At the turn of the century, gas-driven cars were considered a novelty — and a hazard — lacking the reliability of a horse. Motorized fire equipment merely supplementary to the more respected steed.

Required an "A" ticket in 1964-1965

From Main Street Vehicles Story Guide 1966:
The motorized fire engine is similarly a composite design of a hose carrying fire engine of the turn of the century. The primary change we made was to place seats where the hose was carried. The Studio men designed a chassis, then pored through standard catalogue for unlikely, but practical equipment — a jeep rear axle, a three-speed truck transmission, the power plant of a small pick-up truck and standard drive-line parts. The bell and the siren are authentic, purchased after considerable search.

From WED Disneyland Dictionary 1968
Answer "alarms" with visitors aboard. The two 1900-era fire engines begin their bell-ringing journey down Main Street from the Disneyland Fire Department in Town Square.

From 1970's Attraction Showmanship:
The motorized fire engine is a composite design of a hose-carrying fire engine of the turn of the century. The primary change made on the Disney fire engine was to place seats where the hose was carried. "Imagineers" designed a chassis, then pored through standard catalogues for practical equipment--a jeep rear axle, a three-speed truck transmission and standard drive-line parts. The bell and siren are authentic, purchased after considerable search. Lacking the reliability of the horse, motorized fire equipment was merely supplementary to the more respected steed.

Required an "A" ticket in 1970's