Try this:
Ecometer from Auto Meter: Features
The Auto Meter Model 9100 "ecometer" for smart 450 and 451 models, plugs into the OBDII port and is a 2 1/16 inch dia. gauge.
"Customizable to the specific vehicle’s type (compact, full size, crossover/mini-van or truck/SUV.)
Uses proprietary software to calculate fuel economy performance (real time and/or average.)
Display real time fuel economy performance in both a radial energy consumption graph and in a precise, easy-to-read digital format.
Offers 4 different modes that can be easily selected by the driver at the touch of a button.
Instantaneous Fuel Economy, Average Fuel Economy, Digital Tachometer with Instantaneous Fuel Economy, Digital Speedometer with Instantaneous Fuel Economy"
Uses both analog and digital presentation.
I have ordered one of these but it has not yet arrived and the cost to me was $82 including S&H. I have the factory tach and clock and primarily wanted an easily observable fuel economy gauge. Should have more info in a week or so after the unit arrives and will post here.