LOL! Actually, that was *NOT* showing the "first bus". It was showing the totals and averages for *ALL* buses!
It's a peculiarity of MySQL that it allows you to do SUM and MAX and so on queries (called "aggregate functions", by the by) *without* requiring that you get the GROUP BY correct. Any other DB would have given you an error.
Since you didn't bother to show the fields in your table, I'll make a guess:
SELECT busNumber, driverName,
MAX(OdometerReading) AS finalMileage,
MIN(OdometerReading) AS startMileage,
SUM(Gallons) AS fuelUsed,
MAX(OdometerReading) - MIN(OdometerReading) AS totlaMiles,
( MAX(OdometerReading) - MIN(OdometerReading) ) / SUM(Gallons) AS MPG
GROUP BY busNumber, driverName
ORDER BY busNumber
The prinicipal is simple: Any *NON* aggregate fields in your SELECT *must* then be mentioned in your GROUP BY.