Prius: now Jeep puts the boot in
by Simon Handby in Your car on 11.06.08
The problem with putting anything on a pedestal is that sooner or later somebody wants to knock it off. And so it is with Toyota’s Prius, congestion-charge darling and poster child for green cars in general.
First, The Times pitted a Prius against BMW’s 520d on a jolly to Geneva, and it used more fuel. That’s galling enough for the hybrid’s fans, but surely website Clean Green Cars is just taking the Michael by holding a drinking contest between the Prius and a diesel Jeep Patriot?
Saint Bob on wheels versus a boxy four-by-four: the devil incarnate, no less?
Well, perhaps not. Clean Green Cars’ recent analysis found that Jeep was the most improved manufacturer when it came to CO2 emissions, slashing its range’s figures by 21% between the first quarters of 2007 and of 2008.
Clean Green Cars‘ test, which is also published on Fifth Gear’s website, involved a slightly less ambitious round-trip from London to Brighton, along the coast a bit and back. This time the Prius won, but having managed only 39.9 mpg – just 1 mile more than the Jeep for every gallon of fuel.
The most concerning part of the test is that, seemingly, both cars’ onboard computers were talking absolute rubbish when calculating mpg. Clean Green Cars says that the Jeep’s computer claimed 3.1mpg more than the 38.9mpg it had actually managed. That’s 8% more than the reality, which might not be such a big deal with fuel economy, but a similar margin of error in a speedometer could see you getting a stern talking to from the police.
No matter, because the website says that the Prius over-estimated its fuel economy by a whopping 17.1mpg – a 43% margin of error. Surely some mistake?
Pocket change
Clean Green Cars rightly doesn’t draw any conclusions as to the reliability of the Prius’ onboard computer from a single result, but it’s a huge discrepancy all the same. Its publisher Jay Bagley confirmed that they had brimmed both cars’ fuel tanks before and after the test - a method that we’d expect to be fairly reliable.
Toyota GB isn’t so sure, however. Spokesperson David Crouch told us that he hadn’t previously heard of such a large discrepancy between the calculated fuel economy and that shown by the computer. He did, however, explain that the top of the Prius’ fuel tank has a ‘rippled effect’, which could trap air pockets and reduce the amount of fuel it was possible to fit into a fill-up:
If there was the potential for air to be in the tank they may not have topped the tank up in the way that they thought, and we would be guided by the read-out that the car was giving.
If you undertook this test over a period of time then you would obviously build up a picture and get a more accurate reading. We’ve actually had positive comments on how accurate the readings in the car are for the MPG achieved.
Without knowing how many litres of fuel the Prius used during Clean Green Cars’ test, we can’t work out how much air would need to become trapped in the tank to produce a 43% discrepancy. We’re guessing that it would be quite a bit, though, and that Toyota’s theory might not fully explain the result. Indeed, David Crouch added:
This result is extremely out of the ordinary; we wouldn’t expect [the discrepancy] to be that much.
IMAGE by Flickr user Beige Alert
My Prius does seem a bit optimistic, but not that much. The US version at least actually has a flexible bladder inside the fuel tank to minimize evaporative emissions, and it really has no well-defined “full.” It holds at least a few liters less in winter than in summer, and you’ll see lots of scatter in the amount of fuel pumped into “fill” it vs. the computer’s fuel consumption figures.
11.06.2008 at 3:31 pmI find it hard to believe 40 mpg for the Prius however it was measured. The drivers were obviously unfamiliar with the Prius as they could not even engage drive at first, which oddly was at a set of lights. So it seems reasonable to ask if they reset the consumption display before starting out. You know they may have been rushing before the lights turned green? As you say, “a 43% margin of error. Surely some mistake?”
12.06.2008 at 4:28 pmWe get an average readout of 62-65 MPG from our Prius (mostly town driving). One of the problems comparing face to face consumption is that our last car (Golf GTi) measured fuel economy at 0MPG whilst stopped, as does the Prius, but the latter has its engine off during these periods (worth measuring for city drivers).
Nevertheless, I did the only real test which is after TWO OR MORE fills (which eliminates the ‘air in the tank theory) and found the Prius to be getting exactly 56.8MPG, compared to the actual figures for the Golf of 26 (ish). Not mind blowing, but still over twice the economy. The Prius will always be under attack from some quarters, and it is not difficult to drive it in such a way as to give poor figures, but we forget about the economy until we get to the pumps, so I believe our figures are fairly normal.
24.06.2008 at 12:56 pm