That's what I'd expect. You can see the old CAFE numbers on the net, and drive reports show that at 55 mph, a 4 speed manual 3.3 liter 1980 Capri used to get 30 US MPG flat, and 22 mpg diven with economy. Any time else, it'd drop to 16-17 mpg.
I've studied this at length and have three answers. Through the discipline and information on these old Ford and Fox engine combinations, the Corpoarate Average Fuel Economy (CAFE) average fuel consumption figures showed that it is possible to do a fuel consumption prediction for flat roads.
It was here that I learned there are many primary factors for mileage.
Power to weight ratio
Engine tune (Idle speed influences, ignition, HP per liter)
Gear ratio, tires, and aerodynamics
Climate (warmer climate uses less fuel)
Driving style (getting optimum mileage takes skill)
Terrain (flat terrain gets a lot better mileage than hilly terrain)
I have a computer simulator which calculates field US miles per gallon given some basic details. I've used it to forecast different miles per gallon for different engine combinations. And then backed it up by realworld testing.
It stems from an argument that arose in the early 80's when Chrylser Australia sold a four 1.6, 2.0, 2.6 and 4.3 liter sedans, and they all got the same real world fuel economy figures. Wheels May, 1980 found no difference in 2.6 vs 2.0 fuel consumption, and then the blunt old 3200 pound 4.3 liter Electronc Lean Burn CM Chrysler Valiant posted US 26.4 mpg (33 imp mpg) at 62 mph and used to get 21 US mpg (26.2 imp) city to higway.
The adds in Australia and the United States were crazy. Fords Mustang 2.3 with T5 got 38 mpg at 55 mph,
while the Aussie 3.3 Cortina got 28.2 mpg US (35.3 imp) at 62 mph
Ford Cortina verses Sigma verses Faclon verses Valiant fuel economy wars were extingished when the Total Economy run came out.
adds
The three points are:-
1. In 1978, Ford started responding instantly to CAFE regulations by making its cars
a) 20% slicker through the air (better coefficent of drag, reduced frontal area through downsizing, 400 hours of wind tunnel desgin for every new design from 1978 onwards), and
b) over gearing everything 25%.
The areodynamics results in a huge reduction in engine power needed at speed, and at 65 mph, a 25% reduction in engine revs needed results in a 8% reduction in fuel use. So SROD's, 5 speed over drives, and the Fox body automatics with Moon Shoot gearing were how Ford got the 25% reduction in engine revs. This was the era of the 25% (0.81 and 0.79 in the SROD 4 and 6 and T5), 39% (0.72 in the SROD V and 49% over drives (0.67 in the AOD 4 stage), and a 50% overdrive is enough to save 15% in highway miles per gallon.
Ford did this because maximum fuel economy happens when gearing and capacity is optimised to the car... a modern Chevy, Ford or Mopar 6 liter capacity engine can match a 4.2 if its geared right, and the Corpoarate Average Fuel Economy figures 1981 5.8 LTD got better economy than the 4.2 or 5.0 LTD.
2. Due to the need to mandate stoichiometric air fuel cruise ratios in the Federal high way and city emissions cycle, some 1, 2 and 4 bbl carb enginess seem to do better than EFI for MPGS on certain engine combinations (300cube /4.9liter F100 and F150's verses a 4.9 EFI or 5.0 carb or EFI spring to mind), especailly when there is a 4-bbl or EFI with 30 or 65 more hp...In the old days before closed loop and open loop EFI, some 1 and 2-bbl engine combinations met the Federal emission manually at leaner than 14.7 or the oygenated 15.2:1 that is stochimetery, and this allowed them to stay out of the later 12.5:1 open loop air fuel ratios. The modern 3 way cat, feedback system on oygenated fuel doesn't hurt the fuel economy one bit, but they have some inbuilt limits the old engines don't, and occassionally, the later EEC managed engines suffer just as many inservice problems as the early 1978 to 1982 engine. After 1983 in all states, feedback non lean burn engines became mandatory, even if they were called ELB, High Swirl, or Lean Burn, they weren't ever lean burn again because 14.7:1 became the ideal air fuel ratio. EFI engines can run at 22:1 in low load situations if the exhast valves are tough enough, but the electronics after 1983 forced car makers to adoped non lean burn air fuel rations, and so every engine is 10% less efficiecnt than a non emissions, lean burn engine. So some of those 1978 to 1982 engines could really fly great fuel numbers
3.The ideal gearing for economy was cracked back in th 50's but we had to wait till the late 70's and early 80's for the formula to be re-arranged for best miles per gallon for a given average cruise speed. It is based on the from the Index of Thermal Efficiency, found in French Grand Prix and English RAC litrature by Hodges,
and was revised in 1980 for the Australian Total Economy Run, to use the average frontal area and drag fact for a lates seveties, early 80's car. See http://www.snooksmotorsport.com.au/i...d=31&Itemid=39
Here is my computer out put for a 3.3 and 4.1 engine in a Fox Mustang, using the Snook variation on the Index of Thermal Efficiency
That last one is like a 1979 to 1982 4.2 V8 with a 1.83:1 diff ratio. If it was lowered to the factory 2.47 or 2.26, and rerun with a 4.2 engine, the fuel consumptionat 62 mph would be 22.4 or 23.7 mile per US gallon at 62 mph.Originally Posted by xctasy
I re-ran it with the 4.2 4-bbl and AOD with 3.45 diff and got 23.4 US mpg at 62 mph.
The above Capri 4.2 with AOD combo with a 2.31:1 overall final drive would have done pretty well.
The lock-up clutch sure would have helped too. If you can employ the lock-up, there is a potential saving beyound that, maybee an extra mpg at 62mph.
Oh, and a 2.3 with T5 running just a 3.45:1 diff gets 38.6 mpg at 62 mph using the same information. The point is that as an engine gets bigger, it can carry higher gearing, and to do that, the camshaft duration and lift can get dropped. A 4.2 runs 244 degrees of duration and has maximum power and195 lb-ft of torque at 2200 rpm, which is very low in the rev range. At 100 mph, its only turning 3400 rpm with 2.47:1 gears, so its pefectly geared for 115 hp.
It comes from the above formulae above, and it works.
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