Gas price going up // Rethinking the new project car

If just reaching 20 mpg hwy is your goal, a 318 with OD can probably do it. Probably have to be good at tuning the carb and ignition for that but that's me.

Had a 360 Magnum, standard A-833, 3.23 ring and 225-60-15 tires (25.6 OD) -> drove it to work for a week before I moved, some of it was 50-55mph and the rest was 70. Got 17 mpg with a 600 holley. That combo turned 3000 @ 70.

Currently I have a 340 with the T56 magnum with the 2.97 1st and the .63 OD which turns about 1900 rpm at 70. Also have EFI so its much happier at low RPMs and I have overrun fuel cut but even moderately "in town" stuff when you are nice to it is over 15. The pandemic put a damper on some of my plans for going on a trip with it but I would never expect it to get more than 25 mpg no matter what, 21-22 would be quite good actually.

I will say that it has no problem pulling any hill at above 1400 rpm so 50+ it can be in 6th. I use the .8 OD gear down to 40 mph quite often. So an A-833 OD is about 1/2 way in between. You'd pull about 2200 at 70 with the A-833 OD if you had the same gear and tires as me. I think much above that your fuel economy will really drop off fast.

I have to also say what was achievable from a fuel economy standpoint with non-ethanol gas isn't achievable anymore. Its at least a 3-4% fuel economy loss due to the energy.

If you have dreams of getting 30+ MPG with a V8, I just don't see that as realistic with one of these bricks unless you only drive like 50 mph everywhere.

I had a 1994 Oldsmobile Cutlass Supreme Coupe with a 3.1 V6 160hp version, Sequential EFI, EGR, ran 17:1 AFR at cruise, 0.29 Cd so very slippery, similar weight to at least the fatter end of the A-bodies, 4-speed auto, with 0.705 OD gear, and a 3.33:1 final drive with basically the same size tires as my Duster. I think the drag coefficient helps that car a lot, but if you went 65 on an entire tank of gas and were nice to it on the not especially hilly terrain here in the great lakes region, it could get 32-34 mpg fairly easily on a full highway trip. Freeway commuting to work it would get like 23 when you're going 75 and a trip going 75 was like 29 or 30.

My Dad had a 1990 of the same chassis car, but with the 2.3 Quad 4 HO 180hp, Getrag 282, 3.61 final drive and a 0.72:1 OD. 65 mph it would get 38-40 mpg and if you went on a trip at 75 it would get 34-35, commuting it got 29. Mind you, other than the powertrain and the front bumper, it's the same car body.

I currently own a 2012 Chevy Cruze 1.4 Turbo (so this is an 83ci engine), M32 6-speed manual (this is not an ECO with the super tall gears), 3.83:1 final drive and a 0.74:1 6th. That will get 36-38 on pretty much any highway trip and if you went 65 it would get 41-43. Now at the same time this has enough power to maintain speed in 6th at 45mph. Also roughly the same size tires as my Duster. 30 mpg city. These cars have to run richer due to emissions because lean= NOx

In summary, a smaller engine that can do the work and keep the RPMs down at the same time will get better MPG. Also our A-bodies are not aerodynamic. You stay with a 318 or 273, I can see maybe low 20s. I don't see much of an improvement with a slant six if you want to go 70-75 with traffic because its not a very powerful engine, especially for its displacement. There's a balance there. This is why anyone would consider the pentastar. You feed the same displacement and it makes double the power, it would follow its more thermally efficient.