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04 Silver Highlander
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For any one who is interested:switched to shell premium gasoline & for whatever reason I'm getting 90 mpg per qtr. tank,before I had always used plus Highlander 04 V6 at over 75k miles I felt as if the milage was silpping a little,so I said what the heck...now I must say I have never tried the experiment with different grades before but for now I will keep using the 93 octane to see if ant thing changes I guess I will stick with shell for now...Comments welcome.

-Ray.
 

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I have suspected that higher octane fuel specifically in the HL engine should result in higher mpg because the exact same engine is in the Lexus RX and the RX manual says it should be run with premium gas. So while running regular octane fuel in the HL, in theory the engine computer slightly retards the ignition to prevent pre-detonation under load which should result in slightly lower fuel mileage. My guess is that you can pay 10-15% more for premium and get 10-15% more mpg so it may be a wash but I have not tried and measured both types of gas to check actual mileage variance. I may run a couple of tanks of premium and see if there is a significant difference.


As an aside - We get about 18.5 mpg over mostly city around town driving in our 07 Limited with 17 inch wheels. We averaged 19.5 in our previous 06 HL (not a limited) - and it had 16 inch wheels and sat slightly lower to the ground. For a short while we had them parked side by side in the garage and it was clear that the 07 with the larger wheels was at least an inch taller than the 06. My experience is that taller higher ground clearance cars produce lower mpg.
 

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AvConsult
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I did an 87 vs 89 vs 91 octane test a few years back on my '02. With 89 the mileage appeared to improve over 87 about 1 mpg over several tanks. Not exactly a statistical certain value, given variability in gas pump shutoff points, ambient temps, stop light cycles, traffic....However, it clearly indicated *some* improvement. 89 is 15 cents more a gallon than 87 in Dallas. This translates to 17.5 cents/mi for 87, and 17.3 cents per mile for 89.

91 octane looked to be about a 1.5 mpg improvement over 87, same caveats for measurement as above. 91 is 30 cents more than 87. Translates to 17.6 cents/mi.

What I did notice was quite a dramatic improvement in throttle response/low speed torque from 87 to 89, and another incremental change from 89 to 91. Especially when the A/C is on or carrying load.

All said, I've settled on 89 year round, as it provides good throttle response and probably a net neutral cost.

Other might see slightly better improvement in mileage between octanes, depending on their driving habits and conditions.

EDIT: Over a years time, I see 21.5 to 22.5 mpg with 89 octane, calculated by odo reading and pump fill.
 

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AvConsult
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I should mention that the '01/'02 HL have a 3.0L and inferior knock sensor design that makes it more sensitive to octane. The 3.3L seems to to perform better with 87, with less dramatic but still detectable difference as you get to 91.

I know my mom's '07 V6 didn't shift on hills near as much when I switched to 91 octane about half through a trip to western Nebraska/Eastern Colorado.
 
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