Story of a custom 5th gear for a '98 Ford Escort ZX2: +6% MPG

[MetroMPG]

A MetroMPG.com visitor contacted me this week to share a link to his site about a gear swap he performed on his 1998 Ford Escort ZX2 5-speed.

http://sites.google.com/site/diyoverdrive/

It turns out that my car... 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

modified, taller 5th gear

The modified gearing reduced his RPM from 3220 RPM (stock) to 2330 RPM at 70 MPH.

Unfortunately, the story doesn't have an "and they lived happily ever after" ending. The new gear started to fail, and was removed. An explanation is given on his web site.

The site is worth a look. It's really well documented, and the owner posted a number of analyses of the swap, including a comparison of his new top gear to other 2.0L vehicles in the 2006 EPA database. He noted:

As shown in the graphs below, this is significantly taller overdrive than any 2.0L vehicle in production in 2006

His fuel economy graph was also notable:



Much more info:

http://sites.google.com/site/diyoverdrive/

[Daox]

Very interesting, and wow thats a huge drop in RPM. But, yes, all transmission gears are made of alloy steel and heat treated to take the abuse that they do. Welding will remove that heat treating and weaken the gear. Most of the gearboxes we make at work use a pretty big interference fit. We use liquid nitrogen to freeze the shafts, and we heat the gears up when they get pressed together. On top of that, they have a key in them.

[Stan]

Nice work, Tom...and thanks for the link!

It turns out that my car... 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.

This feature is common to many FWD transaxles, including some of those from Mitsubishi and VW. Basically, any FWD gearbox family that came in 4-sp and 5-sp versions usually uses this design approach to minimize tooling costs.

I swapped in taller 5th gears in two of my cars this same way, except that in their cases, taller true 5th gearsets are available without having to machine and weld up 2nd gears.

[diesel_john]

Tom good information. Is it true that most gear sets are made up of slighly different alloys so they don't tend to weld together? I have heard the story of just the wrong number of teeth causing a repeating pattern which evenually wears certain teeth.
My rabbits got a .71 fifth and 3.89 final does anyone know of any combination that's better?

[roflwaffle]

Pull the 3.67 R&P from another 020.

[diesel_john]

Originally Posted by roflwaffle

Pull the 3.67 R&P from another 020.

what is a 020?

[roflwaffle]

Go look at the part number of your transmission.

[janvos39]

Diesel John.
Gears in a gearbox can be of the same material. The lubrication of the gearbox is the separator between the contacts on the teeth. In dry condition indeed to choose different materials is best to avoid fretting corrosion.

[arcosine]

My Saturn SC1 is at 2200 RPM at 70 mph = 31.4 rpm/mph

[niky]

This can easily be done with any BJ Mazda Protege with the 2.0 manual. The MX6/Probe/626 used the same gearbox, and both final drive and 5th gear are longer. Swapping 5th gear alone is painless and lowers cruising rpm from around 3200 at 100 km/h to around 2900-3000.

Even better, if you can order a fifth gear from Europe, the 5th gear from the diesel Familia should lower cruising rpms to under 2500 with the same final drive, and it would likely be much more durable than a custom-ground gear.