather has become shinier in the most exposed spots, but there are no advancesd signs of cracking or tearing.

« Ugly as a Prius ». I have heard or read the comment many times, and it still puzzles me. I would not rate the Prius design as aesthetically seductive, but I still find elegance in its sober lines, especially from the rear third quarter viewpoint. A matter of personal taste, but my genuine interest of first generation Fiat Multipla MPVs undoubtedly places me in a small niche in style matters.

Over the lonf term, Toyota’s Hybrid Synergy Drive demonstrates in a striking manner its ingeniosity. For a complete description of its operating principle, we recommend taking a look at this excellent graphical animation which allows to understand the interactions between its various components. We have never had the slightest problem, the faintest hick-up or hesitation, let alone any breakdown. This technology is refined and mature, a real marvel of contemporary engineering. One has to get used to the continuously variable transmision and the fact that the car never feels in gear. Any change in throttle position translates into a variation of the thermal engine revs when it is turned on. Transitions between the different operating modes are transparent, and piling up miles has had no effect on that. Our California model does not have an EV button allowing to impose electric drive when conditions allow. To enjoy it, the throttle pedal needs to be finessed to contain power requirements, and speed must be kept below 50 km/h. The ‘B’ engine braking mode, accessible with the small gear selector stick. allows to modulate the energy recovery rate in coasting or downhills, a useful feature in comparison with competitive systems which solely rely on a brake pedal dead travel zone.

The second biggest shortcoming of the Prius II is its handling of very low adherence conditions. Traction itself is rather good provided that you have adapted tires, but electronic engine and transmission management forbids the slights slip of the front wheels, and there is no way to deactivate it. I found myself once on a winter night on an icy incline covered with fresh snow, and our Prius unwilling to transmit any torque to the wheels, even with the throttle to the floor. The conditions were challenging, but it is very frustrating to find oneself stranded because the car has detected a hint of slippage. The only work around is to turn around and reverse your way up the hill, something doable in a quiet residential zone but clearly impossible in most road situations. It is possible that Toyota boffins took precautionary measures to prevent the electrical engine torque from chewing the transmission up, but the solution is not acceptable. The Prius II is otherwise fairly benign in snowy conditions, but when the conditions get tricky, the electronic nanny may well leave you stuck at the wrong place, no matter how honed your winter driving technique is.

Road handling is placid, defined by the rather limited grip of the tires, furthermore with the 15 inch wheels. Body roll is pronounced, the tall tire falls are not rigid under lateral forces. There is nothing to entice spirited driving, and attention is best shifted on smoothness and precision while incorporating the ingredients of eco-driving: anticipation and preservation of kinetic energy. The Prius is to be treated as a momentum car to avoid costly accelerations, but within the confines of its cornering speed capabilities. On freeways, stability is a bit wobbly at times, especially on tarmac deformed by heavy trucks. Side wind sensitivity is perceivable and requires attention. Driving within speed limits is never an issue however, except when uphills require to squeeze the small 4 cylinder Atkinson engine, in which case its noise gets easily tiring. The Prius II can easily cruise above 90 mph (150 km/h), but this is neither confortable nor coherent with the definition of the car. Long journeys are easily doable as well, we drove this car from Silicon Valley to the south of Los Angeles in a morning without a hitch, we also drove it down to Tuscany and back, loaded to the gills with luggage and passengers. It is just largely advisable to comply with the wishes of the lawmakers in terms of speed.

As long as you take the Prius II for what it is, a practical, reliable, functional and fuel efficient compact car, the conclusion is positive. I enjoy driving it for its ease of use, manoeuverability, a perfect car for chores, shopping runs, urban commutes, but also for a quiet stroll or to cart around passengers which favor the living space and relaxation of the Prius to the confined and somewhat claustrophobic luxury of our Audi S5 Sportback. Beyond the peace of mind it brings, the Prius is also an interesting car to drive, an excellent tool to practive eco-drive skills, perhaps the only driving technique that remains realistically applicable in daily traffic conditions. If the thrills are not quite comparable to a  lap of the Nordschleife, the discipline is interesting and will reward you with a few extra hundred yards of electric drive. An ephemeral moment of silent and smooth mobility.

Maintenance and costs

Depreciation is massive if computed with the official swiss list price at time of purchase, in February 2008. Our Prius did, in fact, cost only 30915 CHF (19240€ at the day’s exchange rate), thanks to the weakness of the US dollar and the favorable sticker price on the US market. The value of the Prius on the swiss used car market is depressed, perhaps a reflection of unwarranted skepticism on reliability and complexity, which makes it an excellent bargain for a used car buyer. Fixed costs rank second in the total budget, just ahead of tires and maintenance. If computed on the basis of the real purchase price, cost per kilometer comes out at a more reasonable 57.3 cents. The first service in the US was free, and servicing in the swiss official network seem rather expensive due to labor rate. The car has had strictly no breakdown or issue of any kind.

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