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My car is a Citroen Xsara coupe 2000.

I am little tight financially at this point and I try to save money from everywhere I can...

I was wondering what is the ideal speed for my car to achieve the best MPG during my way to work which is 21km... My first assumption is to go in 5th (highest) gear at give or take 1500 RPM...

But I've read online that cars achieve the best MPG at speeds around 55mph... But I can't think of a reason why my car would achieve better MPG running at higher RPM on the same gear, in order to reach 55mph.

What is the recommended approach?

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    $\begingroup$ Can you do 55mph all the way to work? $\endgroup$
    – Solar Mike
    Aug 1, 2021 at 12:07
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    $\begingroup$ key to best mpg tends to be to brake as little as possible rather than to reach the best aerodynamic speed - it doesn't do you any good to accelerate to 55 before having to brake all that away before a stop sign. $\endgroup$
    – Abel
    Aug 1, 2021 at 15:22
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    $\begingroup$ If you are really interested in this search on "hypermiling". $\endgroup$
    – Eric S
    Aug 1, 2021 at 17:38
  • $\begingroup$ Don't race to stop at a red light. Try to drive without stopping. Waiting for lights to change is burning fuel while you're stationary. Look ahead & notice what the traffic & any traffic lights are doing, try to anticipate what the lights will do by the time you get there & if possible adjust you speed, particularly slowing down, to get there while they are green. Adjust your speed by taking your foot off the accelerator & not putting it on the brake. $\endgroup$
    – Fred
    Aug 2, 2021 at 4:43
  • $\begingroup$ If you leave five or ten minutes earlier, do you get significantly less traffic? That would help with driving in a more relaxed style, with less braking needed. $\endgroup$ Aug 2, 2021 at 17:23

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Let me try summarizing what's available in the public for literally half a century or so, in no particular order. (There may be some overlap with other answers.)

Optimal speed

As others already mentioned, this is just fantasy. Whatever the value is, and it exists, you won't be able to apply it in real-life driving situations. E.g. in the city you'll tend to be too fast, on the highway too slow, citing the speed you mentioned.

So what are alternatives?

Don't drive

It's less silly than it may sound. Commuting may be the biggest part of consuming gas, but there are many more routes we take. So, think about avoiding using your car more frequently e.g. by:

  • better planning (one round trip instead of repeated starts from home)
  • team up with colleagues to drive together and agree about cost sharing (which will pay off quickly)

Tire pressure

Check regularly, i.e. more frequently than once a year.

Flexing tires with every rotation consumes considerable energy. If pressure is too low, this amount increases, resulting in more gas consumed per mile. Overpressuring a little isn't harmful, saves gas and avoids wear-down pattern typical for flat-footed tires.

Your cars mass

We all have the tendency to drive around extra luggage, like blankets, the old battery etc. Inspect your cars for items which don't need to follow you each and every day ... and take them out. Keep all legally required items (and make them complete again, while you're at it).

Look ahead ahead

The way you drive is amongst the biggest contributors to gas consumption. Typically you have these three situtions:

  1. start driving (accellerating, $a > 0$)
  2. driving at constant speed ($v = const., a = 0$)
  3. braking/stopping ($a < 0$)

#1 and #3 are the gas killers, while #2 is a steady-state, where gas consumption is much smaller. BTW, time is your friend: you don't need to drive a racing car ($v = a \cdot t$), which drives your MPG up, up, up. So keep $|a|$ small whenever possible.

So the further you can watch geometrically (1-2 miles ahead, at least) and in time (anticipating traffic), the more often you can avoid or reduce the impacts of #1 and #2. Besides your eyes use traffic information. "Read" the other vehicles around you. Etc.

In traffic jam avoid stop-and-go. Increase your distance to the car in front of you, and try keeping a constant low enough speed instead, i.e. keep rolling, flowing, where others don't. With a truck in front of you, watch e.g. shadows ahead or traffic lights to gradually speed up in time, i.e. often a bit earlier than the other impatient "lemmings".

Transmission and gear

I assume you don't have an automatic: then there is little you can do, besides e.g. lifting your foot from the gas pedal to signal the system "there's a change". However, Cruise Control can literally pay off.

With manual transmission you can do many things, with situations #1, 2 and 3 in mind, e.g.:

  • when accelerating, switch gears earlier than usual (roughly, consuption is proportional to $N \cdot v$, where N is your motors rpm)
  • for constant speed use the clutch and reduce gas: your car will roll even with least RPM
  • watch topography: rolling down the hill literally asks for opening the clutch; gradually increase speed in time when approching up-hill
  • this implies using your highest gear as soon and as often as possible
  • when braking, open the clutch and brake, while keeping gear: often using rolling- and air-resistance from an opened clutch is all you need; adjust gear once you know you need to accelerate again

Safety

BRAKE hard and fast (OPEN clutch AND NO off-gas, a two feet operation) when you need to: time lost never returns here, i.e. results in accident. (Yes, and watch the "lemming" in your back always, best in advance).

If you haven't done yet, attend a safety training, which will open your eyes beyond MPG.

Self-feedback

Track you mileage somehow. If you car offers a measurement for MPG, try using it. Else, refuel more frequently, at least for a while, and divide your required gas volume by your miles driven. From your protocol you also know your behaviour. Try learning from it, besides seasonal impacts (more gas needed in winter). E.g. detect your preference for driving with open windows, which eat up gas, and change it somehow.

Repair etc.

Keep your car intact. Listen to their statements regarding your motor, oil, cat and what have you. Try reading the repair list as a forensic statement, telling you about your car, its traffic and ... yourself.

Refueling

You probably know and use it already: gas price varies much during the day, from day to day etc. Use websites or apps to anticipate the best time for refueling now. You don't always win, but will save money overall.

Final

When we were driving with manual transmissions, my MPG was constantly better than my wifes: she just drove.

With our automatic, my MPG still was better, because I used the Cruise Control more frequently than she does.

With the dramatic increase in price from ruZZia's illegal invasion into Ukraine, she found shorter routes and avoided using the highway without sacrificing arrival time ... which reduced her MPG.

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When you hear the engine roaring, it is guzzling a lot of gas. don't do that. When you brake to correct your excessive speed you are wasting all the gas burned to give you that speed.

In traffic don't drive like New York taxi drivers, constantly putting your foot on gas then brake pedal.

Try to create some space between you and the car in front of you( not as big as encouraging fellow drivers to claim it) to avoid having to be jerked forward and back to stay in a formation with them.

Have your engine tuned and tires inflated with correct pressure.

All of the above are common sense, only require discipline.

Not only do you save gas, but you also add years to the life of your car.

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If you really want to get into details, such as optimal RPM and aerodynamics, it might get complicated. Every engine has an optimal "operation" RPM, where it runs most efficently, this means that the engine will produce the most amount of torque at the minimum cost of fuel. You can probably find this information online which is sometimes provided by the manufacturer or measured by car enthusiasts. I found it for my Audi A4 2011, 2.0 TDI which was about 2200 RPM in 6th gear.

Aerodynamics are somewhat more difficult to account for, since weather might change during your trip or on different days of the year. But in general a good estimation of the fuel consumption can be measured on most cars by their own on-board-computer. If you take all the other answers into account and start measuring your fuel consumption, you can get a good estimate of what RPM and drive-style gives you the most MPG.

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The biggest circumstance is CONTROL YOUR SPEED. Air resistance (drag) increases with the square of speed, and therefore the power needed to push an object through air increases with the cube of the velocity. So....a vehicle needs 8 times the engine power to reach twice the speed. In principle, this means that fuel consumption will increase fourfold. if you lower the speed, fuel consumption is decreased by the full 75%. How fast are you going? If you're on a major highway, AVOID going above 90 km/h.

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