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Fuel economy vs speed

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Lots of variables contribute towards your economy changing with vehicle speed.

As in, do I get better milage driving at 80kph or 120kph?

Aerodynamics, engine RPM, how you drive, etc etc all contribute but its difficult to know for sure.

I had a friend drive my car up and down the motorway the other night while I was seeing how far I could advance the ignition timing before registering knock.

I had cruise control turned on, so we could hold the car at the centre of a column of cells and watch it go up and down the column as we approached hills and went back down them. So we repeated the same thing very consistently at different RPMs. (Yes it took ages haha)

Since I've got good data from characterizing the injectors, and I had this huge datalog file from an hour or two of cruise control motorway driving held at various engine speeds in 6th gear.

I thought I'd throw together something to find out the best speed for least gas.

The lower the line, the better the economy:

I'm thinking that since my car is an 80s square brick, with probably a CD of .35 of there abouts. And also that it's diff ratio means it does 3300rpm @ 100kph, it's most economical at a lower speed than I was epxecting.

My daily driver has this peak economy point closer to 85kph, which is where most cars get to that aero cross over point unless they're really slippery like a prius or something.

Most cars are designed to have best fuel economy at highway cruising speeds. Although both Holden (Australian GM) and Ford (Australia) run fuel economy tests on various cars and use these in advertising. When Holden released the JB Camira (1982?) they did a fuel economy run between Sydney and Melbourne (which has since become the standard for doing these runs) and they found the Camira ran most economically at 65 kmh. Ford did the same trip in 2010 with a Ford Territory Diesel and they found below 100 kmh was optimal. All these tests are done on what is basically a freeway with the occasional town/city driving.

Yeah its pretty much a balance of finding the highest speed at which you can be near the ideal BSFC rpm/load point of the engine, but not so fast so that the coefficient of drag is coming hugely into play.

As drag increases at the square of your speed most cars start losing economy pretty quickly generally above 80-90kph.

Based on other similar car shapes/designs from the 80s I'd say my car has a CD of 0.34 - 0.36 kind of area.

I wonder if there's a way you can work it out by looking at operating the engine at the exact same rpm in different gears, then comparing how much load the engine is under. As a more aerodynamic car would increase load with speed at a lower rate.

Interesting David! Just a thought that might help improve the data in your log - If you tried filtering the data based on either rate of change of MAP or TPS, you might end up removing a lot of those outliers and end up with a more representative graph. This is a common technique applied to filtering the data log to rescale a MAF sensor for instance. Basically focus the data on true steady state with little to know sample to sample MAP/TPS variation. I know you're using cruise control but there is still likely to be some less than smooth changes in load that will affect your data.

Interestingly my wife's Holden Captiva (I know, hardly an interesting vehicle) achieves noticeably better economy at 110-120 kmh than it does at 90-100. Obviously this testing was performed on a closed road ;)

Hmmm thanks I hadnt considered that.

Interesting about the Captiva economy, must have some good gearing to keep the rpm low-ish?

I should really get around to setting up on the modelled fuel equation so I can have some realtime economy results directly from the ECU, now that I've got the injectors well characterized.

Captiva are 6 speed manual or 6 speed auto aren't they? with the top 2 ratios being overdrive?

I had a thought the other day.

While I've got a bit of an obsession with fuel economy at the moment, why not repurpose my tacho into a fuel economy gauge.

Currently I'm running a traditional fuel setup instead of modelled, so I dont natively get any fuel economy info.

So I had to put together a map that controls the sweep of the tacho based on effective injector pulsewidth, and engine speed as the axes.

Then work on the basis that it will only work in 6th gear for the moment and base the values in the table around that.

Driving with it is interesting!

Basically you'll be driving along fairly normally at say 70kph, the needle will be on say 6-7k on the tacho.

As you slowly accellerate, the needle will drop and drop, and then when you get to a certain point it just stops and doesnt go down anymore. If you keep accellerating at the same rate from there, the needle starts going back up.

So once you get to that low point, you back off the gas to hold that speed, and the needle will drop down to say 2krpm or 3krpm and you can fine tune the speed up or down to keep the number as low as you can without losing speed.

Regardless of whether you're on an uphill or whatever, ithin the space of about 5-10 seconds you can find the most economical load and rpm combo which is pretty cool.

It's amazing how deviating just a few kph either way of that point takes away economy really quickly though.

I found that my throttle was quite jumpy when trying to fine tune that needle angle, so I have revised my e-throttle curve to be even less sensitive below 70% throttle.

I might setup a second throttle map so that if you ever go above 6000rpm, it will start a timer and switch to the "race" throttle map which is linear. Then if you havent gone over 6000rpm again in the space of a minute or something like that, it goes back to the mushy throttle.

Or maybe just have it activated by one of my centre panel switches I guess.

But congratulations to me, I've just figured out how to make e-throttle as unresponsive and mushy as OEM cars, the thing that everyone hates about it hahaha.



does testing on a closed road yield better fuel economy at 120 ?

As per my findings, in a turbo car on a flat surface, best economy was in top gear off the boost. I believe, that in general it could be said that least amount of gas that results in highest speed returns best economy.

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