A bunch of things affect the design shape of a liquid carrying tank. Perhaps the most important consideration is vehicle stability while making a turn. Ideally, for the best stability the loaded vehicle's center of gravity should be as close to the ground as possible. An elliptical style tank has a cross-sectional area similar to a rectangle, which when compared to a square of the same cross-sectional area will have a lower center of gravity. However, the corners in the rectangular tank create some issues for various producst carried in terms of cleaning the tank and the large number of joints. Hence, the elliptical design is used in a large number of non-pressurized liquid tank designs.

From a stability perspective, the elliptical design also provides maximum stability against liquid movement inside the tank during periods of lateral acceleration. When we encounter liquid tanks with a circular cross-sectional design those tanks are used more for pressurized liquids where tank strength must combat vapor pressure.

From a fire department tanker perspective, choose the design of your tanker wisely...making sure the center of gravity on your new rig is as low as possible and don't discount the use of the elliptical tank design just because it is not as "in vogue" right now.