mpg vs. speed

[thisisntjared]

i think its interesting that they took the time to show that when you go too slow in the top gear of some cars you lose efficiency. in the '86 gti it clearly shows that your peak efficiency is 40mph but any slower is less efficient. as also sited from fueleconomy.gov, there is an efficiency that peaks around 45-50mph in the top gear. even though their source seems somewhat arbitrary...

now i have heard contrary info from some of you guys.... logic tells me that when the rpms go too low some motors, particularly hondas become inefficient.

[SVOboy]

I think you've seen my graph. It's better the slower you go, though not by much. However, in normal driving where you can't just cruise completely unimpeded 30mph at 1k rpms is not as good as mehbe a bit higher because of the little changes you have to make. However, it's still better than being in 4th gear so there's not much to do about it.

[MetroMPG]

Originally Posted by thisisntjared

i think its interesting that they took the time to show that when you go too slow in the top gear of some cars you lose efficiency. in the '86 gti it clearly shows that your peak efficiency is 40mph but any slower is less efficient.

But... look closer at the GTI graph. The efficiency drop below 40 mph is in 4th gear, not 5th. So its peak efficiency is the lowest tested speed in 5th gear:



Also, the EPA graph doesn't say whether it's all in top gear or not, but I doubt it is - it's too similar to the shape of the GTI graph to be coincidental.

I suspect the peak of the EPA graph is for a car with an automatic transmission, which shifts into overdrive around 50 mph. Conjecture. I've yet to see a graph that shows better fuel efficiency at except in top gear at the lowest possible speed.

[MetroMPG]

Originally Posted by SVOboy

I think you've seen my graph. It's better the slower you go, though not by much.

Actually I forgot about it, but yeah, that's a crazy graph (Speed versus MPG (CRX))

If it's OK with you, I might fix it up a bit and post it on my site with the others. But I want more methodology info: what were the temps, and were the readings averages of bi-directional runs, or one-way? All repeated on the same stretch of road? Other cars around? (I cancel runs where I'm in another car's wake - I don't mean drafting).

(Feel free to reply in your graph thread.)

[thisisntjared]

Originally Posted by MetroMPG

But... look closer at the GTI graph. The efficiency drop below 40 mph is in 4th gear, not 5th. So its peak efficiency is the lowest tested speed in 5th gear:



Also, the EPA graph doesn't say whether it's all in top gear or not, but I doubt it is - it's too similar to the shape of the GTI graph to be coincidental.

I suspect the peak of the EPA graph is for a car with an automatic transmission, which shifts into overdrive around 50 mph. Conjecture. I've yet to see a graph that shows better fuel efficiency at except in top gear at the lowest possible speed.

very good point! i totally didnt notice, however look at the changes in 2nd gear and the changes in 4th gear. there is something to be said for that...

[MetroMPG]

Originally Posted by thisisntjared

look at the changes in 2nd gear and the changes in 4th gear.

Good eye! Hadn't thought about that and can't explain it either. I was really just looking at the 5th gear stuff, since that's what I did in my car.

I wonder if I did a range of constant speed runs in all my gears whether the best mpg for each gear would always be at the lowest practical speed the gear could be driven. My first instinct is "yes", but I'm not certain now.

I could see first gear from 0 mph perhaps showing an oddball result, but I'm surprised 4th is different from 5th.

[SVOboy]

We could always hook my MID up to jared's car and test it out for ourselves,

[MetroMPG]

Don't let me stop you. Go get some data! I'll may go around the block a couple of times later in the Flea just to see what the SG says about it.

[SVOboy]

Well, that's all up to jared. I figure he's busy with marriage or something, I'll just send it to him before I leave for school or something, if he wants, that is.

[thisisntjared]

Originally Posted by SVOboy

We could always hook my MID up to jared's car and test it out for ourselves,

i have a feeling itll be good at lower speeds just because of my awesome 4.25 final drive and .702 5th