Stupid transmission related question...

flashburn

So, I'm a bit of a noob when it comes to this stuff, so be easy on me. (or not)

Which is more fuel economical in this scenario:

When driving @ 70MPH, say I want to accelerate to 80MPH, and I'm in final gear.

A) Floor it without shifting.
B) Shift down a gear (or two) and accelerate at the same speed as A. Meaning reach 80MPH at the same length of time as the first case.

Like I said, it's probably a stupid question, but enlighten me!

trent1331

Just by thinking about it in my head it would seem that downshifting would increase engine speed, which would use a lot more gas than just staying in top gear.

flashburn

So simply the fact that the RPM's would be higher in a lower gear would mean that gas usage would be higher? Even if in final gear it was under WOT (but at lower RPM's)?

PapoZalsa

You should be able to keep in 6th gear and accelerate a little more, no need to floored the gas pedal.

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2009 370Z MB Base w/Sport Package 6MT
Modshack's Custom Air Intakes /3M Clear Bra/ Illuminated Kick Plates / F. I CBE w/12" resonators / EVO-R Front Lip / Rear Hatch Wing / Tein S-Tech Springs /Ilegal Tint in the Rep. of VA/4x20mm Z1 Spacers/ Vinyl Roof/Liquid Carbon Designs

flashburn

This is on the automatic tranny. I understand that WOT at 7th might not be needed, but the scenario I presented is somewhat hypothetical.

Basically I'm just trying to understand if going at a given acceleration in one gear would be equivalent in fuel efficiency when compared to going that same amount of acceleration in another gear. For instance WOT in 7th and a matching acceleration in 5th or 6th.

JvKintheUSA

I believe that the engine runs most economically where the torque curve and horsepower curve intersect - around 5300 RPM or so. So keeping the RPM around the 5300 will give you the most bang for your gas-buck.

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PapoZalsa

In 7AT or 6MT cruising at 70 to 80 MPH you should not be at 5300 rpm if that is what yoy trying to say.

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2009 370Z MB Base w/Sport Package 6MT
Modshack's Custom Air Intakes /3M Clear Bra/ Illuminated Kick Plates / F. I CBE w/12" resonators / EVO-R Front Lip / Rear Hatch Wing / Tein S-Tech Springs /Ilegal Tint in the Rep. of VA/4x20mm Z1 Spacers/ Vinyl Roof/Liquid Carbon Designs

JvKintheUSA

Not sure what I was trying to say...

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Jeffblue

getting good mileage is a balance between keeping the engine speed low, but also not lugging and bogging down the engine by trying to run it in too high of a gear. IE shifting up at like 1200 RPM to try and be in the next gear is just gonna bog your engine. Imagine riding a bicycle, and trying to start off in the highest gear. YOU are the engine and it is hard for you. Thats how your engine feels when you try and run it in too high of a gear to try and save gas.

jhh343

it is all about rpms vs. the speed you are going. the answer to your question is A. 6th will get you to 80mph with the least amount of rpms/engine strain so there will be less fuel consumption. you could try to watch the gauge in the car while attempting both scenarios to see which keeps the mpg the highest.

flashburn

Nice, thanks for the answers and explanations.

Vegitto-kun

I think at WOT and you are in 7TH you will use more fuel than when your in say 5th

Its logical.

you are on the last gear and you trying to accelerate hard and it just takes more effort than when you are already in the high revs.

but if you are slowly accelerating to the speed you want the high gear is better.

drdre8424

it depends i guess on why you need to increase your speed...if its to pass someone, especially in a 1-lane hwy, i say coast down to about 55, drop down couple gears and pass the guy in front of you cuz he's making you drive slow

have some fun...unless your driving your car hard all the time, the fuel economy will level out.

flashburn

Okay. Anyway, back to the topic at hand.

Vegitto, your reasoning is kind of what I was thinking in the first place. I switched on the instant MPG readout and it seems to agree.

This all came about because I noticed how frequently when in Auto the car will downshift on acceleration when I really didn't need it to (i.e. going from 70-75, basically small increments), like most auto's will do. So I started wondering if when its doing that it also isn't as fuel efficient as staying in that higher gear, given about the same level of acceleration. It didn't seem efficient because it would shift down a gear or two for a couple seconds worth of acceleration and then after a few more seconds shift back up to 7th.

Vegitto-kun

just means you hit the throttle too hard. I can accelerate perfectly fine without it downshifting.