xRiCeBoYx

iTrader: (22)

I recently took a road trip recently (got back in to Vegas last night, actually) and I decided to experiment with my my car.

Now, the L15B7 (L15BA, apparently, in my case, per various parts websites calling my engine that) found in the 1.5L Turbo civics was tuned to run safely on 87 octane. The L15BA (which, I'm assuming is the extra tune the Sport/Touring models get that gives it an extra 6bhp -- 174 vs 180bhp) says "87 octane requires, 91 recommended," which my friend that has a 2017 EX Sedan says is on his, as well.

That said, I decided to see if and what any perceived or observed performance differences would be if I used both fuel grades, and I figured a road trip with 98% highway miles with minimal elevation changes (exceptions apply, will address later) would be the best way to test it out. The 2018 Civic LX/Sport has a different instrument cluster than the EX and above trims, but all seem to have these bars on the top (seen below) to give you a quick indication of how your fuel economy is. The bars were white when the fuel economy gauge was 0mpg to about 28MPG. Light green from about 28-36MPG, and green above 36mpg. Fancy maths using engine load, throttle position, etc goes into making this fancy pants light show.

Pictured is the instrument cluster from the LX/Sport models. EX and above look much fancier.

Photo source: motortrend.com found via google images. Arrows added by yours truly.

Outlined in red is the course I took back home. The red line at Barstow is where I refueled. Started in CA central valley and finished in Vegas. I'll address the area in green in my findings below. For those that have driven I-15 east probably know that area and why it's significant. lol



The first leg of my trip back to Vegas was a tank of 87 octane gas, filled at a Chevron station in Lathrop, CA (literally right off of I-5). Ambient temperature was fairly constant throughout my drive, somewhere between 55-65F (12.7-18.3C). When I accelerated to freeway speeds, I left the car in economy mode and tried to keep the bars at least light green, not going to white if I could possibly help it. Throughout the drive, cruise control was pretty much set at 83MPH (flow of traffic was at about that speed, some cars speeding by me at probably 90-ish), and lane changes were minimal. During this leg of the trip, elevation was rather flat with very minimal road grade changes. During the slight uphill climbs, the bars went from green to light green fairly often, and average MPG (would be in place of "Range" on the pictured cluster) never went higher than 34.5MPG. The estimated range by the time I got to the gas station in Barstow, CA to fill up was ~17 miles. I record all my fillups, and I netted 354 miles on the one tank, and the refill was for 10.54 gallons (albeit, this fillup was with 91 octane). App with its fancy maths said 33.58mpg average. I guess I should note that I do not top off my tank for "accuracy" of fuel economy calculations. I let pumps stop on their own, wait a few seconds, then remove nozzle from gas tank. Another thing I do to keep things consistent on my end. Varying calibrations of pump shut off is beyond my control.

On the second leg, I stopped at a Chevron station in Barstow, CA, just to keep things consistent. As mentioned above, I filled up with 91 octane this time, and again, with driving to the freeway onramp and getting to merging speed, never let those bars become white. Cruise was also set at 83MPH, with economy mode on. Slight increases in road grade never made the little bars change from green to light green, and my average MPG never dropped below 36.5 (at one point, it stayed at 38.8 for a good hour or so). As my car sits right now (with a bit less than half a tank) my average MPG for this tank says about 34.9.
but Josh... you said the little average MPG meter never dropped below 36.5..? Did you just up and gun it everywhere once you got home..?
Well, this is where the little area in green comes into play. Once you pass Baker, CA on I-15N, the freeway goes from fairly flat to a 6% incline. Killed my fuel economy, and brought it down to 32.5 at its lowest. Mind you, even in economy mode, the bars never became white while cruise was set.

Now, this test isn't 100% conducive since I didn't use a full tank of each, but I feel like my leg of 91 octane was enough to give me results I can rely on. While a tid bit more expensive, engine power was perceptively better, and yielded better fuel economy as a whole.

TL;DR
1.5T Civic is tuned for 87 octane, but 91 recommended.
87 octane seemed to struggle a bit more and got worse fuel economy than 91. 91 played a lot nicer with L15B7 (L15BA) engine.
Take my experience with a grain of salt.