Thanks for the specs information. Other than the really deep gear ratios, there's LOTS of hydrocarbons going out that aren't really contributing to moving the car efficiently. The one T&C seems a tad high, but MORE like it should get. Obviously, gas money is easy to come by for some of y'all, but if the mpg increased by 15% or more, that's more disposable income for other things, even car parts. It used to be that a good Chrysler would get about 16+ mpg on the road, if it didn't, something was wrong somewhere.
In the middle to later 1970s, we sold a good number of Chevy 454 pickups, 3/4 and 1 tons. So many of them never got more than about 8mpg running down the road empty. We generally put 4.10 gears with them as people wanted them "to work and pull". Later, I found out that similar Ford F250s were similar.
Admittedly, where you drive and how affects mileage a good bit. PLUS how much time is spent with the engine idling (for whatever reason).
Still, though, IF the carbs are calibrated for a cruise mixture of 14.2 to 1 air/fuel ratio, mpg should be much better. Regardless of the mph/1000 rpm of the vehicle, that a/f ratio (with ethanol gas), which is optimal for E10, it still sounds like a big lot of inefficiency or a too-rich mixture is happening. The factory vehicles will run a 14.8 to 1 a/f ratio, which is optimum. The factory ELB engines would do more like 18.0 to 1m but needed the electronics to make it work well. Are the spark plug insulators burning "bone white" after cruise or are they darker and sootier? Whether it's a richer carb calibration and/or a strictly mechanical advance distributor (even with a fast curve), there's some inefficiency in there, somewhere.
As for driving style, the old "barefoot with an egg between your foot and the accel pedal" isn't the best fuel economy tactic. Brisk acceleration to cruising speed gets you there and into the most economical cruise economy mode quicker. Accelerating very slowly, staying in the lower gears longer, is not the best way. On every car I've rented (since the mid-1990s) that's had an "Instant Fuel Economy" readout, I've always watched that as I drove. Neat thing about fuel injection is that in "coast" mode, the fuel delivery is trimmed back greatly, yielding 99mpg on the Instant Fuel Economy readout. Carbs don't do that.
On many of y'alls modified cars, I could see 12-13 on the highway, even with the lower gears, but 8-10 strikes me as "there's something wrong somewhere", even if it starts and runs easily. Of course, I'm talking about steady-state high cruise at 60-70mph. The deeper gears might mean the engine is turning 3000rpm, which should be right in the meat of the torque curve with cylinder filling being more optimum, along with engine efficiency . . . unless the engine is calibrated with too-rich mixtures in non-power cruise situations.
In the later 1970s, each year Chrysler (along with other manufacturers) published books with listings of their vehicles' EPA mileage ratings. In the Chrysler books, they had information on driving for best economy AND charts of how much thing like axle ratio change would affect fuel economy. Some great engineering information of how efficient and EPA compliant Chrysler vehicles were . . . including the B/RB cars.
ONE thing I've found is that Chrysler set the transmissions to upshift too soon at part throttle. I'd always thought our '66 Chrysler did good, but when I went to a west Texas city, it more like a drag race at each red light. I started to manually shift just to keep up. I ended up adding 2 more turns preload on the kickdown rod to raise the shift points in "D". That worked well! The adjustment was later a sliding interface on the '70 Monaco and the '72 Newport. On my '80 Newport, the adjustment is hidden under the floor pan, so I got a small black wire tie and put it in the end of the slot of the kickdown rod, so that the carb pin contacted the wire tie, effectively adding more preload to the adjustment. Raised the shifts points just right. More acceleration with less throttle input, keeping the carb out of the power mixture.
Now, for another point of reference. My work vehicles are Chevy pickups, 1/2 tons through the years. The '76 I had was a 250 six cylinder with 3-speed manual, 3.73 rear gears, and L78R-15 tires. Always got 14mpg in a mix of mainly freeway driving with a little surface streets. The '78 C10 was a 350 4bbl, 3.42 gear, P235/75R-15 tires. Best it ever did was 13.25mpg on its second full tank of gas. From there it went to about 11 and stayed there, no matter what. But when we got the first throttle body injected V-8s in '87 people raved about how much better they ran, but complained about the fuel economy at the first oil change. LOL On fuel injection, the fuel is added as you move the accel pedal downward, but on a carb, when you move the throttle, it takes additional air flow to pull the fuel from the float bowl into the engine. I was asked to log my mileage and I did. It was a 350 TBI with 3.08 rear gears and P235/75R-15 tires. Speed limit was 55mph, although I sometimes went p to 60. Using the old fuel economy driving tips, I was able to log many tanks at 20mpg average, using the cruise control. Considering how thirsty prior pickups had been, this was a fuel sipper! Better than the 305 4bbls of the middle 1980s with 2.56 and P235/75R-15 tires. At the present time, I've got a 2014 GMC Sierra with the LS 4.3L V-6. In MDS, it goes into V4 mode as it's a 90 degree V-6. In the same driving orientation, it generally gets 20.6 mpg with a cumulative total of 19.9mpg for over 10K miles. It's now got about 145K miles on it. Now, when I spend a good while with it idling with the a/c on, the average mpg drops to about 15mpg on the trip computer. By the way, I do work for a GM dealer, which has supported my Chrysler "habit" nicely for the past 40 years.
One truck I had was a G2500 box van. Gearing was such that it ran 1700rpm @ 70mpg in OD. With the stock Bridgestones and a 70mph speed limit, it would average a best of 13.8 mpg on strictly highway runs. When the speed limit went to 75mph, it dropped a bit. When they put some deep-treaded Firestones on it, mileage dropped to 11mpg and could never be made to get back up, even with increased inflation pressures. The tires were absorbing too much power and resultantly, fuel, compared to other tires! In the 1990s, one of our salesguys bought an S-10 4.3L V-6 5-speed. As he fancied himself "a hunter", he went down and got some Buckshot Jones Monster Mudder tires. That little V-6 never got past about 14mpg, no matter what. Difference was the bias ply tires and their flexy deep tread lugs. He finally got another truck and an engine that would better pull those flaky tires.
Thanks for the answers and tolerating my amazement at the low mpg figures.
CBODY67