how do i get 30 mpg in a 1992 deville?

Be sure all your driving is downhill.

You can improve economy some by keeping tires inflated to the max recommended pressure, never drive over the speed limit, pretend there's an egg between your foot and the gas pedal -- ease away from stops, and anticipate slowdowns-- every time you use your brake, you're giving back the energy you purchased getting up to speed -- take your foot off the gas, and coast almost to the stop sign -- begin deceleration sooner when approaching stop signs and lights.

I had an 82 deville, back in 83, and it was thirsty, yours is a bit lighter and better engine, but not designed for thrift. Do the math -- if you're getting 13 mpg and a new econobox would get 39, and if you drive 25,000 miles per year with gas costing $3.50 / gallon, you would save $374 per month on fuel -- about enough to make the car payment on the econocar. Trade in the cad on a used small car, and the math gets way favorable.

It will be really hard. You drive a brick and it has a large engine.

Don't go over 45-55mph ever and try to stay within this zone. Other than that buy a different car.

use a chain and tow it with a toyota. ha haha.