Dim PrevMiles
PrevMiles = DMAX("Mileage","Table","Mileage<" & MileageField)
MPG=(MileageField-PrevMiles)/TotalGallonsField

You will need to add more to the criteria like the vehichle ID or something to uniquely identify it if you do multiple vehicles. Otherwise this will take your Mileage field and subtract the biggest number that is below that number (which would be your previous mileage record), then divide by the gallons.

Thank you so much, it help Immensely.

Check out the Subforms video as well.  Richard shows an example of Mileage Tracking in that too.

Adam Schwanz, I am following this thread with interest since I have been trying to figure this out for myself.  My question is, where do you place the "Dim PrevMiles....." code?  Also, what is meant by "MileageField"?  Would this be the same as "Mileage"?  (In my database, I am using the label "Odometer", which refers to the what the odometer reads at the time of gas fillup.  I think that in your example, I would substitute "Odometer" for "Mileage", correct?)

Sorry for all the questions, but I'm still very new to this.  Thank you for your help.

MileageField would be the name of your field that you put the Mileage in yes, whether that be Mileage or Odometer or whatever you called it.

You could put it in a lot of places, I guess the simplest would be to make a button and put that inside the code editor in the on click event. Again with the second part, MPG assumes that MPG is the name of the field on your form that you want the result to go into, if yours is called MilesPerGallon that would be MilesPerGallon=(YourMileageField-PrevMiles/YourGallonsField)

This only works with one car, if you have multiple cars you need to add additional criteria to the DMAX statement to get the unique values for the car you're working on.

Thank you.  Will give it a try.  I appreciate you getting back to me so quickly.

A follow-up, as it seems to be working for me.  I created a form, based on a query for gas fill-up.  On the form, I have a lookup box which selects the proper vehicle.  That lookup is the criteria in the query so I only have the gas data for that particular vehicle.  Back on the form, there is a subform which displays the data from the query.  I then have added unbound text box on the subform where I placed:  =DMax("Odometer","qryGas","Odometer<" & [Odometer])  in the Control source.  For me, this is working perfectly and when I go to enter a new "fill-up", the proper value is displayed automatically.  I also have a 2nd unbound text box where I calculate the MPG, also working correctly.

Again, thank you for your suggestion regarding "DMax".  I had tried that before your suggestion, but couldn't figure out the 3rd portion ("Odometer<"[Odometer]) part.  That was the missing solution.