Change reference cell in formula based on selection in drop down list

I'm working on a way to estimate gas costs for a trip.

This is the formula:

G = Gas Use Estimate
D = Distance to Travel
M = MPG (no load)
W = Load Weight

G = D / (M - (W / 100) X (M/100))


This formula gives me an estimate of how many Gallons gas I will need to make the trip there. Now I change the type of vehicle using a drop down list. I wonder if there is anyway that I can tell Excel to change the M value based on what vehicle is selected in the drop down list..

If cell A1 is car type1 in the drop down list and cell B1 car type1 MPG (no load)
If cell A2 is car type2 in the drop down list and so on.

Right on, this is good stuff to know. Apparently my understanding of it is a little off tho.. Here's how I'm interpreting it:

- (lookup(A1,{"car type 1","car type 2"},{5,10}) -

this is telling Excel to analyze the data in cell A1 (this is the cell with the drop down list where I select the vehicle type).
The formula continues: if A1 contains the txt "car type 1" the value of M is 5, if A1 = "car type 2" the value is 10.

Using this understanding I came up with the following formula:

Formula:

=P31/(LOOKUP(N31,{"Small Delivery Vehicle","Medium Delivery Vehicle","Large Delivery Vehicle"},{40,15,5.5})-(O31/100)*(LOOKUP(N31,{"Small Delivery Vehicle","Medium Delivery Vehicle","Large Delivery Vehicle"},{40,15,5.5})/100))

This formula returns incorrect values:

When Small Delivery Vehicle is selected it returns a value of: 96.36363636
When Medium Delivery Vehicle is selected it returns a value of: 35.3333333
When Large Delivery Vehicle is selected it returns a value of: #N/A

Perhaps someone is catching my error?

Change:

=LOOKUP(N31,{"Small Delivery Vehicle","Medium Delivery Vehicle","Large Delivery Vehicle"},{40,15,5.5})

To:

=CHOOSE(MATCH(N31,{"Small Delivery Vehicle","Medium Delivery Vehicle","Large Delivery Vehicle"},0),40,15,5.5)