Audi's 2.0-liter TDI engine is a real sweetheart, making it simple to maximize efficiency.

Every time a tenth of a mpg would fall off the displayed fuel economy number, it felt like a personal attack... a failure. I wanted to win. And I was trying so hard.

Coasting down mountain roads in Neutral is fun, but doesn't reveal a ton about how a car is performing when all of its systems are firing together.

The TDI's interior boasts good refinement, a tight back seat, and all the onboard tech you could ask for in a compact Audi.

The onboard fuel gauge told me I had run out, and yet the car kept going.

I was incredibly proud of my score of 59.4 mpg. I bragged about my achievement to anyone who would listen.

With a TDI engine underhood, an already good package gets even better.

I officially gave up after 758 miles. The 15 or so miles leading up to this decision were spent in the right lane of Southern California's I-8 freeway, hazard lights blinking, climbing uphill at just over 40 miles per hour. After two days of sweating to the oldies (okay, a mix of SiriusXM Classic Rewind and First Wave), I had covered those 758 miles in a 2015 Audi A3 TDI on one tank of diesel fuel. And when I say sweating, I mean it quite literally. In order to maximize fuel efficiency , my co-driver and I kept the air conditioning off, even when the direct sunlight in the California desert had outside temperatures hovering around 90 degrees. I had been doing this hypermiling exercise for two days, the car was getting stinky, and I was ready to hear the sweet "thhhhhhhwack" of satisfaction that would finally come from peeling my sweat-soaked self off the A3 's leather seat. Sexy, I know. Audi had challenged me to drive 834 miles from Albuquerque, NM to San Diego, CA, on just one 13.2-gallon tank of diesel fuel. If you believe the EPA's highway fuel economy rating of 43 miles per gallon, this means I should have sputtered to a stop after 568 miles. But I went a grand total of 758 – that's 59.4 mpg – and I could have kept going. In fact, two teams made it the full 834 miles on their one allotted tank of fuel. That's over 63 mpg. That's twenty miles per gallon better than EPA estimates.The TDI Challenge took me through three states over the course of two days, and the 834-mile journey wasn't just a simple highway cruise. I negotiated uphill climbs, long series of involving switchbacks through the mountains and elevations that ranged from 220 feet below sea level to nearly 8,000 feet above. I learned that super-crazy-efficient driving like this an incredibly challenging game that takes serious skill. But I also learned that if you're going to attempt to stomp all over the EPA's numbers, the Audi A3 TDI is one heck of a car for the journey.The 2015 Audi A3 TDI is a pretty simple package. Take one A3, shove the redesigned Volkswagen Group 2.0-liter turbodiesel inline-four under the hood, and you're good to go. Audi will only offer the A3 TDI in sedan guise for now, priced from $32,600, not including $925 for destination. But folks seeking more functionality will be able to opt for a five-door TDI Sportback next year.Clearly, the vast majority of credit for the 59.4-mpg observed rating goes to the mad skills executed by my co-driver and I over the course of our journey, but I'll admit, the 2.0-liter TDI engine is a real sweetheart, making it simple to maximize efficiency. The same can be said for the standard six-speed S-tronic dual-clutch transmission, which was easily kept in manual mode to ensure I was in the highest gear possible (Audi does not offer the A3 TDI with three pedals, sadly). This engine is essentially a redesigned version of the VW 2.0 TDI powerhouse I've always loved, now rated at 150 horsepower at 3,500 rpm and 236 pound-feet of torque that comes on strong from just 1,750 rpm. That's more than enough grunt for the 3,241-pound A3, and while it would have been foolish to test such claims on the efficiency challenge, Audi says the TDI sedan will hit 60 mph in a relaxed 8.1 seconds on its way to an electronically limited top speed of 130 mph.Like all diesels, the beauty of this TDI setup is its way-down-low torque thrust, which would be an incredibly helpful asset for eking out every last mile per gallon on this trip. Before departing Albuquerque, I sat down with mileage expert Wayne Gerdes you've read his name on the pages of Autoblog before – who explained how to really take advantage of the car's low-end power. I was instructed to get into third gear at 16 mph, fourth at 26 mph, fifth at 32 mph, and the finally into the top gear at about 40 mph, never letting the Audi get below 41 mph on uphill climbs (otherwise the transmission would downshift on its own, regardless of me having it in manual mode). At these speeds, I used the maximum available torque at the lowest rpm without digging too far into the turbocharger. And in fact, on the entire 758-mile trek, I don't think the tachometer needle ever crested 1,800 rpm. Now that's efficient.On the drive out of Albuquerque, feathering the throttle was key. It was sort of weird, suddenly being the guy I normally yell at who's going 50 mph in the right lane of the freeway, taking forever to merge into the fast-moving traffic. Wayne explained that, in situations like this, where you're going slower than the flow of traffic and you're on a busy road, "ridge riding" is incredibly helpful. Drive in the right lane, with the passenger-side tires riding the white line of the highway. Since most drivers tend to focus on the center of each lane, positioning yourself to one side makes you more noticeable. And if you see someone coming up behind you at a faster clip, throw the hazard lights on way in advance, giving them a heads-up to pass.Oh, and you'd better get used to being passed. With the speed limit at 65 mph and me putting along at 50, literally every other motorist on the road wanted to go around. Gross old pickups, motorhomes, semi trucks, and even the odd oversize load convoy all passed by (I can now say I've been overtaken by a house, so that's neat). But Gerdes' fuel-mizing techniques were paying off. What started as 40 mpg became 50, and as the day progressed, I was well into the high-50s, feeling pretty darn proud of myself, exchanging high-fives of congratulations with my navigator because we were fully confident that there were no men in the world more efficient than us.The uphill section of Day 1 was mentally tough. It's really disheartening to watch your fuel efficiency drop consistently – those high-50 numbers became 40s with a quickness. I pleaded with the Audi to stay in sixth gear on these uphill runs, and every time a tenth of a mpg would fall off the displayed fuel economy number, it felt like a personal attack... a failure. I wanted to win. And I was trying so hard.But what goes up must come down. As I rolled into Sedona , AZ for the night, the sun started to set, the temperature dropped, the elevation lowered, and the roads got really, really good. Managing the mountains in the A3 was an incredible experience, being able to throw the car into Neutral and just coast for miles and miles down mountain roads against some of the most beautiful backdrops I've ever seen. Normally, I'd be working the throttle and brakes, putting power down whenever possible, and using it to plant the A3 solidly in the turns. But this time, with the car in Neutral, I relied solely on the Audi's tires, chassis and steering to get me through bends – the engine was off (well, idling). But the A3 performed admirably.The A3's steering was predictably light, with the typically dead on-center feel of most compact Volkswagen /Audi products. I found more feedback through the wheel the farther it turned, and the chassis remained relatively flat and composed through corners, even without me sending power to the front wheels. Speaking of which, don't bother looking for Audi's celebrated Quattro all-wheel-drive setup here. The German automaker isn't offering the A3 TDI with Quattro right now, and has no plans to do so in the future.All-in, the A3 TDI was truly a pleasant vehicle to drive – just like it is with its gasoline engines. I'd wager that it's better to steer than a Mercedes-Benz CLA250 , but of course, my only time spent with the CLA was in conditions where I could, you know, really drive it. Coasting down mountain roads in Neutral is fun and all, but it doesn't reveal a ton about how a car is performing when all of its systems are firing together. Coasting did allow me to explore the limits of grip offered by the Continental ContiSportContact 225/40R18 tires, as I tried to keep as much speed as possible through the turns. But really, all I cared about was using as little fuel as possible, as I still had a grueling second day ahead of me.Day 1 ended with a 58.4-mpg average, which put me in fourth place among the nine teams. Not bad, but not great. Still, I was proud, and I was eager to work even harder the following day.At 5:30 AM the next morning, I left Sedona under a sheet of darkness and cold temperatures, leaving the windows cracked to get as much of the chilly outside air circulating into the the TDi's minimalist cabin as possible. The TDI's interior is unchanged from the rest of the A3 lineup, which means good refinement, a tight back seat, and all the onboard tech you could ask for in a compact Audi.Day 2 was both the hot one and the tough one – long stretches of nothingness through the desert, sunlight and heat causing fatigue. But I wouldn't settle for 58 mpg, especially when other teams were already seeing numbers over 60.In speaking to Gerdes once again, he warned me that the "pucker factor" would be strong on Day 2. That's what sets in when the distance to empty gets lower and lower, and I'd just have to power (coast?) through, keeping an eye on the current fuel economy readings and doing mental math to predict just how much diesel was still left in the tank. Wayne also offered tips about managing the route's long stretches of "whoop-de-doos," roads that would roll over hills, offering opportunities to pick up efficiency and momentum while going downhill, and then let the speed drop while coasting uphill. On top of that, because elevation would gradually be dropping through the day, I was told to use a "pulse and glide" technique: accelerate briefly on stretches of flat road, getting up to about 50 mph, and then let the engine coast back down gradually to about 40 mph, staying in sixth gear.These tips, plus a constant cruising speed of no more than 45 mph through the desert offered massive gains, as I saw the observed fuel economy average run as high as 64 mpg at times. But the last stretch of the journey, entering the hills just before San Diego on I-8 westbound, combined with the heat inside the car, the sweat, and the tension of trying to get every last mpg out of that Audi, made it so incredibly tough. The onboard fuel gauge told me I had run out, and yet the car kept going. I was left wondering just how much diesel was in the reserve tank, and whether or not I should bail out at one of the planned Audi checkpoints or test my luck and just hope I didn't come to a stop on the side of the freeway. If I succumbed to the latter, I'd effectively be betting that a support car could find us before I had died of heat stroke and was eaten by buzzards. (That's totally a normal thought process, right?)Actually, at this point, I was just glad that I'd be finishing in any place other than last. You see, Audi threatened us a bit, saying that the first team to run out of fuel would have to finish the drive in a vehicle simply named, "It" – a 1994 Ford Aspire with a missing hubcap and a four-speed manual transmission. In fact, to sweeten the deal, Audi officials added more and more odd bumper stickers to the back of "It" as it followed us on the journey, on a trailer behind a Q7 . And while there's part of me that really kind of wanted to roll into swanky San Diego in that Aspire with proud "I Brake For Butterflies" and "Cowgirl" stickers on the back, I figured my best bet was to push the A3 as far as it would go. This, despite running in a team named, "Gunning For 'It'."But alas, there I was, 758 miles later, saying goodbye to the A3 TDI that had just carried me from Albuquerque on one tank of diesel, preparing to finish the drive to San Diego in a shiny new A3 Cabriolet (stay tuned for that writeup). And even though I didn't win – my partner and I finished somewhere in the middle – I was incredibly proud of what had been achieved. 59.4 mpg. 16.4 better than the EPA's own highway rating in a bone-stock A3 TDI (some of my competitors opted to modify the sedan with wheel spats and the like, but I chose to leave it alone). I bragged about my achievement to anyone who would listen."You're gonna blow the EPA out of the water," Wayne told me before departing on this journey. And I did – handily. My journey was difficult, and the speeds and techniques I used aren't necessarily things I'd consider working into my everyday driving routine – let alone recommend for yours. In fact, while we weren't out there drafting semi trucks, hypermiling clearly isn't the safest way to drive. It creates closing speed issues with other traffic, and frankly, you can spend so much time thinking about when the fuel line will run dry that it lessens your mental bandwidth for other tasks associated with driving.However, it did show me just how capable the A3 TDI is when really pushed – you can drive this car 834 miles on one tank of fuel. Think about that for a minute. Even in real-world scenarios, these TDI cars are commonly good for better economy than their window stickers would suggest, which is more than we can say for most gasoline-powered cars, let alone hybrids. Autoblog frequently saw numbers over 50 mpg in our long-term 2011 Jetta TDI , if you recall, without even trying.On its own, the A3 is a solid car. And with a TDI engine underhood, an already good package gets even better, provided you're willing to pay the premium for an entry-level luxury sedan (the Premium Plus tester I drove stickered for about $43,000 which, I'll admit, is kind of a lot). Hypermiling – a word Wayne Gerdes created – isn't in the vocabulary of the everyday driver, but terms like "efficient," "comfortable," "well-appointed," "stylish," and "good to drive" are. The A3 TDI checks all those boxes – it's a premium little fuel-sipper that I'd happily drive again and again, even if I'm not sweating my way to 60 mpg.