1. Your table is normalized.
2. For the sake of this example we'll call it Fuel. We'll also assume the only two fields we care about are a data field [FuelDate] and the amount of fuel [FuelVolume]
3. The only days that we currently care about are today and yesterday.
4. The only thing we're looking for is the difference between the fuel volume today and the fuel volume yesterday.
This can be done in one query. You could also modify it to look at days other than today, or even longer time spans by using references to textboxes
in your forms rather than the simple date() function.
SELECT Sum(IIf([FuelDate]=Date(),[FuelVolume]*-1,IIf([FuelDate]=(Date()-1),[FuelVolume],"0"))) AS FuelUsage