Bad gas mileage, need suggestions to get it better.

[nogard]

I have a 1999 5S-FE Auto 110,000 miles. I used to be able to get 28 MPG. I had an oil leak in one of the spark plug tubes. So I fixed the leak, put dealer spark plug wires and factory spark plugs in. That is when the gas mileage went down to 24 MPG and then over the winter to 17-20 MPG. Now that it is warmer all I can get it 21 MPG. I think it is the spark plugs. I don't get it they are new and the stock double electrode Nippodenso.

What is the reason that the car used the double electrode? Can I put in a single electrode plug like a NGK V-Power? That is what I use in my 1988 camry with 3S-FE and I can get up to 31 MPG. Also is it suggested to replace the coil packs on the 5S-FE? And what else would be recommended to replace? I want to give the car a good once over. I remember reading something about the coolant temp sensor should be checked?

Thanks.