Taller top gear makes a real difference (10%)

[Multiades]

I have a fun little project for my first post:

My daily driver of choice isn't quite as economical as some of the cars I see here, but it's hardly a gas guzzler. I drive a 1994 Mazda Protege LX with the DOHC 1.8L 125hp engine. Unlike it's SOHC counterpart, the gearbox has much shorter gear ratios that make driving a frustratingly high-rpm experience at normal highway speeds. After some research, I discovered that a taller gear was available from a Ford Probe (this platform shares many parts with several other cars).

Some pictures:






Check out the full article I wrote about doing the mod.

I've seen a 10% improvement in highway mileage (which varies significantly with my driving style on a given tank). Since the change in gear ratio is also about 10%, this seems pretty reasonable to me.

-Chris

[NeilBlanchard]

Hello Chris,

Is that the final drive you changed out? [Edit: I should have read the link first...] I would love to take the final drive from a Yaris and put it in my xA...it would be about a 15% improvement! (3.72 vs 4.312)

[Daox]

That is just his 5th gear set. The final drive ratio is the gear reduction that takes place at the differential. So, you'd have to swap in a new differential into the xA. Also, lower RPMs don't directly translate to better FE. It is all dependant on the engine design.

[SVOboy]

Nice swap! That's definitely nothing to scoff at.

[Daox]

See this thread for some info I had collected when I was considering doing the same thing.

I do agree with SVOboy though, very nice job.

[louisb323]

Nice!

[ttoyoda]

Pardon a stupid question, but did putting in a taller gear mess up the accuracy of your speedometer and odometer?

[2003protege]

i feel you on the protege. I believe i'm somewhere north of 3500 RPM at 70mph in 5th. Very cool to hear you're getting significant results.

[PaleMelanesian]

In some cases. In other cases, including Hondas like mine, the speed sensor is downstream of the transmission, so it runs at wheel speed, regardless of engine speed.

[bhazard]

I just wish my turbododges had overdrive.... 3 speed autos FTL!