A stock '57 Imperial in top shape should deliver as much as 18 MPG on the highway. Unless you are planning on crossing the country on city streets, or driving very hard, your mileage should be similar. There is little to be gained from this conversion. I have often experienced 17 MPG out of my '62 Imperial on long drives using the Auto Pilot, and over 18 MPG in my '56 just using my foot. My '81 Buick with a 200 4R gets around 21 MPG on the highway, but it weighs MUCH less than a '57 Imperial. Paul In a message dated 4/22/2004 11:25:03 PM Eastern Daylight Time, Overland14 writes: > Can anyone tell me what year rear brake backing plates can be used to convert >my 57 emergency brakes so I can install a GM overdrive transmission for >extended cross country driving.....any help will be appreciated. Thanks, >Frank Whitney, > Binghamton, NY