195/60-15 Tires, premium and MPG

[Henry G.]

So the stock Goodyear LS were toast at 29k. Or damn near, driving thru a sprinkler leak was scary not that these tires were any good in the wet regardless. So I bought some 195/60-15 Eagle GT's, the new V-rated tires. At 40 psi, (they are rated for 51 psi max) I noticed they were, well squidgy. The turn in and general feel were not tight like the old ones, even worn out. Up to 45 psi: better. So up to 51 we go. Much better, grip is good, and feel is better but still not as tight as the old ones. I think its the taller tire, even though they are 60 profile they are are taller as the width has increased. But I can deal with it, even though I drive 55 miles of canyon a day. Wished I had got the 195/55 tires instead now. So to the point: I immediately noticed an MPG decrease. It got better when I went to 51, up to 33 from my usual 36 taking into acct the small speedo error due to the taller tire. But heres the kicker- as it got hot( 95-100 degrees) recently, I am running the engine extra hard as usual but with the A/C on a lot more, so I went from 89 octane to 91 premium. Right away I noticed the fuel gauge bars going down slower, and the calculator doesnt lie, I'm now getting 35 mpg last 2 tanks. I'm thinking the car's engine computer noticed the increased octane and changed the timing or am I just getting imaginative? Either way the mpg has definitely improved. Any ideas ? I keep reading that octane doesnt improve mpg but in this heat and engine load it sure seems to.