Eureka,
BLUF: for a given power requirement, generally best BSFC occurs at the lowest engine RPM wide throttle openings can be maintained without stalling, in other words when engine manifold vacuum is greatest. Engineers choose vehicle gear ratios in an effort to keep a particular engine in its BSFC "sweet spot."
Compare the rated power outputs of the Transit's available engines:
3.7: 275bhp @ 6000rpm, 260 lb-ft @ 4000rpm (high rpm, lower torque)
3.5EB: 310bhp @ 5500rpm, 400 lb-ft @ 2250rpm (med rpm, torque monster)
3.2D: 185bhp @ 3000rpm, 350 lb-ft @ 1500rpm (very low rpm, torque fiend)
Now compare the RPM's of the 3.7 with different final ratios (and if you have any mpg data, please feel free to add it):
130, LR, 3.7 w/ 4.1
75mph = 2545rpm (tach reads ~2700rpm), 17mpg
70mph = 2375rpm (tach reads ~2500rpm), 18mpg
65mph = 2205rpm (tach reads ~2300rpm), 19.5mpg
60mph = 2036rpm (tach reads ~2100rpm), 20.5mpg
55mph = 1866rpm (tach reads ~1900rpm), 22mpg
130, LR, 3.7 w/ 3.73
75mph = 2315rpm (tach reads ~2500rpm), ? mpg
70mph = 2161rpm (tach reads ~2300rpm), ? mpg
65mph = 2006rpm (tach reads ~2100rpm), ? mpg
60mph = 1852rpm (tach reads ~1900rpm), ? mpg
55mph = 1698rpm (tach reads ~1750rpm), ? mpg
130, LR, 3.7 w/ 3.31 (if it were offered)
75mph = 2054rpm (tach reads ~2200rpm), ? mpg
70mph = 1917rpm (tach reads ~2000rpm), ? mpg
65mph = 1780rpm (tach reads ~1900rpm), ? mpg
60mph = 1643rpm (tach reads ~1700rpm), ? mpg
55mph = 1506rpm (tach reads ~1550rpm), ? mpg
Finally, compare the frontal area of the three configurations:
LR: 39.3ft^2
MR: 48.4ft^2 (23% more aero drag force than LR)
HR: 53.3ft^2 (36% more aero drag force than LR)
(aerodynamic drag varies directly with frontal area & the power requirement (fuel burn) varies with the cube of velocity)
A 3.7 pushing a heavy 148, HR would simply not have the juice at the given gear ratio provided by a 3.31 rear (which puts the engine well below its optimal BSFC power band <2000rpm) to overcome the enormous rr & aero drag. A heavy 148, HR motivated by a 3.7 & a 3.31 rear would essentially become a 4spd- as the 6spd auto's programming would have a hard time selecting 6th & even 5th gears to keep the engine in its optimal BSFC range. The 3.7 simply can't put out sufficient bhp/torque efficiently that low in its rpm range & the vehicle load requirements would be too high for this particular final drive configuration.
Hope this labored explanation helped a little.