The best way to determine fuel economy is to use the open road and a steady road speed. Too many variables in trying to do it other ways.
For example, in 1974 Fenner Tubbs Chrysler-Plymouth in Lubbock, TX took each one of their demo fleet cars on a little trip south to the edge of the caprock and then back to Lubbock, each one with the factory cruise set to 55 mph after they got out of town. It was about a 56 mile round trip and they used the Shell station across from the dealership for initial top-offs and fill ups when it was over.
By that time frame, Chrysler had refined their "efficiency" orientation approach to exhaust emissions from the earlier times of the middle 1960s. End result, the cars were usually more fuel efficient than their similar Ford or GM competitors.
When their test was complete, they printed up signed and notorized sheets of what each car did. The New Yorker (440 ThermoQuad motor) averaged 20.66 mpg and the Newport 400 2bbl did 20.33 mpg and a big Fury 360 2bbl did in the 19.5 range. All of those cars would have had similar rear axle ratios and tire sizes and probably not a 400 pound weight spread between them. Of course, weight would not affect mpg that much in that weight class of vehicle and in that type of use, I suspect.
Many GM owners were skeptical of those claims. The dealership people stood behind their results and handed prospects the keys for a weekend to see just how truthful their claims were. Lots of GM and Ford cars were traded in on new Chryslers that year!
I know that everyone has their own driving patterns and habits, plus warm-up strategies for cold engines, all of which can impact the resultant fuel economy. But to me, the real test is to get the car warmed up, top off the fuel tank, and hit the expressway for about an hour or so (a metro loop of about 60 miles works fine during non-peak traffic times). Using the same filling station and pump for each of the fillings is a good strategy too. In this manner, you get the vehicle into it's more efficient rpm range and keep a reasonably constant speed.
If the carb's working correctly, you might be surprised at the fuel economy. It doesn't take much street driving to erode those highway economy figures, though. Similar with an automatic choke that is a little slow coming off. Sometimes, there have been some engine/carb/vehicle combinations that don't seem to do as well as they probably should, but knowing that everything's tuned-up and operating correctly is good too.
Some people like to brag about their vehicle's low fuel economy. In the case of Chrysler products, that's not an automatic situation as it is with many Brand X vehicles. By the same token, some Brand X vehicles seem to do better than they might have a right to.
If you really want to find out how much some things might impact fuel economy, find a late model Chrysler LH car with a trip computer that will read out "Instant Fuel Economy". It seems that somewhere in the 2003 or 2002 model year, the "Instant Fuel Economy" feature was deleted, with just the "Average Fuel Economy" readout. Some of the upscale GM and Ford vehicles also had trip computers with "Instant Fuel Economy" functions too.
For ages we've been told to accelerate slowly away from a stop sign, yet get into high gear as soon as possible. If you do that and watch the Instant Fuel Economy readout, you'll see that all of that extra time spent in the lower gears results in something like 12 mpg with a very slight throttle input for quite a long time. Even after you get up to 30 mph or so, it still takes a good while to get the fuel economy readout into the 20 mpg range. To get the similar readings with the "Average Fuel Economy" feature, you'll have to "zero" the computer at a stop sign.
I read in a '75 Chevrolet owner's manual that it was sometimes better to accelerate moderately instead of slowly from a stop. This way, you got to the more efficient fuel use range quicker for better overall economy, even if it took a smidge extra to do it faster. After watching the fuel economy readouts, it makes sense, just as long as you stay out of the power mixture in the carb or fuel injection.
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