* Specific conversion: The weight of the conversion (added features, interior materials, etc.) significantly impacts fuel economy. A heavily customized van will get far worse gas mileage than a more basic conversion.
* Driving habits: Aggressive driving, excessive idling, and frequent acceleration drastically reduce MPG.
* Maintenance: Proper maintenance (tune-ups, tire pressure, etc.) improves fuel efficiency.
* Road conditions: Hills, city driving, and highway driving all affect MPG.
* Engine condition: A well-maintained 350 engine will likely perform better than one that's neglected.
While you might find some average figures online, they'll be broad estimates. Expect to see figures in the range of 8-12 MPG for city driving and perhaps a few more MPG on the highway, but it's entirely possible to be outside that range depending on the factors listed above. Real-world MPG could be considerably lower.
