The fire department should get a cheaper car for these calls then.

That isn't cost effective. They need the large trucks just in case - even though realistically they are never used. A large truck isn't that much more expensive to drive than a small one, and in any case if the large truck isn't used for more than a couple days you need to take it for a drive - ensure all the operators know how to drive it and if something broke (sitting is harmful to moving parts - wear is worse, but sitting is harmful) you want to find out and fix it right away.

Which is to say the small cheaper car only makes sense after you have enough large ones and conclude you still need more cars.