Gear Change MPG Affect
 

After a recent change in my daily commute I've seen my gas mileage drop ~5mpg (from 40ish to 35ish). The new commute puts me on the interstate for the majority of my drive, typically around 80mph.

My '01 Civic Ex is geared very short. At 80mph I'm right at 4000rpms. There appear to be several other transmissions available to bring the RPMS down. An LX/DX tranny should run 3500rpms @ 80 and a HX tranny should run 2700rpms @ 80.

I've tried searching, but haven't had much luck finding results to a change in gearing. Do you feel like longer gears would significantly improve mileage (enough to bring it back up to what it was before?) or at those speeds is wind resistance simply going to kill mileage regardless of cruising rpm?

80- are you serious?

Yes, 80, with the flow of traffic and I do not have any intension of driving slower. I aspire to improve gas mileage, but not at the expense of driving 40mph slower than everyone else or creating unnessary drive time. I guess I should have stated that in my inital post.

Is everyone late for a fire? Oh well.

Dropping the RPMs will improve your gas mileage.

Dropping the speed by 5 MPH would improve it as well.

-soD

Out of curiosity, how far is the 80 MPH freeway portion of your commute?

It's such a shame too, that those are your only choices! :rolleyes:

Thank you for the input. I felt like lower RPMs would help, but I hate to invest significant time/effort/money into something that isn't going to make a substantial difference.

32 miles a day.

As for your gearing question: do you have a fuel economy gauge?

If you do, you could get a rough idea of how much a gearing change would affect your fuel economy by comparing MPG at a constant speed in, say, 3rd, 4th & 5th. Plot RPM vs fuel consumption, and then extend the plot into your theoretical new, lower RPM range for that speed.

It will not predict perfectly what the drop in RPM will accomplish (because engine efficiency for a given load changes as RPM changes, so you're changing multiple variables). But it'll give you a ballpark idea.

EG:

http://www.metrompg.com/posts/photos...-rpm-mpg-z.gif

from: http://www.metrompg.com/posts/rpm-mpg.htm

I suggest you to try 65mph instead of 80 for one complete tank and compare your MPG.

The cheapest way to change the ratio is changing the size of your tire, You need to use a GPS or something else to give you your real speed in that case.