Combining internal combustion energy and electricity does the GS real favours when it comes to the official emissions and mpg tests. The economy is important, but it’s that CO2 figure that really counts here, the 109g/km the GS 300h emits - when fitted with the smallest wheels - makes for a very compelling company car if you’re far up the slippery corporate pole to be picking such vehicles. Even the 19-inch wheeled F Sport 300h manages 115g/km. Those numbers translate to official combined economy figures of between 60.1- and 56.5mpg. Real world use is likely to shave at least 20mpg off that, but you’ll be saving so much in tax you’re unlikely to really care.
The GS 450h doesn’t do quite so well, even if its 141g/km isn't bad when you consider its output; neither is its 45.6mpg official combined economy figure. There are plenty of rival diesels that can trump the GS 300h’s combined economy figures, however, and better them in real world usage, too, while they get close enough on emissions as to make a negligible difference on the overall running costs. Servicing isn’t that cheap in the GS though and is a fairly regular requirement, too.