1. No. Sport mode merely makes the shifts at a bit higher rpm, and allows downshift to occur sooner when you step on it, cannot possibly affect mileage when you cruising along in 4th gear.

2. Not sure what you mean by regular and high test. In NC we have 3 grades available 87, 89 and 93 (or is it 91, can't remember). I run 89 and have used 87 with no problems. Your car will adjust timing if pinging occurs with the 87. Running higher octane will neither increase mileage or power.

3. That's fantastic mileage! I have never done better than 22.5 highway on my 97 V90. If AMM was outputting an out of range signal that could not be corrected by the O2 sensor you would have a check engine light on. What year do you have?

4. As above your mixture is rich to the point where the O2 sensor signal does not allow for fine mixture adjustments you will get a fault code.
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David Hunter