Is your MDX getting worse gas mileage than it did before?  If so, then it needs to be fixed to get the better mileage back.  Getting it repaired will cost you much less than a new car.

If it's getting the same MPG it always got, you put up with it's fuel economy for about 12 years if you got it new.  We're all dealing with outrageous gas prices and it's a major hit to the wallet even if you have a car that gets 30 mpg.

A smaller SUV or sedan will get more miles to the gallon than your MDX.  Consider it's smaller brother, an Acura RDX.

It depends. If you need a car with that much space, that much cargo capacity, and that much passenger capacity, there isn’t much better you can get on gas mileage. 

But if you want to downsize to a compact SUV, or even a mid or compact sedan for the gas savings, then it would make sense. 

It depends on your needs. Yes a sedan or smaller suv will give you better gas mileage. And if you do a lot of city driving then a hybrid will give you even better gas mileage. But the point is with a new car (depending on where you live) you'll be paying higher insurance, car tabs, sales tax, and well definitely the cost of a new car which I don't see anything under $24k ish. So you have to figure out all of these to see if it makes financial sense or not.