Please cite the specific years for your example. I somehow doubt that the two models you're citing with a difference of 20 mpg have the exact same everything, and that Honda made a whole new model just to put out a different intake.
I was talking about the 5th generation from 1991 to 1995.
The VX has VTEC-E(economy) engine, different than the famous VTEC engines.
This article from Wiki. not all the numbers are 100% correct unless you drive the car and calculate the MPG.
: The economical CX was the base model equipped with all-manual features, and power brakes. In the U.S., it came with the 8-valve 70 hp 1.5L D15B8 engine and manual transmission
. With 42/48 miles per gallon (mpg) (city/hwy) [revised to 2008 EPA rating: 35/43 mpg city/hwy
] or 40/47 mpg (city/hwy) [revised to 2008 EPA rating: 33/42 mpg city/hwy
], the CX was the second most fuel-efficient Civic model of the fifth generation, after the VX. CX models in Canada came with the same 16-valve 102 hp 1.5L D15B7 engine as in the DX model, but could also be ordered with automatic transmission
which also came with power steering. Some later CDM CX models (colloquially known as the "CX-Plus") could be ordered from the factory with a rear wiper, side mouldings and manual passenger-side mirror.
: Fitted with the same manual transmission, the VX was identical to the base model CX except that it gained improved fuel efficiency through a 92 hp 1.5 L VTEC-E
engine yielding 48/55 mpg (city/hwy) [revised to 2008 EPA rating: 39/49 mpg city/hwy
] or 44/51 mpg (city/hwy) [revised to 2008 EPA rating: 36/46 mpg city/hwy
]. In Canada, it was rated by Transport Canada fuel consumption estimate: 4.7L/100 km city and 4.3L/100 km hwy.
Other added features were an 8K tachometer with redline at 6K RPM, lightweight 13-inch (330 mm) aluminum alloy wheels, as well as additional front & rear under-body trim additions to improve aerodynamic flow.[citation needed
] The VX was also equipped with an aluminum alternator bracket, an aluminum front driver side engine mount, and a lightweight crank pulley. In addition, the instrument cluster featured a shift indicator light that would notify the driver when to shift upwards in order to achieve optimum mileage. To this day, the CX & VX models are lauded as one of the only gasoline-powered cars that rival the fuel economy
of today's hybrids and diesels. In the March 2010 issue of Car & Driver
for example, it mentions its long-term test car, a 2009 VW TDI Jetta with 6-speed dual-clutch
auto transmission, got worse fuel mileage (38 mpg) than their 1992 Honda Civic VX test car (which got 41 mpg) and 2000 Honda Insight hybrid (48 mpg).