On the official test, the 2.0-litre turbo-diesel engine in the Q5 2.0 TDI, used least fuel, consuming 6.1 litres/100km (urban and country combined).
In use this is a fine engine that most drivers would find satisfying, especially in country driving. However, the Q5 2.0 TDI was suspended from sale for several months in 2016 while Audi parent Volkswagen Group sought to rectify its engine control software, which was found to include a device for cheating emissions tests. (Only a software update was needed.)
Only marginally more thirsty is the 3.0-litre V6 turbo-diesel supplied with the much more expensive Q5 3.0 TDI. This engine uses 6.4 litres/100km on the standard test, and is much more powerful than the 2.0 litre.
A reason not to choose either diesel is that both have particulate filters, and therefore are not suited to predominantly short trips around town. The filter prevents diesel soot from getting into the air, and you need to do a 15-minute drive above 60km/h regularly to keep it clean.
A second reason not to choose either diesel is that you prefer the extra zip and friendliness that Audi’s petrol engines deliver, especially around town.
The 2.0-litre turbo petrol engine in the Q5 2.0 TFSI uses about 25 per cent more fuel than the 3.0-litre diesel. But it’s a very nice engine and works well in the Q5.
The 3.0-litre supercharged V6 petrol engine in the more costly Q5 3.0 TFSI is not much thirstier than the 2.0 petrol, consuming 8.5 litres/100km on the official test. Considering the impressive performance available, that’s quite frugal.
All petrol Q5s have a traditional eight-speed automatic gearbox. In contrast, the 2.0 TDI and 3.0 TDI diesels get a seven-speed S-Tronic gearbox, which uses computer-controlled manual-type clutches. The S-Tronic is an excellent gearbox once you are moving and it saves fuel, but it does not provide the fluid, elastic take-up from rest that you get from a traditional automatic, instead feeling snatchy sometimes on hills, and jerky occasionally in stop-start traffic. So that is a third reason you might not want a diesel.
The exception here is the SQ5. This is the quickest Q5, using a twin-turbocharged version of the 3.0-litre V6 diesel that provides even more power than the 3.0 TFSI petrol. And it does so while using less fuel. The SQ5 drives through a conventional eight-speed auto gearbox like that in petrol Q5s.