[JohnAh]

I´m planning to build my own fuel economy computer för my 1975 Saab 96. I´m a big friend of vintage electronics so my design will have a four-digit seven segment LED display, as if the FEC is of the same age as the car. Compared to a standard LCD module I will get big digits from a small package. The display will be a compact unit with it´s own microcontroller to allow a thin three-wire connection.

The FEC main unit will be placed below the dashboard where I can reach it comfortably and the display unit will be placed next to the speedometer. Both units will have an Atmel ATmega88 microcontroller that I program i C.

I will try to keep this project simple. A small flow-sensor for liquids (like a small turbine/paddle-wheel) gives about 9000 pulses per litre fuel. (may be about 2000 per gallon) The FEC will also have input from the ignition system to read engine speed. The third input will be pulses from one of the wheels to meassure distance.

The Saab 96 have manual transmission with a unique feature: A freewheel that allows the engine to idle when the throttle is released in gear and to change gear without using the clutch. This freewheel makes it easier to drive by "pulse & glide" and save a lot of fuel. The freewheel is the reason for meassuring the engine speed. -By comparing that with the road speed the FEC will see if the engine is burning fuel on proper work or wasting fuel on idling.

Since I live in Europe (Sweden) I will meassure fuel economy in litres per 100 kilometers (inverted value compared to mpg). If I drive like most people do the car consumes about 8 L/100 km but my way of driving mostly results in figures around 6,7 ("L100").

I plan to meassure and count a lot of different parameters based on time, distance, road speed and engine rev. The most important is offcourse the L100, litre per 100 kilometers. I think it may be interesting to present the L100 as average during last 30 sec, last three minutes and last 30 minutes.
The FEC will be able to display traveled distance, total ammount of used fuel and also calculate what my savings is or would have been if I hade shut off the engine instead of idling.

There will also be two similar "register pages" with several parameters to use as "trip meters" that I can reset. The last function is to meassure the time it takes the car to coast down from highway speed down to almost zero. By dividing this meassurement into four or five equal intervals I hope to have a way to see if an aerodynamic modification is a success or not.

I think most meassurements need only two or three digits of value, there´s no need to tell fuel consumption, traveled distance or traveled time with a precision of ppm. An automatic decimal point will allow the meassurements to lose precicion as they grow.

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Now my headache:

In total there may be about 20-30 different figures/parameters/numbers that can be displayed one at a time on a simple four digit 7-segment LED-display.
But how to arrange and manage such a lot while driving? A standard alphanumeric LCD module could make the job a lot easier by the help of text labels and a good menu system but I´m quite sure that I´ll be safer in traffic with a clever way to find the right data without browsing a menu system.

My idea is to group the parameters into "pages" containing 5-8 parameters each depending on their use and purpose. I then intend to choose parameter within one "page" by turning a knob with distinct position. To choose "page" I will use another knob or two push-buttons. If I use buttons, one will increase the page number, the other button will return me to the first page.

I hope such a system will be quite easy to learn. The LED display will have a very limited ability to show text labels to guide me in the beginning. Each time i change parameter/page a text label will be displayed (one second if I choose to switch on the label function). The decimal point will help me to recognise different types of L100-values.

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But is this idéa with 3-5 "pages" of 5-8 parameters each a good solution or is there a more clever way to find what I need while driving safely?

Is there any valuable information or calculations I have forgot? My FEC will have an accurate timebase and pulse inputs from consumed fuel, travelled distance and engine rpm.