gear ratio and MPG

[The_Jeff]

Stuck a "wrong" MP3 transmission in a new to me SOHC saturn, 5th gear ratio is 13.5% shorter. Have not yet computed MPGs but trying to guesstimate its effect, to make sure the rest of the motor tune is in good shape.

Car is supposed to have .643 OD with 4.063 final drive. On 175/70/14 tires went 65 mph at 2300 RPM. FWIW the motor's torque peak and presumed MPG sweet spot is 2300 RPM, which would now hit at 57 MPH. Am wondering if I stick to country roads at or below this speed, if I actually would gain. Yes I know about lugging, volumetric efficiency, etc.

New trans is .730 with same 4.063 F/D. The worst case scenario I can figure would be a 13.5% mileage hit but I expect less. After all my drag coefficient didn't get any worse. How much less? Must be some figures from long ago when rear axle swaps were common, and more dramatic than 13%.

Naturally I'll figure this out when I run a couple tanks through... but logistics (inspection appointment a week out) have me twiddling my thumbs.

 

[Johnny]

This is way above my knowledge level, but will be interesting to hear what others have to say and what you fine out.

 

[IndyIan]

http://ecomodder.com/forum/showthread.php/saturn-s-series-5th-gear-swap-2779.html
If you're bored you can read through this and see what the result was for this guy. He just swapped the mp2 5th gear into a mp3 trans.

 

[The_Jeff]

Thanks, that was interesting, I have never seen the gear swap process illustrated. Now I'm kicking myself for trading my old scrap transmission (case cracked due to diff pin exploding) for two used tires at the junkyard.

I drive sanely but don't do any pulse and glide or other hypermiling stuff.

 

[getnpsi]

The pulse and glide can increase your gains far more than a 1-2mpg loss from extra rpms on the highway.

Getting into 5th around town when you may have not been able to before should offset the worry of extra RPMS on the open road. Slight hills may not require the same downshifts as well.

 

[Bob Woods]

If you have room in your wheelwells get some bigger diameter tires.

Have verified that your speedo is still correct.

 

[Shannow]

Hard to say what the end result will be.

Classically, the taller the gear, the greater the throttle opening, and the more efficient the engine uses fuel.

But I've also seen on carbed cars the reduction in throttle opening with the higher RPMs can be more efficient, as it uses different ports in the carby that can be tuned differently.

If you've been keeping records of before, then please let us know what you find.

 

[The_Jeff]

42 MPG at a steady 65 MPH all warmed up. Not bad with E10 gas and near freezing temps.

The diff is the same, so the speedo/odo pickup is the same.

 

[PT1]

What are the rpms at 65mph? If under 2,000 you are fine.

 

[The_Jeff]

2800 and there's an annoying exhaust resonance.