Opinion: every car does 35mpg... with a few corrective factors

You keep reading that PHEVs never manage to hit their WLTP fuel economy figures. In view of which I need to take a new look at Horrell’s Universal Law of Fuel Consumption.

Horrell’s Law was outlined years ago, and goes like this. “Everything does 35mpg. With a few corrective factors.” The Law’s origins were in the days of pure petrol and diesel cars and the corrective factors were simple. Driving fast costs 5mpg, or driving gently saves 5mpg. Diesel improves things by 10mpg. A non-plug hybrid also adds 10mpg. Superminis add 5mpg while a big car costs 5mpg. A crossover body costs 3mpg, a true SUV costs 7mpg. Four-wheel drive costs 3mpg. GTI-type performance costs 5mpg, very high performance 10mpg.

You simply take 35mpg, then correct cumulatively. Eg a small diesel crossover will get 35mpg plus 10 (diesel) minus 3 (crossover) plus 5 (small) equals 47mpg. A big petrol SUV with 4WD and high performance will get 35-7-3-10 equals 15mpg. Admit it: you get no better in your Range Rover Sport SVR.

[Any mathematicians among TG’s readership will be up in arms at this point, screaming that you can’t do these cumulative calculations with mpg because it’s an inverse. You could do them with l/100km, because that’s consumption whereas mpg is economy. But I say heck try my formula and you’ll see it really is accurate enough to be useful.]

Interestingly, although downsized engines and reduced cylinders and turbos and 7spd transmissions all improved the official test figures, in the real world of Horrell’s Law, nothing really changed for three decades of those and other innovations. Every Golf-sized petrol car still got 35mpg. Today though, popular opinion says that the thirst of PHEVs just doesn’t obey any logic, and certainly not the one the tax authorities use.

Well, after many recent experiments, I can pronounce that my law still applies. You just have to factor in a simple codicil. It becomes a calculation on the basis of the two parts of the journey: the part where you’re draining the battery and the part where you aren’t.

Imagine you start fully charged. First, note how far the battery took you. Let’s say 25 miles. Then deploy Horrell’s Law for the rest of the trip. Say it was a 70-mile trip in all, so for 45 miles the battery was no longer depleting and the car was acting as a non-plug hybrid. Since a mid-sized petrol hybrid gets 45mpg according to Horrell’s Law, you used a gallon. But you also had 25 electric miles. So the gallon goes across the whole 70 miles, meaning you get 70mpg. If instead it was a 200-mile trip, it’s 175 miles in hybrid, which is 175/45 = 3.9 gallons. And 200/3.9 equals 51mpg.

That’s why a bigger battery means better mpg, and shorter trips also mean better mpg. A 50-mile battery and a 60-mile total trip means just 10 hybrid miles, so 270mpg. And the WLTP cycle is a short trip. A PHEV can save petrol. And tax. Though it doesn’t save energy because you also have to charge the battery.