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| One of the best ways to improve highway fuel economy is to change gearing so that the engine runs at lower RPM. Traditionally this is done by changing the axle ratio or final drive ratio, but that involves a trade-off with acceleration performance in all gears.
It is also possible to install a "taller overdrive" just for top gear, but few vehicle manufacturers push overdrive very far. If the engine RPM is too low on the highway, a downshift will be necessary for every little hill. This is a real problem if the tall overdrive means a big step between the top two gears. Also, the EPA "Highway" test averages only 48 miles per hour, so there is little incentive for manufacturers to use taller gearing which would be helpful at higher speeds.
These factors led to my quest to build myself a taller overdrive. It turns out that my car (a 1998 Ford Escort ZX2) is especially suited for modifying top gear. The transversely mounted transaxle has a 5th gear mounted *outside* the main transmission housing and bearings. This means that I can change the 5th gear ratio without removing the transmission from the vehicle - I can just reach in the wheel well and remove a sheet metal cover from the transmission.
I bought a junkyard transmission for an Escort sedan, which uses slightly different gear ratios from my ZX2 (5th gear is 0.717 instead of 0.755). Swapping 5th gear from that transmission into my car changed my engine speed from 3220 RPM to 3060 RPM (at 70 miles per hour). As a fangler, that just whetted my appetite!
I determined that I could take 2nd gear from the junkyard transmission and use it "in reverse" to get a really tall overdrive. By using the small diameter 2nd gear from the input shaft, and putting it on the output shaft, the 2nd gear ratio of 1.830 became a new 5th gear ratio of 0.546. This brought my engine speed down to 2330 RPM at 70 MPH!
Of course, some machine shop work was required to make the new gears work. The original 2nd gear was an integral part of the transmission input shaft, so some cutting was required. Also, the synchronizer from the original 2nd gear won't work with the shift linkage for 5th gear. Luckily, a friend offered to do the machine shop work for free!
I decided that the easiest way to make it work is:
(1) Use the hubs, splines, and synchro from the original 5th gearset, but cut the gear teeth off them.
(2) Use the gears from the original 2nd gearset, but cut out the center hubs and synchros.
(3) Press fit the original 2nd "gear rings" onto the original 5th hubs and weld them together.
My machine shop plans are in a link above. I can also email you a Word or PDF version which shows things actual size, and lined up properly. If you want the file (it is less than 800 KB), send me a note: TGLEONE AT GMAIL DOT COM
Photos of the finished gears are to the left. The welding on the smaller gear is especially beautiful! I'm nervous that the weld on the larger gear is close to the gear teeth - hopefully the heat treatment of the teeth wasn't affected.
Also to the left are photos of the scraps from machining. The shop used wire EDM (Electrical Discharge Machining), and you can see the narrow slot left by the wire.
I'm very pleased with the new overdrive. Lower RPM makes the car much quieter on the highway, and there is still plenty of torque reserve for small hills. It is a fairly large step from 4th gear to 5th, but I don't find it objectionable.
My analysis indicates that I should expect about 10-12% improvement in highway fuel economy. My driving is about 80-90% highway, and I have measured an average benefit of 6% during 2100 miles of driving. Details are in the "Fuel Economy" link above.
UPDATE July 2007: I learned that the Escort "Pony" and "Fuel Saver" models from 1986 to 1990 had a four-speed manual transmission (MTX-II) with overdrive similar to my project. The acceleration in lower gears would not be as good as my car. Also the halfshafts lengths are different for the old four-speed, so it would not be a simple transmission swap. If anyone tries this, please let me know how it turns out!
UPDATE January 2008: The new 5th gear failed and I had to go back to the stock gearing. Oh well, it was fun while it lasted (15 months and 22,000 miles). See the "Durability Failure" link above for photos and failure diagnosis...
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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