You'll likely find figures ranging from 8-15 mpg (miles per gallon) in city driving, and perhaps slightly better on the highway. The lower end of that range is more realistic for heavier vehicles, less-efficient engines (like the larger V8s commonly found in these vans), or poor maintenance. The higher end might be achievable with a lighter load, a well-maintained smaller engine and careful driving.
To get a more accurate estimate for a *specific* 1986 Chevy G10, you'd need to know the engine size (e.g., 4.3L V6, 5.0L V8, 5.7L V8, 7.4L V8) and transmission type (automatic or manual). Even then, the actual mileage will vary.
