Ok here goes, apologies as I’m sure all of us know all this so it’s just for information
Avoid the traffic lights grand prix, smooth acceleration and deceleration is more fuel efficient, plus don’t over rev the engine, change up to the next gear as soon as you are able. Automatics sort this out for you automatically
Most modern engines have an engine management system that checks its own operation and adjusts fuel mixture and ignition timing to ensure efficient fuel usage. If you have an older vehicle, check your air filter, if its dirty or clogged the engine will consume more fuel.
Diesel engines are usually reliable although for higher mileage vehicles fuel injection systems are prone to partial or complete failure. Our school minibus although only just over a year-old developed rough idling plus a red engine management warning light on the dashboard. It was caused by a faulty fuel injector that was replaced by a mobile mechanic. I watched him plug his interrogate gizmo in and get the fault code, he also replaced the fuel filter at the same time.
Servicing is important so if you spot a service light on your dashboard its usually there to warn you of impending doom, sadly it’s probably time to open your wallet.
For the DIY shed type mechanic, with the aid of an OBD (On Board Diagnostic) dongle and a phone app you can interrogate the vehicles systems and get the code that is causing that management warning light to illuminate. And if you are brave enough and know the location of the faulty part you may be able to replace that part. I've done this a few times for friends and family with good results.