Sure. The calculators are emperically derived. They are not based on a black and white physics analysis. Transmission and TQ are the best examples. Efficiency changes matter, e.g. more time in the peak power band, more gears, lighter weight spinning internals......equate to more average power to the tire and faster and quicker car without more power under the hood. Change the weight of the tires/wheels will change et and mph, possibly in opposite directions.

My point is, lots of variables, and even if you attempt to account for them, it's still an estimate. The calculators are tools, just understand their limitations, and why.