Fuel Economy Update for July - Good Gains Over 5,000 Miles - 2015 Audi A3 Long-Term Road Test

Edmunds.com

Fuel Economy Update for July - Good Gains Over 5,000 Miles - 2015 Audi A3 Long-Term Road Test

We added almost 5,000 miles to the 2015 Audi A3 since the last time we measured its fuel economy. This time around we've seen impressive gains in both lifetime average and best-fill MPG.

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ebeaudoin

0.8 MPG below EPA really isn't that bad. Better than most Edmunds' rides!

locke42

Have you guys noticed if your A3's built-in mileage calibration is off?

I've noticed that the mileage calculator in mine is consistently overestimating my mileage by about 10%. For example, it would report my mileage to be 30mpg at 270 miles, which would mean I should have only used 9 gallons, but when I fill up my tank, I would end up filling 10 gallons, meaning my actual mileage is 27mpg.

Note: I don't "top off" my tank. I let the pump go until the handle clicks off. Also, I have the diesel A3. My mileage is so much lower than the EPA estimates (31/43/36) because (a) LA traffic sucks, and (b) I usually keep it in sport mode and drive with a heavy foot.

legacygt

That mileage is not bad at all. You're pretty much at the EPA mileage. When a turbo accompanies an appropriately sized engine you can get pretty close to the EPA numbers. You also get to see the "dual nature" of the powertrain in wide spread between the best and worst tanks. Drive aggressively relying heavily on boost and you get 17.8 mpg. Drive conservatively and stay away from boost and you get 35.1. It's nice to have the right amount of power when you need it but the ability to drive efficiently when you don't. Contrast this with other vehicles (ahem F-150 in the current fleet for example) that rely on turbos in otherwise undersized engines and they need to rely on boost nearly all the time, making the EPA numbers a fantasy.

socal_eric

Many people automatically jump on the bandwagon that smaller displacement turbocharged engines in today's larger vehicles will always yield poor mileage. Sometimes that's true but often it isn't. Take this heavy for a compact, AWD Audi A3 for example. It seems to have no issues meeting economy numbers while some of Ford's EcoBoost models often have trouble matching EPA numbers (such as the current F-150 in the Edmunds' fleet). It isn't just about a small engine being in boost. If the turbocharger is designed on the smaller side to minimize turbo lag and provide better response it chokes off exhaust flow and reduces efficiency out of boost.

When you go into boost you're essentially capturing exhaust energy that would otherwise be lost and using it to spin a compressor that forces more air into the engine and effectively increase the dynamic compression ratio. With newer turbocharger and turbo exhaust housing assemblies that can withstand substantially higher exhaust gas temps (EGTs) and things like direct injection that minimize knock, the automakers don't have to dump tons of excess fuel into the cylinders to help keep EGTs and detonation in check, so going into boost isn't always the main reason for poor economy.

It doesn't matter if you have a big naturally aspirated V8 or a small turbo V6. To make the same power is going to use a similar amount of fuel. But to make a turbocharged powertrain livable and provide "normal" power delivery many buyers expect, if you size it on the smaller side to provide better transient response you'll have increased pumping losses and less overall volumetric efficiency when off boost.

Take this Audi's 2.0L turbo and compare it to the 2.0L turbo in the '13+ Focus ST. My lifetime average over two years driving a Focus ST was about the same as Edmunds is getting for the A3 but my mileage was a lot more highway miles and on long road trips I could never touch mid-30s. Then look at the turbo. Ford is using a tiny BorgWarner K03 turbo while VW/Audi is using a larger Honeywell turbo in the new A3 that's sized the equivalent of one of BorgWarner's K04 turbochargers.

That bigger turbo family has a larger exhaust housing that when out of boost isn't as restrictive (even with the wastegate open). By carefully tuning valve timing, exhaust manifold, transmission gear, etc. Audi can get a nice power curve that mimics a bigger naturally aspirated engine but it isn't quite as responsive as the smaller turbo on Ford's 2.0L EcoBoost (although very close and things like the DSG trans in the Audi willing to downshift quickly mask this somewhat).

Ford is introducing a new 2.0L EcoBoost for 2015 that will have a redesigned cylinder head with its integral exhaust manifold having coupled cylinders to allow for use of a twin-scroll (divided inlet) exhaust housing on a larger turbo. That will allow for better utilization of the exhaust gasses available to build boost in even though the exhaust housing and turbine wheel are bigger and less restrictive (i.e. better efficiency off boost). Combined with even higher static compression ratio you'll probably see significant mileage improvements, so before jumping to the conclusion that "all small turbocharged engines in bigger vehicles will equal poor mileage", there's a lot more variables that come into play and that isn't always the case.