Our P5 is mainly driven on the highway and the gas mileage sucks, 25mpg highway. Cruise is usually set at 70-75mph and the tach sits at 3200-3400 rpm.

I'm thinking a gear swap would help make the vehicle more highway friendly. Maybe just the OD and leave 1-2-3 the same, although 4-3 is already pretty harsh.

Is this possible? How involved would the process be?

 

Thats pretty strange that your mileage is so poor. I get at a little better (~27-28) most of the time and that is mixed in town/highway. Im also a pretty agressive driver (read WOT a lot). I think the gearing in 4th and 5th is too high already, I cant imagine how much it would suck with a taller gear... Either way I bet that by the time you pay for a higher 5th gear to be machined and installed, you could pay for a LOT of gas...

 

i guess this is one of those moments where you should whack your self in the head, and say "i should have bought the manual tranny." from mazda's website, the automatic has a higher final drive ratio when compared to the manual. however, i would assume that because of the greater losses inherant to automatics the milage is suffering. again from the mazda site, the estimated milage is 30 for the auto and 31 for the stick, both on the highway.

 

I agree that the gears are too short on my 5 speed. I don't think the enginge should break 3grand cruising at highway speeds.

 

I've checked into this. Our old 86 626 with the 2.0L revs about 3000 at 70-71 MPH. Our 2001 ES revs closer to 3400 at the same speed. The protege has a fairly short 5th gear ratio (not much of an rpm drop from 4th to 5th) and the final drive is a 4:10. The only other ratio final drive I've been able to find is a 4:30 for using on short course or rally. Haven't been able to find a slightly taller 5th gear yet. I'll keep looking. I'm going to have the tranny out eventually to put a LSD in and maybe a lighter flywheel and stronger clutch in anticipation of a decent turbo kit.

 

Traveller---Let the board know if you do find a taller 5th gear. I'd like to slow the engine down.

 

You may not like it if you do. The reason Mazda went with lower gears in the Protege is because of the engines power curve. Below 3000 RPM our engines don't have much HP or torque. If the gears dropped the RPM below that at highway speeds, the engine would have trouble maintaining the speed, and would require a downshift to accelerate at all.

 

Um, have you checked out other cars with similar engine displacement or smaller? There is no reason that an engine shouldn't be able to run down the freeway in the 2500-2800 range. I don't know about your cars, but mine pulls like a tractor from 2000 up. The same engine has been used in the Ford Probe and the 626 from about 93 to the current model. The Ford revs about 2500 with the overdrive auto on the freeway at 70.

The main reason for ours revving higher (that I've been able to find anyway) has more to do with keeping the 2nd catalytic converter hot and therefore more efficient. If the engine rpm goes lower on the freeway then the exhaust will be cooler. Keep in mind that these cars are LEV (low emissions vehicles) and as such are meeting stricter emission standards. They also have 2 catalytic converters (one attached to the exhaust manifold, one farther downstream). The 1st cat soaks up a lot of heat due to it's location. The reason it's there is for quick 'light off'. It starts to clean the exhaust very shortly after engine start. This is done because it was found that a 'cat' needs to get to a certain temp to become efficient. On older systems it might take 3-5 minutes to 'light off'. So, the cars were a lot dirtier for the first few minutes.

The new early light off cats are much cleaner, but they are restrictive and they remove a lot of heat from the exhaust. So the rpm has to be higher at cruise to keep the 2nd one that is downstream lit off. That's why the rpm is so much higher than a 10 year old car. Emissions.

 

Hmmm my 1978 320i was up around 3000 RPM at 55. Go figure, it didnt have cats...