Olds Engines!
49-53...303 CID, 54-56...324 CID 57-58 ...371 cid,59-64...394CID...65-67 ...425 CID..68- 455CID, Some Dynamic 88s and Jetstar 88s used different sizes, as for gas mileage,they really were not too bad, thru 56 they would get about 18 to 20 on the road, the 57-59 would get about 16 or 17 , I had a 63 Starfire , it would get about 15 or so if you took it easy, but the best Olds I ever had was a 65 Ninety Eight, it was geared 2.91, and would get between 18-20 on a trip, plus it would do almost 100 in passing gear,it would do almost 60 in low,I was about 20 when I had it so it got rough treatment! never a problem, the 63 was fast...if you could keep the tranny in it, 61-64 used a AWFUL thing known as a Slim Jim, positively TERRIBLE,My favorite cars though..Chrysler! Once you drive a pushbutton Torqueflite with full time power steering....I did and NEVER bought a GM product again, still, I like the old stuff!