You're more or less describing a golf cart.

The reason computer controlled is used is because in order for electric vehicles to be efficient they need a special transmission, nobody has designed a fully mechanical transmission that compares to computer controlled (not least of all because mechanical transmissions are reactive, not proactive, meaning you're always less efficient than you could be, because that very inefficiency is what drives the transmission to shift).

You'd also lose regenerative braking which is a major "win" for electric vehicles.