My relatives bought a diesel car and I found out that their gas mileage is better than my petrol gas mileage. Now, the price of petrol is soaring. Can I increase my mileage to match their gas mileage? My engine and tires are well maintained. I stop the engine at the traffic signal but I still lag behind by miles per gallon.
The simple answer is:
No. You can't.
To expand a little, both diesel and petrol technologies are continually improving, so you can make incremental changes to your engine in terms of engine management, input air flow etc., and also things like use low rolling resistance tyres etc. But you cannot change the basic fact that your engine is less efficient than his - whether or not one is diesel or petrol.
You can learn to drive more efficiently (see hypermiling for an extreme version of this - some of the techniques may reduce your safety, so be careful)
But if you really want to increase your mileage significantly, you will need to change your engine or your car. Newer cars are generally much more efficient than older cars, and diesel technology currently has a mileage advantage.
Here are a few simple ways to increase gas mileage, but not at par with a diesel:
Any edits or edit suggestions are most welcome.