Fuel Economy with IS38

One look at that title must have you wondering – “What kind of lunatic tracks fuel economy in a modified GTI?” THIS GUY does. HAHA! Coming from a Volkswagen turbodiesel background and having a lifetime of fuel economy testing under my belt, you KNOW I took the car on a road circuit to see how the IS38 affected my fuel economy. I got 39.6 mpg when the car was stock and descending from Tennessee to the coast during its first trip to Tail of the Dragon. Later, I developed this 200-mile road circuit (see photo) where I could make consistent drives to test fuel economy at efficient highway speeds in support of this article about how different loads affect fuel economy. I drove the circuit again after upgrading to APR Stage 2 and achieved 38 mpg. I had hoped to do better with APR’s high-flow catalytic converter. Unfortunately, I didn’t test fuel economy after upgrading to the Baun Performance cat-back exhaust. I wish that I had.

I had planned to test fuel economy after the Baun upgrade, but I had visited him in July 2020 and wanted to do my testing in October. I prefer to do my fuel economy testing in the spring and fall when I can drive comfortably without the air conditioner. Sure, that boosts my fuel economy numbers, but it also enables me to achieve consistent results. October came and I was ready to test; but then I had to make a pair of road trips to California. Before I knew it, I had jumped into an IS38 upgrade without testing the Baun exhaust. Moving forward, I wanted to see if the IS38 presented a reduced exhaust restriction or if the slower turbo spool might have improved my fuel economy on a well-behaved trip. The IS38’s turbine exducer opening is 3mm larger than the IS20. That’s only 6% larger, but it’s bound to yield slightly less exhaust restriction. Less restriction leads to better fuel economy, which was why I wanted to test the Baun exhaust setup.

I drove the 200-mile circuit, shown above, at the posted speed limit, early on a Saturday morning in May to avoid driving in traffic and running the air conditioner. The fuel gauge needle was baring moving as I drove. Usually, I think I’m doing well if I can get 50 miles per 1/8-tank on the gauge. I was doing better than that… MUCH BETTER. I drove exactly 200 miles and filled with just 5.1 gallons, resulting in (drum roll, please) 39.2 mpg! I hadn’t beaten my previous record, but I was still amazed with getting 39 mpg from an IS38 GTI! However, there was a problem: It’s not hard to argue that even a small deviation during the fill-up could have resulted in a change. After all, 5.1 gallons isn’t much. Squeezing-in another tenth of a gallon would knock-off almost a full mile-per-gallon. Could I have squeezed-in another 1/2-gallon? I don’t know. I needed a longer route to narrow my margin for error.

It’s difficult to leave my area without being forced onto the Interstate. I refuse to “hypermile,” which often requires driving below the speed limit, especially on an Interstate highway. Instead, I drove my same test route, but continued farther west along rural Hwy 58 to Danville, VA. That more than doubled the distance of my test (see photo). Traffic was light during my morning drive. But Hwy 58 west of Emporia has more rolling hills. I was certain that the hills would have a negative impact on my overall fuel economy. I also encountered a slow-down at a local tunnel on I-664. “What if I told you that the speed limit inside the tunnel is the same as outside the tunnel?” As I got closer to home, I realized that I could stretch the drive and make it my longest single tank of fuel! I continued along I-64 past my exit to see if I could get 500 miles out of a single 13-gallon tank. My extended route took me to 502 miles! I still had room to drive farther, but decided against taking additional risks. I refueled with just 11.9 gallons, resulting in (GASP!) 42.2 MPG!

Of course, driving the speed limit on 55-60 mph roads is BORING. Plus, it’s not realistic in most areas. I decided to take the car on another trip to Emporia, this time on the Interstate and with the air conditioner. I drove between 70 and 80 mph, mostly 75 mph. The drive was 242 miles. I needed just 6.6 gallons upon my return. That tank, even with mild speeding and air conditioning, still returned 36.7 mpg! But what about with a trailer? I had just finished a 7600-mile trip with my cargo trailer. Most of my worst tanks were in the 19-20 mpg range due to high speeds and the best were at lower speeds in CA, which were around 24-26 mpg. How could I do on my 55-60 mph circuit with the same trailer after an IS38 upgrade? With the cargo trailer in tow, I used just 7.8 gallons for the 214-mile drive. That’s 27.4 mpg! Some GTI owners don’t get that with just the car alone. HAHA!

At this point, I’m sure plenty of you are calling BS on this testing. To be clear, I did not rely on the dash display for reporting my numbers. All of my calculations are “miles driven” divided by “gallons dispensed.” I used the same fuel dispenser at each fill-up. For my refueling method, I ran the pump until it clicked-off, waited 30 seconds for any foam to settle, then pumped slowly until it clicked again. After that, I’d squeeze it up to the next tenth of a gallon. That’s not as perfect of a fill-up as when I could fill to the brim on my TDIs, but it’s about as consistent as I can get without damaging my evaporation canister. Sure, there’s still room for error. But I cut that error in half on the 500-mile tank. I would have been satisfied if the reduced error had knocked my economy to a lower number. I had no idea that I would be the best I’d ever seen!

It’s worth noting that these were not freak tanks! If you look at the data for all of May, you can see that I had a good month, mostly because I wasn’t driving aggressively. Even before my IS38 upgrade, I was regularly asked, “How do you get such great fuel economy?” After all, even *I* am skeptical when I see someone posting a high fuel economy number, especially if it’s accompanied by a photo of the instrument cluster readout. Well, other than having a turbo that doesn’t pull the car into action below 3000 rpm, here are six of my most effective tips:

1) STOP SPEEDING – Yeah, that one’s a tough one! But speed is the #1 killer of fuel economy. Higher speeds lead to greater wind resistance, which requires more engine power and fuel to move the car.

2) MAXIMIZE THE USE OF TOP GEAR – The “shift” indicator on the instrument cluster is prompting us to drive efficiently. I used to ignore that indicator because I like driving closer to 2000-2500 rpm so that the engine was a little more responsive when I wanted it. Now that I’m driving with a less responsive turbo, I’ve been content to just drive in sixth gear anytime I’m driving faster than 45 mph. Sure, it’s a dull way to drive, but it’s efficient! See this video by “Engineering Explained” if you want more details about how lower engine speeds lead to better fuel economy.

3) TAKE HIGHWAY ROUTES – This may not work if you’re a city driver. But the engine will run far more efficiently if you can get on the highway and drive at steady speeds. The longer you drive at a steady, economical speed, the more your fuel economy will climb, to a point. In some cases, taking a longer highway route may be more efficient than driving a shorter, stop-and-go route.

4) AVOID DRIVE-THRU WINDOWS – I spend less time in drive-thru lines now that I no longer have a daily commute. Idling in drive-thru lines wastes A LOT of fuel. The same can be said of traffic lights. So, run those lights when you can! Just kidding! Think about the math: When you sit and idle the car, you’re getting ZERO miles per gallon. Anytime you average “zero” with any other number, the average of the higher numbers will fall. The more “zeroes” you add, or the longer you idle, the lower the average will be. Along the same line, don’t warm-up your car. That wastes fuel. Start the car, idle it while you fasten your seat belt, plug-in your smartphone, and choose your music, then drive gently until the oil temperature reaches 180°F.

5) ACCELERATE GENTLY – Brisk acceleration requires more fuel. If you’re not in a rush, then accelerate with half-throttle or less and shift at 2500 rpm for good fuel savings. “Engineering Explained” has a video about that, too.

6) MINIMIZE YOUR BRAKING – Some people put their car in neutral and coast to a stop, using the brakes to stop the car. Instead, downshift and let the engine slow the car. When the car is in neutral, the engine needs fuel to maintain idle speed. If the car is in gear, the rotation of the wheels keep the engine turning and the injectors are nearly shut-off. Don’t believe me? Watch your “instantaneous economy” readout on the instrument cluster and see what it does when you coast in neutral vice coasting in gear.

“But Scott, my car is made for fun. This is BORING!” Yes, I agree. The thing I like about my GTI is that it’s a great compromise between power and efficiency. Sure, it can’t get the fuel economy of a Prius; but a Prius can’t surprise anyone with brisk acceleration. On the other hand, my GTI cannot outrun a Corvette, but a Corvette can’t touch the GTI’s fuel economy or utility… plus, it’s far more expensive. Where do you draw your line?

Have I given you anything to think about with regards to fuel economy or economical driving habits? Will you change your driving habits to see if your fuel economy improves? Comment below if you give it a shot and see results. See a video version of this entry here.

Bored, but Flush with Gas Money!


About Scott

I grew up near Houston, TX and served in the U. S. Coast Guard for over 30 years. I have an electronics background and continue to work in the electronics engineering field. I taught myself the basics about automotive systems as well as how to perform some of my own maintenance (cars and bicycles). I became involved with Amateur Radio and computers in 1995. The explosion of technology has made my job and several of my hobbies quite interesting. My hobbies include Volkswagens, bicycling, photography, electronics, amateur radio, web management, and reptiles. Visit my websites to learn more.
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4 Responses to Fuel Economy with IS38

  1. Maggie says:

    Thanks for the post, Scott! Is it bad that I have a hard time trusting the cluster gear recommendation? I mean, I can’t believe that VW would recommend anything to the driver that would damage the car long term, but it just feels so odd being in 6th at such low speeds. What do you think?

    • Scott says:

      Thanks for writing, Maggie! I trust the display’s gear suggestion when cruising on relatively flat roads. But I drive a gear lower as soon as I hit rolling hills. And I downshift when I expect to accelerate.

  2. Ricardo says:

    that was an amazing article!!! i only have 1 question tho, whats your hp when added the is38?

    • Scott says:

      Thanks, Richardo! Depending on how you measure, APR claims just under 400 hp at the crank and ~360 at the wheels. Some say that my exhaust system adds a bit more. I’ve never measured; so I just stick with APR’s claim. Thanks for visiting!

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