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Old 01-02-2014, 03:09 AM   #1 (permalink)
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Engine load vs rpm? (Climbing: hold top gear at very high load, or downshift?)

Which is better? I live in California, and I have to drive up the Cajon pass everyday (I think it's a 3% grade maybe more idk) and I can do about 70 @ 3krpm with really high engine load, my guess is somewhere around 80%, or 3.8k rpm with around 35% ish engine load. My question is, which one is better on fuel economy? Low rpm with high engine load or high rpm with low engine load?


I bought a cold air intake for my car a while back and it honestly has helped me a ton with judging engine load, I gained a 2 mpg increase once I put it on. I don't think it was the air intake itself, but me being able to adjust my throttle foot more sensitively to keep engine load down.

I have a lot more questions but I'm just starting with this one.

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Old 01-02-2014, 05:04 AM   #2 (permalink)
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There's no way the load will drop to 35% going from 3000 to 3800. Unless that "load" isn't actually the correct load. Higher load is better. 100% load with engine running rich is often no less efficient than highway cruise at low load. If you don't need to downshift, don't unless the rpm is like 1000 and the engine is struggling.
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Old 01-02-2014, 05:13 AM   #3 (permalink)
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Quote:
Originally Posted by serialk11r View Post
There's no way the load will drop to 35% going from 3000 to 3800. Unless that "load" isn't actually the correct load. Higher load is better. 100% load with engine running rich is often no less efficient than highway cruise at low load. If you don't need to downshift, don't unless the rpm is like 1000 and the engine is struggling.
Exactly the answer I was looking for, thanks. And maybe it's not 35%, I don't have the smarts to install a mpguino (at least i don't think I do) and my car is 1 year off from being able to use scangauge. It's the reason I bought my cold air intake, so I could hear the engine more and kinda try and guess where my engine load is at.
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Old 01-02-2014, 09:11 AM   #4 (permalink)
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Going less than 70 would help a lot. If those people that come barreling up behind you can pass and don't like it, that's their problem.
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Old 01-02-2014, 01:03 PM   #5 (permalink)
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If you can keep it in closed loop at highest load would be the best.
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Old 01-02-2014, 01:14 PM   #6 (permalink)
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Here's some good discussion on the subject: http://ecomodder.com/forum/showthrea...ear-23957.html

I remember driving up that grade in an overloaded six cylinder 1955 Ford 1/2 ton at WOT in first gear, while watching the speed slowly decrease. No choice of gears on that trip. Barely made it.
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Old 01-02-2014, 03:46 PM   #7 (permalink)
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I did practically the same thing up Afton Mountain in my 170 CI 63 Valient. Had to downshift to second gear, then back to third until speed dropped to 35 MPH, second to 50 then third again, rinse and repeat.

Got 28 MPG in that old Valient. Probably had the carb set a little lean.

Oh yeah, bias ply recaps .

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Old 01-03-2014, 02:09 AM   #8 (permalink)
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Any idea what kind of increase in mpg I will get between 70 and 75 mph?
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Old 01-03-2014, 06:51 AM   #9 (permalink)
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Quote:
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Any idea what kind of increase in mpg I will get between 70 and 75 mph?
Seventy and sixty-five? How soon do you plan on investing in an MPGuino?
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Old 01-03-2014, 06:56 AM   #10 (permalink)
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Quote:
Originally Posted by Baltothewolf View Post
Any idea what kind of increase in mpg I will get between 70 and 75 mph?
You will get a negative increase. Try taking it down a few MPH.

Gas Mileage Tips - Driving More Efficiently



Mpg For Speed - Fuel Efficiency Vs. Speed says: "You'd be surprised to learn that a slight decrease in your highway driving speed can significantly reduce your gas consumption, while only adding a few minutes to your travel time. "

They have a calculator you may find enlightening. Plug in your MPGs, miles driven, and the price for a gallon of gas, and it will show you the difference, in dollars and minutes spent, across a range of average speeds.

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