Before I put the 4200 inthe truck I built a stout 292 NA motor - well for the money I spent on it the stock 4200 ran rings around it and burned 1/2 the fuel of the 292. The only thing the 292 has going for it over the 4200 is bottom end grunt. Put a bit more gear in and an OD to compensate.
Without any driveline changes the highway mileage went from a best of 13 MPG (with a reasoanble tune in the holley 600) to 25 MPG with the 4200.
