Before you do that, you need to calculate the additional costs of new purchase, the change in monthly car payment, a potentially higher interest rate on your current car loan, etc.
You might be ahead keeping the bigger vehicle.
Absolutely!
15 MPG SUV driving 20,000 miles per year will consume 1333 gallons of fuel.
30 MPG Subcompact driving 20,000 miles per year will consume 667 gallons of fuel.
At $3.00 per gallon, the difference is $2001 per year.
Depending on your income, that might be OK, but let's assume that the subcompact costs you $5000 more than what your SUV is currently worth.
That means it would take 3 years to recoup your losses---if gas prices hold at $3.00 per gallon.
It also is a big change. Is the safety of an SUV worth $2000 per year?
Lots of questions.