You are an agronomist and only put 15000 miles per year on a pickup?  You are very efficient.  If your only putting 15000 miles per year on a truck my thoughts do not apply.  

I was assuming as an agronomist you were putting 40000+ miles per year on a pickup.  

I was also thinking about a VW Jetta TDI or a Chevy Cruz as a car.  Both brag about 40 MPG+ and I know from 1st hand experience from owners that they both will do that.

So my math is:

10000/15*3.5= 2333 for truck
30000/40*3.5= 2625 for car for gas or 30000/40*4=3000 for TDI
+$700 insurance for car
total cost is $5658 gas $6033 diesel

40000/17*3.5= $8235 

In my assumption I assume insurance is the same for the truck in both scenarios.  

Cost for the car would have to be thought of also, but I don't know how often vehicles are replaced.  My thoughts are that the pickup would be good for 10 years and the car would be good for 4 years.  As opposed to having to replace the truck only scenario every 3 years (120,000 miles).

So basically it all comes down to miles traveled in your truck.   

If it is worth it or not depends on what image you want to show to your customers.

Andy