There is no easy answer to this. It all depends on how well the car has been taken care of. Each one will be different.
here is a best case chart.
Fuel Type Regular
MPG (city) 22
MPG (highway) 32
MPG (combined) 26
probly around 28-30 MPG
I have an 04 Grand Prix and I average around 22-24 mpg city, and as much as 31 mpg highway.
MPG (city) 19 MPG (highway) 28 MPG (combined) 23
22 City/30 HWY mpg
The 1999 Pontiac Grand Prix GT typically achieves around 18-20 miles per gallon (mpg) in the city and 27-29 mpg on the highway. Its fuel efficiency can vary based on driving conditions, maintenance, and individual driving habits. Overall, it provides a balance of performance and fuel economy for its class.
The EPA mileage estimate for the 2001 Suzuki Grand-Vitara is 17 mpg in the city, 20 mpg on the highway.
The EPA mileage estimate for the 2001 Jeep Grand-Cherokee is 14 mpg in the city, 20 mpg on the highway.
The EPA mileage estimate for the 2001 Dodge Grand-Caravan is 15 mpg in the city, 22 mpg on the highway.
19 City 31 Highway
The highest mileage was provided by the Pontiac Grand Am Regular and it gave 32 mpg on the highway and 21 in the city.
Around 18-19 mpg city and 28-30 highway is the best you will get.
19 mpg city and 28 mpg hyway