2010 Volkswagen Golf TDI fuel economy: sometimes things just don’t add up

I’m working on my reviews of the wonderful 2010 Volkswagen Golf TDI, for both Canadian and US readers. I always double-check my facts and figures to ensure things are kosher.

But when I was trying to figure out the driving range of the new model, something didn’t add up. Specifically, Canadian figures pointed to a theoretical range of 1195 kilometres on the highway, while the US figures promised about 609 miles…which is 980 kilometres. WUT.

(Yes, I know there are different gallons. I used the correct ones.)

Here’s the math:

• Canada: 55 litre tank / [4.6 (Highway L/100km)] = 11.95 x 100 = 1195 km range
• US (the math is slightly different because you’re eliminating the denominator): 14.5 gallon tank x 42 mpg = 609 miles
• Imperial: 12.07 gallon tank x 50 mpg = 603.5 miles

All is right…right? Well no. Because 609 miles is only 980 kilometres. So either the US cars get worse mileage than their Canadian counterparts, or Canadian cars get better mileage. This doesn’t make sense.

Converting the Canadian figures of 6.7L/100km city and 4.7L/100km highway directly into US mpg and you get 35 US mpg city and 50 US mpg highway. That gives the vehicle a theoretical range of…you guessed it: 725 miles or 1166 km. That’s less than a few percentage difference.

To isolate the problem, I plugged in the quoted Canadian L/100km economy numbers (10.4 city, 7 highway) for the 2.5-litre gasoline-powered Golf into my calculator. They came up as 22 US mpg city and 33 US mpg highway. Pretty much exactly what the EPA numbers for the car are (22/30) in the US.

Oh, and the Canadian figures are identical to what Volkswagen Germany is quoting for the 2010 Golf TDI.

There is a way the numbers will work. If you take the combined L/100km number for the Golf TDI (5.7) and convert it to US mpg, you get 41.2. Close.

More likely, the EPA test cycle skews the figures away from diesels. Differences of 8 mpg in highway and 5 mpg city cycles are about 15 per cent “off” from what Canadian and German fuel economy numbers are. This could be down to how their test cycle brings out the strengths of some powertrains (gas and hybrid) while unfairly penalizing others (diesel.)

But that doesn’t explain why figures for gasoline-powered versions are practically identical between countries and testing methodologies.

Even on Volkswagen America’s site, their “Tank Wars” section that compares the fuel consumption of users sits at an average fuel economy of at least 46.1 US mpg combined — pretty much the “combined” mileage of 5.6L/100km that you’d get by using Canadian or German figures.

Either the EPA gave Volkswagen messed up numbers for the Golf or their testing unfairly penalizes diesel engines. So let’s look at some other vehicles.

Mercedes-Benz ML350 BlueTEC diesel SUV: Canadian figures are 11.8L/100km (20 mpg) city, 8.2L/100km (28 mpg) highway. In the US, they’re 18 mpg city and 25 mpg highway. That’s a 10 per cent gap.

Volkswagen Touareg II diesel SUV: Canada: 11.9L/100km (19.7 mpg) city, 8L/100km (29.4 mpg) highway. The US figures are 18 mpg city and 25 mpg highway. That’s about an 11.5 per cent difference.

BMW 335d sedan: Canada: 8.5L/100km (27.6 mpg) city, 5.3L/100km (44 mpg) highway. US figures are 23 mpg city and 36 mpg highway. Difference? About 17 per cent.

Toyota Prius hybrid: Canada: 3.7L/100km (63 mpg) city, 4L/100km (58 mpg) highway. US figures are 51 mpg city and 48 mpg highway. That’s about an incredible 18 per cent per cent gap.

My conclusion? My personal, real-world fuel economy figures for each of those vehicles are much closer to the numbers quoted in Canada (and Europe.) For these vehicles, the EPA figures seem comically low. So what gives? My real-world numbers say the new Golf TDI achieves mileage much closer to 50 mpg highway and 35 mpg city.

In an industry and buying public now as obsessed about miles per gallon as they used to be about horsepower, there can’t be wildly varying fuel economy figures for identical vehicles between countries. An mpg here or there skews not only the public perception between gasoline, hybrid, and diesel-powered vehicles, but also sales and, ultimately, government mandates for emissions and fuel economy requirements.

Helping me with all this was Mark Stevenson, another Canadian automotive journalist. I’m terrible at math, so Mark helped run some numbers and double-check my findings. Props.