Fuel Economy Question

Post by **racer12306** » Wed Oct 03, 2007 3:44 pm

Something I have been thinking about some.

Would it be more fuel efficient to use more throttle and a higher gear, or less throttle and a higher gear. For example

35mph in 3rd gear is ~2500rpm
35mph in 4th gear is ~1700rpm

going up a little hill in 3rd gear will require less throttle than in 4th gear.

Thoughts?

j0n · Post by **j0n** » Wed Oct 03, 2007 4:18 pm

I was actually thinking about this the other day...

It ultimately boils down to whether or not fuel is getting dumped in the motor while it's bogging like hell...and thats something I cannot answer

Post by **racer12306** » Wed Oct 03, 2007 4:51 pm

Yeah, I was wondering if the fuel map is based more on load, rpm or throttle position.

If its load or throttle position then we should downshift to 3rd (in this situation). But if its rpm then we should leave it in 4th. But I'm betting load and throttle position would have a bigger impact.

anomalous0 · Post by **anomalous0** » Wed Oct 03, 2007 5:24 pm

Cars are more efficient when running in closed loop mode, that is, relying alot on input from the O2 sensor to keep the air/fuel ratio stoichiometric.
TPS does come into the equation. When it senses above a certain % throttle, it goes into open loop mode, which runs a bit richer because it has a different set of fuel maps.
going up a hill in 5th is like using one of those "low-water" toilets.
it only uses 3 gallons instead of 5, but you have to flush it twice to get anything done

You end up using more water

MoxHair · Post by **MoxHair** » Wed Oct 03, 2007 6:36 pm

fuel economy? what's that? I think the unburnt gas fumes have made me lupy.

hansken_yo · Post by **hansken_yo** » Wed Oct 03, 2007 6:51 pm

3rd gear would be the better gear because though your at a higher RPM, your torque/hp is well balanced and the car does not have to work AS hard: hence saving fuel.

I figured this out on my long drives through the desert when i was getting close to 40mpg doing 85mph. She was running at about 3200rpm steady whereas when i would do 65 i'd get about 36-38.

*Disclaimer* All of this is based on my own personal experience, but it all i have to offer you on this.

fixitmattman · Post by **fixitmattman** » Wed Oct 03, 2007 8:38 pm

Typically, when loaded it's better to run a lower gear and higher RPM. Since they don't publish BSFC maps for cars in their manuals, it's kind of a guessing game, although lower RPM does not always equate to better fuel economy. When lightly loaded around town, the lower the gear/rpm the better. When heavily loaded, contrary to popular belief, lower gear and higher RPM can be better for fuel economy by keeping the engine in it's more efficient operating regions and using less throttle.

Half my trip from home school is in some pretty substantial hills where I'm running over 3k, sometimes as high as 4400 when climbing in third and I get as good milage in that portion of the trip as cruising the level highway. (note, on the downward side of the hill I dont use any fuel at all

)

Adionik · Post by **Adionik** » Wed Oct 03, 2007 10:17 pm

So for me being an ATX guy, on long trips i'd save gas if i kept it in 3rd?

Post by **racer12306** » Wed Oct 03, 2007 10:25 pm

i would say no

let the car do the work.

Adionik · Post by **Adionik** » Wed Oct 03, 2007 10:38 pm

BUT I WANT TO SAVE GAS!

T_T

Post by **racer12306** » Wed Oct 03, 2007 10:47 pm

im talking about low speed situations. not highway speeds.

you would suck up gas in 3rd at highway speeds

Mad_Medeiros · Post by **Mad_Medeiros** » Wed Oct 03, 2007 11:05 pm

I have to always use a lower gear, anything higher then 3rd and under 2500 rpm my car sounds really funny... and makes odd noises.

Post by **occasional demons** » Wed Oct 03, 2007 11:41 pm

I would have to say higher rpms w/less throttle. My Mom's Caliber has the CVT and it seems like the motor is always screaming with every little load. Going up a hill is unnerving, but it gets excellent fuel mileage. Can't say how longevity for the engine will be though. To me it seems like a lot of wasted RPM's and a loud exhaust would sound like ass, IMHO. But back to Neons - less load would = less fuel even if spinning faster, within reason anyway. My 2 cents...

kc2005ptgt · Post by **kc2005ptgt** » Thu Oct 04, 2007 12:45 pm

I think Matt hit the nail on the head... I have always been told that when under heavy loads you will use more fuel in a higher gear at lower rpms, where as at lower gears and higher rpms say when travelling up a hill, having fat chicks in the back seat, whatever, you will use less.

I drive a lot, and missouri (believe it or not) has tons of hills, and I have been driving in a lower gear at higher rpms and find I put less throttle down when trying to maintain speed.