The smaller the engine the less fuel it will use. But then again an engine too small for the application can use more fuel than a larger engine that is appropriate for the application.
A very small vehicle with a small engine is most likely to use less fuel.
In the United States the most fuel efficient pickup truck is the Ford Ranger with a 4 cylinder engine.
Many SUV's are not fuel efficient.
In the United states the Toyota Prius is the most fuel efficient car using a hybrid of electric and fuel in a 4 cyclinder engine allows close to an average of 50 miles to a gallon on any commute.
The honda cb 110 is the most fuel efficient motorcycle in it's class
The most dependable fuel efficient car of 2014 is the Ford Focus.
The 2012 Mitsubishi is one of the most fuel efficient SUVs in the market. It has 112 MPG and is very fuel efficient. I recommend the 2012 Mitsubishi for your needs.
Www.utoGuide.com has a list of the top ten most fuel efficient pickup trucks for 2013. Www.shopautoweek.com also has a list of its top four most fuel efficient pickup trucks.
The most fuel efficient GMC Truck you can get it the GMC Banarge, it can come with a small 1.8ltr Diesel Engine offering upto 25Mpg, tho this truck is rather slow it can be great for starter businesses to save on fuel costs
The most fuel efficient vehicles on the market today are usually hybrids. However, many small and economy sized vehicles on the market are not hybrids but are very fuel efficient as well.
A bicycle!
The most fuel-efficient roof rack available on the market is the Thule Aeroblade Edge.