Since Vanessa and I have fairly similar cars, one gasoline and one diesel, I found it instructive to compare their observed fuel efficiency now that we’ve put a few thousand miles on each.

One is a 2012 Audi A4 wagon (4-cylinder 2.0 liter turbocharged gasoline engine). The other is a 2011 VW Golf TDI (4-cylinder 2.0 liter turbocharged diesel engine).

For both of these, I’ll list real-world numbers for the speeds I actually drive, averaged across a bunch of real driving. (I wish it were easy to get numbers like these before buying; the EPA test circuit isn’t all that reflective of our real world driving). I’ll list the fuel mileage for 3 cases which I find instructive:

• best-case I’ve seen (long-distance highway driving)
• a typical commute from San Francisco to Palo Alto (37 miles, mostly highway)
• the average over the entire time we’ve had the car (a mix of city driving, medium-distance commuting, and longer road trips).

One thing I already knew: the diesel engine is more efficient than the gasoline engine. Another thing I already knew: highway mileage is better than city mileage (excluding hybrids with regenerative braking, which these cars are not). And one thing I didn’t know, but learned from reading the Audi owner’s manual and confirmed by real experience: short trips get worse mileage than long trips, because mileage is worse before the catalytic converter warms up.

On to the numbers (all of which come from the cars’ own trip computers).

Audi A4 (gasoline): average over our entire mix of driving is 25 mpg. Typical mileage on the commute is 29 mpg. Best mileage I’ve seen for an extended period of highway driving is 33 mpg.

VW Golf TDI (diesel): average over our entire mix of driving is 37 mpg. Typical mileage on the commute is 44 mpg. Best mileage I’ve seen for an extended period of highway driving is 51 mpg.

Conclusions and observations:

• The Golf TDI definitely gets better mileage than the gasoline A4. About 50% better across the board, actually. (Though there are many variables other than the fuel type; the A4 is bigger and heavier, and has all wheel drive and an automatic transmission. On the flip side, bigger isn’t necessarily worse; it’s likely more aerodynamic. And Audi claims their 8-speed automatic transmission delivers equal or better fuel economy to the 6-speed stick.)
• The Golf TDI is more sensitive to speed than the A4; there’s more upside to driving at or below the speed limit.
• There’s an interesting comparison to be made, for both cars, between the “typical commute” and “best I’ve ever seen” numbers. That typical commute is 37 miles starting and ending with a few non-highway miles; the average mpg starts low (non-highway), climbs as I get on the highway, then climbs further as the catalytic converter warms up. By the time I arrive, the running-total average has pretty much stabilized, but only barely. And the going rate on Highway 280 is often around 75 mph, well above the optimum speed for maximum efficiency (closer to 60 mph for both these cars, as far as I can tell).
• The “best I’ve ever seen” results derived from getting on the highway and driving just over 60 mph for a longer distance — so the steady state average mileage is higher (because of lower speed) and contributes more to the final average (since most of the time was spent in the steady state after the catalytic converter warms up).
• Our observed around-town mileage for both cars is significantly worse than the EPA “city” estimate, because these trips tend to be very short, full of stop-and-go, and at suboptimal speeds. I think the EPA cycle tries to account for the latter 2 factors but not the first one. This is true for the diesel car as for the gasoline one, and is a further argument for electric vehicles for in-city driving.