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.

I have a Kiwi MPG, I'll give that a try. Excellent suggestion.

32 miles round trip, eh?

Well, going 70 instead of 80 would add less than 2 minutes to each leg of the commute. Whether that creates "unnessary drive time" is your call. Slowing down is the fastest, easiest and cheapest solution.

Slowing down also proportionally reduces your rpms.

I was simply trying to ask a physics question to people I thought might be knowledgable or have personal experience. If I wanted someone to tell me to drive slower I could have just asked my mom.

No offense intended.

But "adjusting the nut behind the wheel" is often the first and most appropriate bit of advice given in many threads around here.

Basjoos claims to have good fe at high speed using a combo of aero, lean burn, and gearing.

There are several gearing threads that the search function will find. There is an optimal rpm range for engines and I'm sure at 80 mph you are well past it with stock gears.

Tell your Mom "Hi" for me.

To be fair, advice like "drive slower" is a physics answer , as Wikipedia will tell you.

"Note that the power needed to push an object through a fluid increases as the cube of the velocity."

Maybe the reception was a bit harsh in your eyes, but you are asking a question that has been discussed a few times.

Will aero or gearing mods help? Definitely. But they'll help even more if you combine them with lower speeds.

Since few of us drive 80 MPH, the following experiment would be of interest to many of us:

1) Measure trip time (door to door, not just car moving time) and gas mileage for a week driving 80 MPH.

2) Repeat, only at 70 MPH for a week.

3) Go back to 80 MPH for a week.

This is an ABA test.

Report the results. I'm curious myself.

The Aerocivic gets low 60's mpg on 80mph interstate trips in the summer but to get that mileage requires a combo of exteme aero, tall gearing (2000 rpm @62mph), and the low Cd allowing me to run in lean burn at that speed.

Yea, 4000rpm is pretty high. If you ever look at the few BSFC charts out there you'll notice light vehicle engines tend to start losing efficiency above 3000rpm at any load, presumably because friction is getting pretty serious. For short geared cars people tend to report pretty big mpg changes with taller gears because of this; my MR2 runs a little under 4400rpm at 80mph (no, I never cruise at 80, I do 55 and let people blow past me at 70), people who swap the 5th gear to an 11% taller one have claimed pretty big improvement (like 9%ish) when doing 70mph.

80 on the freeway, in Floridia I have been there a few time and have seen it.

Do aero mods now, regear when you can.