I've read that you should get the best fuel mileage if you can gear your vehicle to cruise at the rpm where the engine gets max torque. For the 235 the torque curve is pretty flat between 1600 and 2400. Assuming you're cruising at 55, your engine speed will be near the bottom of that range in 5th gear and near the top in 4th gear. I have a 292 in my 68 with 3.73 rear and A833 od transmission. It cruises at about 1800 to 1900 rpms, a little above peak torque speed. I get around 17 to 17.5 mpg. I have gm electronic ignition and I used a vacuum gauge to set the timing. It's quite a bit more advanced than what the manual calls for. I also have a Holly 390 cfm 4 bbl carb. I haven't adjusted it yet; I'm not a carb guru; but I do plan to adjust the idle mixture when I have time. I'll use the vacuum gauge again for that. Based on sound and vibration, my engine feels like it's happiest at 1600 to 1700 but it's not too bad at 2000. It has plenty of torque to take the rolling hills around here without downshifting. I've driven it at 70 mph but both the engine and I feel better at 55-60. I try to get ethanol free when I can and it seems to like it better. Some people say it's a waste of time to try to get better mileage out of these old trucks but to me it's just another performance aspect and I'd rather spend time and money trying to improve efficiency than trying to go faster or sit lower. I'm also a tight wad so I like to save anywhere I can.
