1. A diagnostics and prognostics evaluation method for aircraft electric taxi systems (ETS), comprising: receiving, with a computer, an electronically recorded first performance parameter of a first ETS of a first aircraft during a taxi operational event of the first aircraft at an airport; receiving, with the computer, electronically recorded first comparative performance parameters corresponding to the first performance parameter from electric taxi systems of multiple aircraft during corresponding taxi operational events of the multiple aircraft at the airport; generating, with the computer, a first performance factor statistical model in response to the first performance parameter and the first comparative performance parameters; comparing the first performance parameter of the first aircraft with the first performance factor statistical model; and calculating a first performance parameter difference based on the comparison; generating a first aircraft electric taxi system maintenance problem report when the first performance parameter difference exceeds a predetermined acceptable first performance parameter difference value; wherein the taxi operational event of the first aircraft and the corresponding taxi operational event of the multiple aircraft include an ETS operation at a steady state speed; wherein the first performance parameter includes one of a speed obtained during ETS operation at steady state speed and power used by the APU to power the ETS during the ETS operation at steady state speed, or fuel used by the APU to power the ETS during the ETS operation at steady state speed; receiving, with the computer, electronically recorded taxi data and aircraft identification information from the first aircraft with an electric taxi system, the taxi data including position of the first aircraft and corresponding auxiliary power unit status, electric taxi system status, and aircraft speed for each takeoff of the first aircraft and each landing of the first aircraft for a predetermined fuel reporting time period; determining, with the computer, electric taxi system use time periods during all first aircraft taxis to takeoff or first aircraft taxis after landing for the predetermined fuel reporting time period during which motive power was supplied to the first aircraft from the electric taxi system, in response to the taxi data; determining, with the computer, an electric taxi system configuration, an aircraft configuration, and a main engine configuration of the first aircraft in response to the aircraft identification information; estimating, with a computer electric taxi system performance model or in response to a fuel level signal, an amount of fuel used by an APU (auxiliary power unit) of the first aircraft to power the ETS system during each electric taxi system use time period in response to the taxi data and the electric taxi system configuration; estimating, with a computer aircraft performance model, an amount of fuel which would have been used by the main engines of the first aircraft during each electric taxi system use time period if main engines of the first aircraft had supplied motive power to the first aircraft, in response to the taxi data, aircraft configuration, and main engine configuration; calculating with the computer, a first aircraft amount of fuel saved based on the amount of fuel used by the APU and the amount of fuel which would have been used by the main engines for at least one first aircraft takeoff or first aircraft landing during the fuel reporting period; determining, with the computer, main engine use reporting time periods during all first aircraft taxis to takeoff or first aircraft taxis after landing for the predetermined fuel reporting time period during which motive power was supplied to the first aircraft from the main engines, in response to the taxi data; estimating, with the computer aircraft performance model or in response to a fuel level signal, an amount of fuel used by the first aircraft during each main engine use reporting time period in response to the taxi data, the aircraft configuration, and the main engine configuration; estimating, with the computer electric taxi system performance model, an amount of fuel which would have been used by the first aircraft during each main engine use reporting time period if the ETS system of the first aircraft had supplied motive power to the first aircraft, in response to the taxi data and the electric taxi system configuration; and calculating with the computer, a first aircraft amount of fuel which would have been saved based on the amount of fuel used by the main engines and the amount of fuel which would have been used by the APU for at least one first aircraft takeoff or first aircraft landing during the fuel reporting time period.