Without catching up with what has been written before about this topic I feel It's time for an update:
The DIY fuel computer project has been sleeping for a long time. Meanwhile I have improved my eco driving skills a lot and now feels like i'm participating in a true high perferomance sport where every little detail counts. With a fuel consumption of 4.5 L100 (52 mpg) at a speed where most people get 6-7 l/100 km (30-40 mpg) I think it's definitely time to get a more detailed look at the details!
I'll skip the vintage looks and aim for a simple design with a standard alphanumeric LCD module. To get started as quick as possible I will try to keep everything at a minimum, both functions and hardware. The computer will be a little box ontop of the dashboard with a D-sub connector and a magnetic bracket to make it realy easy to remove for software upgrades.
The system will have inputs from the fuel flow-meter (40 year old carburetted engine), wheel rotation and most likely also sense engine speed. The most important to display is average fuel consumption (L100) during the last 5-10 minutes, average speed during the same period and total travelled distance. If I manage to get these things working I can later improve the software with a lot of fantastic features.
The physical box is ready. Many details about computer hardware and software have been decided so I guess it may be time to stop talking and actually start building the thing now!
I will have an AVR ATmega88 on perfboard and use C as the programming language. Very few external components will be used. -LCD, two push-buttons, 5V voltage regulator, some transient filtering, two LED's and perhaps something more. If I succeed making something useful I will share everything needed to know so other electronics DIY'ers can copy and/or improve my design.