To determine acceleration g-forces, input starting speed, final speed, and the amount of time elapsed from start to finish. Starting speed does not have to be 0mph. Starting speed could be from a roll at 5 mph perhaps. For most accuarate results, the less distance between start and end and most accurate time elapsed will give the most accurate acceleration calculator results.
Vehicles usually have maximum acceleration at the start. Wind resistance, friction, and other factors cause acceleration rate to deter at higher speeds. Visually as a graph it would not be a straight line, but instead a curve that decreases acceleration over time because acceleration is not constant.
You could use this to get a very rough estimate of g-forces during 0-60 acceleration.