This would be a challenge because although steam cars are similar they really aren't. Most steam cars don't have gears although some white steam cars do have a 2-speed transmission
IN GAME CHANGES
Steam cars don't operate on a rev gauge as such because all the system is doing is heating water although constantly adjusting the throttle to control steam to allow enough steam pressure for movement.
So instead of including a rev gauge on Gran Turismo it would be better to have a steam pressure gauge.
Water and fuel gauges required
STEAM CAR THROTTLE CONSERVATION
Holding the throttle right open will allow a huge increase in speed, but a huge loss in steam pressure so if you don't coast or release throttle you won't have enough pressure for movement once you accelerate out of the next corner as you used all pressure on the straight
STEAM CAR LIMITATIONS
Steam cars require Fuel & Water to be running so pit stops may be required for over use of steam
Range of steam cars varies hugely depending on multiple factors like - if a steam car has a greater boiler size it can generate more steam pressure faster & How heavy the vehicle actually is.
Fuel and water tank size can also contribute to the range a vehicle may have.
Temperature of environment, if its a hot summers day a steam car will use more water, Cooler temperatures will supply the cars with greater operating conditions more range.
STEAM CAR ATTRIBUTES
New Range, New Innovation for Gran Turismo
Steam engine has instant torque like an electric car
New driving Challenge (Controlling throttle to increase range of car)
Great Variety of cars
Gran Turismo has an awesome smoke system (Look Great Pluming out the back of the cars)
Adds some great historical cars to the game (Lacking Cars before the year 1950)
Examples of cars and range:
Doble Special, 1925 - Range: 110 - 485 kms
Ford Falcon SC, 1967 - Range 48 - 108 kms
Stanley 20HP, 1922 - Range: 50 - 80 kms
Stanley 30HP, 1906 - Range: 48 - 193 kms
White 20HP, 1907 - Range: 50 - 485 kms
White Model 0-0, 1910 - Range: 24 - 161 kms