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| My analysis indicates that I should expect about 10-12% improvement in highway fuel economy. I have measured an average fuel economy benefit of 6% for the new 5th gear.
The average benefit is based on 7 tanks and 2100 miles since changing 5th gear. Of course there is a lot of variability from tank to tank, but there is still clearly a benefit. If I use 2-tank averages to remove most of the variability, here are the results:
Old 5th gear min/avg/max 2-tank FE: 37/38/39
New 5th gear min/avg/max 2-tank FE: 39/40/41
Note that all these comparisons are between my newly-machined 5th gear (.545 ratio) and the Escort sedan 5th gear (.717 ratio). The original ZX2 5th gear (.755 ratio) gave even worse fuel economy, but my driving style was different so I can't make a direct comparison.
Below is the raw fuel economy data, all for the summer of 2006, with similar weather and driving habits.
Original Escort sedan 5th gear (.717 ratio):
Month Day Year Odometer Gallons MPG (1 tank) MPG (2 tank avg)
5 4 2006 121740 7.631 38.7 36.6
5 9 2006 121918 4.485 39.7 39.0
5 15 2006 122272 9.575 37.0 37.8
5 17 2006 122447 4.170 42.0 38.5
5 22 2006 122634 5.141 36.4 38.9
5 27 2006 122915 7.455 37.7 37.2
6 2 2006 123232 8.061 39.3 38.5
6 14 2006 123485 6.649 38.1 38.7
6 20 2006 123722 6.198 38.2 38.1
6 26 2006 124013 7.884 36.9 37.5
7 11 2006 124353 8.629 39.4 38.2
New 5th gear (.545 ratio):
Month Day Year Odometer Gallons MPG (1 tank) MPG (2 tank avg) Comment
7 13 2006 124574 5.439 40.6 39.9
7 26 2006 124925 8.938 39.3 39.8
8 1 2006 125183 6.678 38.6 39.0 some trailer towing
8 8 2006 125551 8.895 41.4 40.2
8 12 2006 125930 9.294 40.8 41.1 some trailer towing
8 14 2006 126123 4.839 39.9 40.5
8 22 2006 126461 8.085 41.8 41.1
Below is a plot of all my fuel economy data for 10,000 miles before and 10,000 miles after the change in 5th gear. Note that fuel economy is about 8% to 10% worse in winter, due to increased aerodynamic drag (denser air) and increased friction in the engine, transmission, etc. because the viscosity or "thickness" of oil and grease increases dramatically at lower temperature. 
To make this fuel economy data meaningful, I should describe my driving habits. For all the data shown, about 90% of my driving is on the highway (approximately 75 MPH). I routinely drive for best fuel economy, by shifting into higher gears as soon as possible (to keep RPM as low as possible). I also shut the engine off for long traffic lights and other extended stops.
For comparison to gearing of typical production vehicles, I downloaded N/V data from the EPA database (www.fueleconomy.gov/feg/download.shtml). Note that N/V is engine speed divided by vehicle speed (in top gear). With my new gears I get an N/V of about 32. As shown in the graphs below, this is significantly taller overdrive than any 2.0L vehicle in production in 2006 (lowest production N/V is 36 for manual transmission, or 33 for automatic).
You can also "normalize" gearing to engine displacement by studying engine swept volume per mile. Again, based on data from EPA, my modified gears put me at one extreme of production vehicles:
If you find my web site worthwhile, please link to it from your web site.
- Tom Leone TGLEONE AT GMAIL DOT COM
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