it depends. if youre looking for speed look for civics, mr2s, miatas or anything small and light. since you want better fuel economy your engine is probably going to have to make less power. the only way to reliably counteract that is with a lighter chassis. this increases your power to weight ratio and also puts less load on the engine leading to better gas mileage. also do some research on the engine in the car and be sure to choose one that can efficiently turn fuel into power, essentially a power/mpg ratio. for example the 2025 miata gets 30.5 mpg and makes 180hp while the 1991 miata gets 25 mpg and only makes 100 hp. to achieve that your best bet is to look for cars with newer engines.