What kind of gas mileage would a 1987 gm suburban get with 350 cubic inch engine?

A 1987 GMC Suburban with a 350 cubic inch (5.7L) engine would likely get very poor gas mileage by today's standards. You're looking at 8-12 miles per gallon (mpg), possibly even less depending on driving conditions, vehicle condition, and whether it's a 2WD or 4WD model. 4WD models generally get worse fuel economy.

Several factors affect this significantly:

* Engine size and type: The 350 is a large, relatively inefficient engine.

* Vehicle weight: Suburbans are large and heavy vehicles.

* Aerodynamics: The boxy design isn't aerodynamically efficient.

* Maintenance: A poorly maintained engine and drivetrain will significantly reduce fuel economy.

* Driving habits: Aggressive driving will lower mpg.

Keep in mind that this is just an estimate. Actual mileage can vary considerably.