How I made my Geo Metro get 66.5 miles per gallon

The car is just like from the factory except;

Engine modifications;

Economy grind camshaft from 3Tech/Mike Cove
Cylinder head modifications shown here plus exhaust ports polished

Fuel system modifications;

I made a heat exchanger from hardware store parts to heat the gasoline from the engine coolant.
fuel heater

It's located first thing in the heater core coolant line so it gets warm before anything else does when warming up the car.
fuel heater location

I installed an EFIE (it adds an adjustable floating voltage to the output from the oxygen sensor making the car run a leaner air/fuel ratio).
EFIE

Air intake modifications;

I removed most of the housing and plastic tubing of the original air intake path from the front fender.
I installed a K&N air filter made for the car.

I removed the bridge in the throttle body for greater airflow.

I threaded the intake ports in the head.
threaded intake ports

Exhaust modifications

I polished the exhaust ports in the head. I installed a racing header from Suzuki Racing Development. I installed an aftermarket replacement catalytic convertor. I replaced the factory exhaust pipe and muffler with Thrush glasspack mufflers and shortened the exhaust making it exit in front the rear wheel.
header and cat

Ignition modifications;

I replaced the factory spark wires with Magnecor wires. I replaced the spark plugs with Halo spark plugs.

Instrument modifications;

I replaced the instrument cluster with one from a Metro convertable which has the tachometer. I adjusted the miles readout to be the same as the car had at the time of replacement. I added a vacuum gauge.

Aerodynamic modifications;

I covered the vents in the front bumper with lawn edging except where the radiator goes.

I recently drove the car 55 mph on the Interstate during a windless, sunny day till it was 648 miles since the previous refueling. When I filled up with gas it only took 9.75 gallons.