On paper the are tied directly to each other. Increase Velocity and energy will increase. In the field, Sectional density, bullet construction and always shot placement are whats important. Its a combination of weight, density, construction, momentem and velocity. Also bullet frontal area and shape play a part. Its a complex subject with many opinions and not to many things written in stone. For deer, I like a bullet like the Partition and Ballistic tip in a medium caliber size at about 26-2800 fps. Sectional density of about 200- 230. For elk Move the SD up to 220 -290. For really big stuff the 280 and up SD bullets are probably best. One exception can be bullets with large frontal areas like the 44 and 45 calibers. Here bullets of less SD still seem to do a good job of killing game. Big bullets create big wound channels even with very low velocity. Many big game animals have dropped to the old black powder bullets like the 45-70 lumbering along at 1100-1200 FPS with 500 grain bullets. Ask the remanining Buffalo.