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2018 GT Sport, 10,300 miles. I have been consistently getting 36-37 mpg computer reported gas milage on So. Cal freeways.

I decided to finally check the milage via a gas tank top off for a trip to Rancho Bernardo (San Diego) I filled up with 91 octane at Costco which is directly off the Freeway in Ontario/Eastvale. The trip was 187 miles (about 15 miles was street) Cruise Control was set at 70 mph for a majority of the Freeway miles.

When I made it back to Chino it took 5.22 gallons to fill the tank = 35.82 mpg. The computer calculated mpg was exactly 36.0. The outside temp was mid 60's for both fill ups. When filling up the tank I let it automatically shut off. When filled I waited for five seconds and continued to fill until it shut off again. Essentially trying to get the exact amount in the tank for both fill ups. Certainly not foolproof, but the best I could come up with.

That trip is anything but flat terrain with some significant climbs. (and descents) The cool temps and 91 octane probably helped, but non the less I was impressed with that milage vs the EPA rated 32 mpg. Probably most amazing was that the Computer generated milage was near dead on with the actual. Others have reported 5+ percent lower mpg actual vs computer.

Not that it would matter much but I am running a fresh fill of 5w-30 Shell Rotella Syn. oil. (Marketed for SUV / Trucks but perfectly fine Dexos 1, gen 2 SN+ oil for any car requiring those specs)
 

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2018 GT Sport, 10,300 miles. I have been consistently getting 36-37 mpg computer reported gas milage on So. Cal freeways.

I decided to finally check the mileage via a gas tank top off for a trip to Rancho Bernardo (San Diego) I filled up with 91 octane at Costco which is directly off the Freeway in Ontario/Eastvale. The trip was 187 miles (about 15 miles was street) Cruise Control was set at 70 mph for a majority of the Freeway miles.

When I made it back to Chino it took 5.22 gallons to fill the tank = 35.82 mpg. The computer calculated mpg was exactly 36.0. The outside temp was mid 60's for both fill ups. When filling up the tank I let it automatically shut off. When filled I waited for five seconds and continued to fill until it shut off again. Essentially trying to get the exact amount in the tank for both fill ups. Certainly not foolproof, but the best I could come up with.

That trip is anything but flat terrain with some significant climbs. (and descents) The cool temps and 91 octane probably helped, but non the less I was impressed with that milage vs the EPA rated 32 mpg. Probably most amazing was that the Computer generated milage was near dead on with the actual. Others have reported 5+ percent lower mpg actual vs computer.

Not that it would matter much but I am running a fresh fill of 5w-30 Shell Rotella Syn. oil. (Marketed for SUV / Trucks but perfectly fine Dexos 1, gen 2 SN+ oil for any car requiring those specs)

I don't get quite that mileage with my N-Line. But then, I'm not gentle on the go-pedal. Just as a note, though, the ECUs on these cars are relatively fixed. No doubt you may notice a difference in how it drives and it's responsiveness with 91 Octane, but it'll pretty mild and could possibly just a handful of HP gain total. If at all. The ECUs are calibrated for 87, and unlike some manufacturers, doesn't deviate very far at all in it's mapping and timing advance. At worst, you're just wasting a bit of money every time you fill up.


That being said, ECU tunes and add-on "chips"... actually unlock quite a bit of power in these engines.
 

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2018 GT Sport, 10,300 miles. I have been consistently getting 36-37 mpg computer reported gas milage on So. Cal freeways.

I decided to finally check the mileage via a gas tank top off for a trip to Rancho Bernardo (San Diego) I filled up with 91 octane at Costco which is directly off the Freeway in Ontario/Eastvale. The trip was 187 miles (about 15 miles was street) Cruise Control was set at 70 mph for a majority of the Freeway miles.

When I made it back to Chino it took 5.22 gallons to fill the tank = 35.82 mpg. The computer calculated mpg was exactly 36.0. The outside temp was mid 60's for both fill ups. When filling up the tank I let it automatically shut off. When filled I waited for five seconds and continued to fill until it shut off again. Essentially trying to get the exact amount in the tank for both fill ups. Certainly not foolproof, but the best I could come up with.

That trip is anything but flat terrain with some significant climbs. (and descents) The cool temps and 91 octane probably helped, but non the less I was impressed with that milage vs the EPA rated 32 mpg. Probably most amazing was that the Computer generated milage was near dead on with the actual. Others have reported 5+ percent lower mpg actual vs computer.

Not that it would matter much but I am running a fresh fill of 5w-30 Shell Rotella Syn. oil. (Marketed for SUV / Trucks but perfectly fine Dexos 1, gen 2 SN+ oil for any car requiring those specs)

I don't get quite that mileage with my N-Line. But then, I'm not gentle on the go-pedal. Just as a note, though, the ECUs on these cars are relatively fixed. No doubt you may notice a difference in how it drives and it's responsiveness with 91 Octane, but it'll pretty mild and could possibly just a handful of HP gain total. If at all. The ECUs are calibrated for 87, and unlike some manufacturers, doesn't deviate very far at all in it's mapping and timing advance. At worst, you're just wasting a bit of money every time you fill up.


That being said, ECU tunes and add-on "chips"... actually unlock quite a bit of power in these engines.
I can definitely say there have been times on reg. 87 I have had mild knocking. The ECU adjusting the timing for that robs a bit of HP. No such knocking on 89/91 octane.
 

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I try to only use Shell's V-Power Nitro, however its hard to find up here in the NorthBay, so Shell's premium is what I mainly use. Stock Tune, hopefully I'll change that soon. I did a nice roadtrip ~200 miles; which I was doing about 65-70 mph. I got up to ~36mpg which is way off the EPA estimate, but I also have an intake & catless downpipe ... sooo that might help with somethings, who knows?
 

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I can definitely say there have been times on reg. 87 I have had mild knocking. The ECU adjusting the timing for that robs a bit of HP. No such knocking on 89/91 octane.

Could have more to do with gas quality (Top Tier). I'm not disagreeing, I'm just saying that it may not be as effective as one may think.


Also, smoother power delivery can be had with a better boost solenoid. The factory one isn't that great.
 
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