Virtually every modification I have made to a car has been in the name of increased performance, whether improving power, handling, or braking. A few months ago, in light of ever increasing gas prices, I decided to try something that would help my fuel economy. My 1994 Protege LX, like so many other economy cars, has a very short fifth gear. So short that the engine runs at 3,500rpm when driving at 70mph. If fifth gear were taller and the engine ran slower, the highway milage could be improved significantly.

After some research, I discovered that taller gears (pictured above with the stock 5th gear) were commonly available. Luckily the Mazda Protege shares major parts with other cars including the Mazda 626/MX3/MX6, Ford Escort/Probe, Mercury Tracer, and Kia Sephia. It so happens that many of the above cars have a similar transmission. The second-generation Ford Probe 2.0L is one of the ones that has a slightly taller 5th gear — 0.717:1 instead of the factory 0.795:1 — and it is 100%-compatible with the Protege LX transmission, so I bought one of those.

Installation was really easy. Transmission in-car, it is possible to remove the rear housing of the trans and access the reverse and fifth gear assembly. Simply remove nine 10mm hex head bolts and pull the steel housing off, then remove two large nuts from the end of the gear shafts (32mm and 27mm hex nut). Remove the reverse/fifth shifter fork by driving out its roll pin, then slide both reverse gears and the rev/fifth syncro assemblies off the input and output shafts. After this you’re left with the pair of gears for fifth. Remove by hand and replace with the taller part from a 1992-1997 Ford Probe 2.0L, which can be removed from a Probe transmission in the same way, no pressing required. Reassembly is the reverse of disassembly, but the mating surface for the housing must be cleaned and fresh silicone automotive sealant applied to keep 2.7qt of fresh gear oil in without leaking.

The results:

So what’s a 341rpm difference worth? In my experience over the past 3 months — a lot. Before installing the taller fifth gear I couldn’t crack 30mpg pure highway no matter how hard I tried. Admittedly I didn’t resort to driving 50mph in the pursuit of the best possible fuel economy, but within reason, I tried. Now I regularly see 33mpg in mostly highway driving, have exceeded 35mpg a few times on long trips, and in one especially motivated attempt where I crossed the Appalachian mountains driving slowly uphill and coasting out of gear downhill, a hair away from 40mpg. Call it a realistic 10% improvement in gas mileage. If only taller ratios were available!

Technorati : fuel economy, gas mileage