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In one of my project I need to display the current gear in which my vehicle is running at that moment and also need to suggest a best preferable gear for that particular vehicle speed to get the best of my motor efficiency. I am finding difficulty in doing so.. need help...Thanks.

Assumptions: 1. It is an ordinary petrol-engined car. 2. It is manual shift. 3. You only need to consider running on a level surface. 4. You know the gearing. 5. This is an academic problem with no safety considerations. Put a set of switches on the gearbox to indicate on leds which gear you are in. Monitor the speed with an electronic device. Decide an efficient speed range for each gear. This is usually with the engine running at 2000-3000 rpm. Indicate which gear gives this engine speed.

PS - My speed range of 2000-3000 rpm is for old long-stroke car engines. Modern cars with short-stroke engines tend to be most efficient at 2500-3500 rpm. See: https://i.stack.imgur.com/Rl5zP.gif But it doesn't really matter, your prof will accept any reasonable numbers.

PPS - It is reasonable to assume first=0-15mph, second=15-25mph, third=25-40mph and top=40+mph as efficient, but your prof may require you to work these speeds out from gear ratios and wheel diameter. Typically the gear ratios might be first=3.3:1, second=2.5:1, third=1.5:1 and top=1:1. Final drive=4:1 and wheel circumference = pi x d Hence you can translate engine efficient running to road speed in each gear.