by Franck Latrémolière on 4 December 2017

Part of the motoring section of dcmf.co.uk.

This is not a conventional car review, since I have never driven that car. It is just a paper review of fuel economy claims.

The detailed claims are in the BMW 7-Series iPerformance brochure (29 pages, PDF).

The numbers also appear, with useful context provided by other manufacturers, in a guide produced by a German car manufacturers' association (88 pages, PDF), available from https://www.dat.de/en/offers … uel-consumption.html.

The 740Le is a substantial saloon car with a petrol/electric hybrid power train. It weighs 2 tonnes. The four-cylinder two-litre petrol engine produces 190kW, and the electric motor adds up to 83kW (the total maxes out at 240kW).

The pure petrol comparators include:

- the 730Li, which has the same petrol engine and weighs 220kg less.
- the 740Li, which weighs 170 kg less and has a three-litre, 240kW engine.
- the 750Li xDrive, which weighs 25kg less and has a 4.4-litre, 330kW engine and a four-wheel drive system.

BMW quotes a range of figures for different configurations. I use the mid point in every case.

BMW does not tell us what the pure petrol economy of the 740Le would be. So I have to guess. My guess is 6.4 litres per 100 km, which is 44 mpg imperial, on the European combined test cycle. That is 7 per cent worse than the simpler and lighter 730Li, and more than 20 per cent better than the bigger-engined 750Li xDrive.

On that basis, the combined fuel economy of the 740Le of (mid point) 2.1 litres and 12.95 electricity units implies that electricity saves 4.3 litres for 12.95 units, i.e. a substitution rate of 3 kWh per litre. That is a plausible figure for engine efficiency: just under a third of the combustion heat is recovered as mechanical power.

This substitution rate gives us a guess of pure-electric consumption: 19.2 units per 100 km, which is 3.25 miles per unit, which is plausible although not great (a Tesla Model S, a little heavier at almost 2.2 tonne, returns 18.1 units per 100 km or 3.45 miles per unit).

The 9.2 kWh gross, 7.4 kWh net battery is supposed to provide 46 km (28.5 miles) of range on the same European combined test cycle. Assuming that only the net capacity is used to achieve this range, this implies better EFRU economy: 16 kWh per 100 km, or 3.85 miles per unit. This would be very good.

Extrapolating the pure petrol consumption from a pure electric of 16 efrus per 100 km and a combined of 2.1 litres and 12.95 efrus would give a substitution rate of only 1.45 units per litre (16 per cent engine efficiency), and a pure petrol fuel economy of 2.1*16/(16-12.95) = 11 litres per 100 km, or 26 mpg imperial. That is implausibly bad.

If the electric range is based on using more than the stated net capacity of the battery, then the 3.25 miles per unit electric efficiency can be reconciled with a 28.5 miles range on the basis of using 8.8 kWh, 96 per cent of the gross capacity. That does not sound impossible.

My next guess is based on electric efficiency being 18 units per 100 km or 3.47 miles per unit. Then the battery range implies that 90 per cent of gross capacity is available, and the combined fuel economy implies 2.4 units per litre and a pure-petrol consumption of 7.5 litres per 100 km, or 38 mpg imperial.

Maybe the reconciliation factor is that the petrol engine operates more efficiently in a pure-petrol car than in the plug-in hybrid. This would be a little surprising given that the plug-in hybrid has the benefit of regenerative braking, but perhaps that is outweighed (on the European combined test cycle) by the stop-start operation required in hybrid mode?

Some bonus links to fuel economy test cycles: