higher octane gas is harder to ignite hence the longer cold-startup time. the economy gain probably is from increased ignition advance since it isn't getting any knocking with higher octane (knock sensor input to ecu). with a high-mileage engine this is common because of carbon deposits taking up combustion chamber space (higher compression ratio) or glowing (or both)
1991 Toyota Pickup 22R-E 2.4 I4/5 speed
1990 Toyota Cressida 7M-GE 3.0 I6/5-speed manual
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