2015 Jaguar F-Type R: Start-Stop Fuel Savings Test
September 25, 2014
Yes, our 2015 Jaguar F-Type R has a start-stop system. I know, right? But a lot of new European cars have one. Gasoline is outrageously expensive on the other side of the pond.
A representative of a U.S. carmaker that sells over there told me European-market buyers actually demand this feature. If it's not standard, he said, they'll pay for the option more often than not. This wouldn't be the case if it didn't work, and there's no arguing with the pure logic of the basic concept.
Indeed our initial test with our 2014 BMW 328i x-Drive Gran Turismo (say that three times fast) suggested there's real money to be saved. Use of the system earned us another 2.5 mpg, an improvement of 9.4 percent.
But was that real? What would happen if we tried it with another car, our 2015 Jaguar F-Type R, for instance?
Last week I returned to our Orange County city course with the F-Type and made one lap in each mode on consecutive mornings.
Here's how the Jaguar fared:
Start-Stop Disabled: 19.7 mpg (5.1 gallons per 100 miles)
Start-Stop Enabled: 22.1 mpg (4.5 gallons per 100 miles)
Like the BMW, the Jag delivered 2.4 more mpg with the start-stop system in play. But because the 550-horsepower supercharged 5-liter V8 is generally thirstier, this amounted to a 12.2-percent gain on a straight miles-per-gallon basis.
As I've said before, MPG is a bad unit. It's mathematically upside down. In terms of fuel used, the thing we really want to know, the savings amounted to 0.445 gallons (4.082 vs. 3.637) over 80.4 miles, and that works out to a 10.9-percent reduction in the amount of moolah spent at the pump — a slightly smaller figure, perhaps, but still quite significant.
Once again, I ran this as a best-case test like the BMW. I'm still trying to wrap my arms around what's possible. So I didn't run the climate control and I didn't pack up if the cars ahead crept forward. Next time I'll alter these elements to see how much of a negative effect they have.
It wasn't that hard to live with the system. The Jag's engine re-fires relatively quickly when the brake pedal is lifted, so it's not as if I was still waiting for it to come back on line by the time I shifted my foot over to the other pedal.
The Jaguar owner's manual is more specific about the conditions it takes to reset the system. It re-arms itself when the creep speed exceeds 2.5 mph, which turned out to be useful insight in more than one traffic situation. In retrospect, the BMW's threshold seemed higher.
The biggest drawback to use of the system has to do with the very essence of the Jag's glorious-sounding V8 powerplant. This is the kind of engine you want to hear, the kind you rev for no reason. A tiny part of me died each time it shut down. On the other hand, it does give off an intoxicating roar when it fires back up again.
It got me thinking. Start-stop systems would be easiest to take on a luxury car like the Lexus LS, which is so quiet at idle you can't tell if it's running or not in the first place. Who would know? Who would care?
Dan Edmunds, Director of Vehicle Testing @ 7,540 miles