Towing with a plug-in hybrid
Do plug-in hybrids make good tow cars, and how much of an impact does towing have on their electric ranges? We investigated as part of this year's What Car? Tow Car Awards...
Plug-in hybrids are increasingly popular as tow cars; they can run near-silently on electric power alone on shorter journeys, with petrol power and quick refuelling to fall back on for longer trips. But what happens to economy and performance when the batteries run low?
To find out, we carried out acceleration and economy tests in three plug-in hybrids with caravans in tow: the BMW X5 xDrive50e, the Honda CR-V e:PHEV, and the Peugeot 308 SW Hybrid. The first tests were completed with a healthy battery charge, then we repeated the route with the electric range readout showing zero miles.
Two of the three cars were quicker from 30-60mph with the electric range at zero. This might seem odd, but it’s important to understand that when a plug-in hybrid is no longer able to run on electricity alone, it doesn’t mean the battery is completely flat. Instead, it will rely more heavily on the internal combustion engine once the battery is low. It seems that the quicker acceleration with a low battery is due to the fact that the engine is already running; there isn’t the brief pause you’d otherwise experience before the engine starts when the driver presses the accelerator pedal to the floor.
While performance remains healthy with a low battery, there is a significant difference in fuel economy. Again, we tested the three cars with a healthy state of charge then again with a low battery, driving up and down the parallel straights of our Horiba-MIRA test track at 60mph and slowing to around 40mph on the connecting bends. The X5 achieved 37.5mpg at first, with this dropping to just 17.9mpg once the battery range showed as zero miles. The CR-V achieved a remarkable 75.8mpg at first, which then dropped to 21.1mpg. Meanwhile, the 308 SW changed less dramatically, but still fell from 28.2mpg to 20.5mpg.
In short, drivers of any plug-in hybrid tow car can expect far worse fuel economy on a long drive without recharging en route, than on a short trip with a healthy charge.
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