What is DTE (distance to empty)?

  • DTE is a common trip computer function, offered on most cars
  • It shows the remaining range, based on your current fuel economy
  • For electric cars, it's one of the most reassuring - or worrying - gauges

Before the widespread adoption of fuel injection or diesel cars with electronic engine management, working out your range relied on an often iaccurate analogue measurement of fuel in the tank, a vague understanding of how economical your car was, and a lot of experience.

At that time, many cars lacked even a low-fuel warning light - so that little, unpredictable needle on the dashboard had a huge role to play.

These days you'll struggle to find a new or used car which can't tell you your remaining range in miles or kilometres, alongside instant and average fuel economy and average speed. Some make it easier to see than others - a button on the end of a stalk, a steering wheel control for a menu, or a dedicated page are common controls to switch trip computer into, but some cars put it on the dashboard, or in a configurable line display (such as Volvo) that might double up for radio/media or telephone functions.

Older cars may simply have a DTE logo on the LCD, but most will display a small icon of a car, some dots or an arrow, and a fuel pump.

Where does DTE come from?

In the 1970s and '80s more sophisticated engine management systems allowed accurate measurement of the fuel used, and better in-tank measurements of fuel as well. Further pressure - pun intended - came from the need to avoid running dry on diesel cars, where the highly pressurised pump system could get airlocks and need to be primed. On cars without an obvious mechanical solution (some have a squeeze-bulb under the bonnet) this means cranking the engine, risking draining the battery before fuel reaches the injectors.

Better to avoid such problems entirely by warning drivers before the tank is empty! So, after the appearance of the 'low-fuel' warning light (and sounds, or even electronic voices on some '80s cars) the next logical step is to tell the driver roughly how far to go before 'empty' - allowing easier route planning, less anxiety and safer driving overall.

When is empty not really empty?

If you've played fuel-light roulette before, you know that your DTE display is likely to go from 30 miles or so to zero - or even more worrying, a line of dashes. Yet your car is still moving? There's probably enough fuel in the system from tank to engine to cover a few miles, but it's more likely that the manufacturer understands human nature, and has allowed a reserve.

Even if that reserve was included in the initial range, once there's less than a gallon or so of fuel left, the car's going to tell you it's empty. Car manufacturers soon learned that drivers will see how far they can get 'on fumes', before the onerous task of paying for more fuel, and have factored that (plus emergencies) in to protect the precision components in a modern engine.

It's worth remembering that even if the 'next services' sign was in your remaining range, when you're stuck in congestion or very slow moving traffic you'll get far fewer miles per gallon. Switch off the engine when stopped, get off the motorway if it's safe to do so (you will often find petrol stations at a lower price within a couple of miles of the sliproad - check your navigation system - but they may not be 24 hour). Ultimately it's better to pre-emptively pull over where it's safe than run out of fuel in the outside lane.

What about remaining range on electric cars?

Electric cars will display remaining range as a primary function - few will hide that information, and drivers will not want to ignore it. There's no reserve; once the gauge is at zero, or empty, that's it. Unlike conventional engines, where fuel can be picked up and delivered to you at the roadside, recovering an electric car that's run out of fuel means physically moving it to a charging location.

Teslas, and other higher-end EVs, will warn you when you're getting out of range of a charging point - and if you're certain that you don't have enough range to reach a charging location, find somewhere safe to pull over and call for assistance rather than risking it.

Most electric cars make it extremely hard to run out of charge without being aware of it well in advance. However, unlike petrol or diesel cars, they only use the energy they need to move - sitting in traffic with only basic ventilation uses very little power. So if the next services are in range, even if it might take a couple of hours, hang in there - it could be quicker overall to reach that high-speed charger than risk a low-rate public charging point and a long wait for recovery or recharging.

Some cars - such as the NIssan Leaf and Renault Zoe - will give you a short heads-up before completely shutting down. In the case of the Leaf, it goes into Turtle mode - and can crawl at 30mph for about a mile, with coasting enabled. Enough to reach the hard shoulder safely.

What about the rumours of reserve capacity on batteries?

All electric cars reserve a percentage of battery capacity for charging and battery life reasons - fully draining the battery regularly will reduce range and shorten its life. That is locked out by software, but for a good reason.

There's an exception to this in that Tesla sold some 60kWh cars with a larger than specified 75kWh battery pack. Owners could pay to unlock the extra capacity, and in a national emergency, Tesla demonstrated the ability (and willingness) to enable the extra range so owners could evacuate an area at risk of natural disaster.

What should I do if I run out of fuel or charge?

Some cars will show signs that they're running low on fuel before they're completely dry - hiccuping, stuttering or reduced power output on electric cars are all signs that it's time to pull over safely and call for roadside assistance.

If you run out of fuel in a town or city, most service stations will sell you an approved five-litre fuel can - restarting your car should be covered in the owners' manual.

 

Electric car drivers will be able to find the nearest available charging point using Zap-map. Most electric car manufacturers offer free roadside assistance in the UK and Europe as long as the car has been serviced by the main dealer according to manufacturer recommendations. Failing that, both the AA and the RAC have started to roll out solutions to add enough charge to reach a public charging station, though these are aimed more at city users.

The AA's current solution is to use a specially modified patrol van as a generator, whereas the RAC has a built-in booster that adds 10 miles.

RAC EV booster adds 10 miles range to stranded electric cars

Some electric cars can act as a powerwall/battery for a house, and in theory, this technology could allow an electric car with a full charge to help recover an electric car without charge.

Should your EV run out of power, the 12V accessory battery will usually keep lights, hazard lights, steering and braking operational to allow you to be seen and be moved - it can be assisted in much the same way as a regular car battery, if it runs out.

If you have a new or approved-used car with roadside recovery, you'll normally be able to get back on the road within a couple of hours - though it can take much longer for electric cars if they need recovery.

If you haven't taken out breakdown cover, some insurance policies offer it as an extra, and it's often offered in extras with bank accounts and other services.

Further reading from Parkers

> > The best cheap electric cars

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