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Maintaining Constant Speed

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Hey Guys!

So at the weekend I drove all the way to Preston and back (about 3.5 hours each way) and one of my passengers who considers himself a good driver (and is, to be fair) pointed out to me that I'm really bad at maintaining speed... so like doing 70 I'll drop to about 67-68 then push it back up again, keep it a bit and repeat... It doesnt bother me and I dont even notice myself but now that he's pointed it out I can't help but notice that I do it!

I have the MP135 and the throttle in 5th is really touchy, so any tips for maintaining a constant speed nice and smoothly? (I dont have cruise control)

I'm also doing a 2.5k-3k mile round trip in a week all round europe so I need to get this sorted :D

Thanks everyone!

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It might be gradients that alter your speed by ~3mph if you keep a steady foot. Anyway, 3mph at 70, what's that 4%?

If it IS gradients, maintaining the speed would cost more petrol as would cruise control. Let your passenger maintain his own speed and cost him more dosh - equivalent to - pffffffffffft ;)

I've followed wa***** varying between ~20 & ~40 on straight flat roads, course, they may have been hissed :D

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It's impossible to try to maintain 70 (unless it's empty). . . It's not even something you should consider as a gauge for how good your driving is. What is far more important is maintaining your space between you and the cars in front, and also anticipating what is ahead and around/behind you, to do all this well you will need to vary your speed accordingly to the circumstances.

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Do you find yourself in the situation of overtaking then being overtaken by the same vehicle? That's the time to consider your speed control - is it you or is it them?

It is annoying to be doing say, 72mph behind someone. Their speed reduces so you overtake and with the road clear ahead you move up to perhaps 75. A short time later they come whizzing past (check speedo, yes they have speeded up). Later you come behind again (check speedo, yes they have slowed down). So the cycle repeats.

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If it IS gradients, maintaining the speed would cost more petrol as would cruise control.

Incorrect. Fuel consumption at speed is primarily dictated by aerodynamics and the aerodynamic behaviour does not change between gradient and flat. The extra instaneous fuel consumption seen in climbing is to gain potential energy, which is a fixed amount set by the altitude gain, not speed of climb. Also, so long as no braking is done on the way down, the energy is fully recovered.

The main problem of drift is with the aerodynamics and the square law. So if you average 70 but drift between say 60 to 80, this will use more fuel than keeping a steady 70. For instance, if the car does 50mpg at a steady 70, drifting like that could reduce it to 48mpg.

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AIUI -

If the aerodynamics remain constant, an uphill gradient will obviously use more fuel to maintain the constant speed of an object being dragged uphill.

"instaneous fuel consumption seen in climbing" is surely not instantaneous, it's a constant extra fuel delivery (for a linear gradient) for the length of the climb.

"So if you average 70 but drift between say 60 to 80, this will use more fuel than keeping a steady 70."

We're talking about trying to maintain a steady 70 but sometimes dropping to say 67. Obviously if we're drifting between say 67 and 73 (instead of 67 to 70) we'll hit more wind resistance at 73. So yes, more fuel.

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Ah this is an interesting physics discussions. . . I'll just chime in my caveman opinion and say I build up speed before the gradient so I don't have to worry too much about applying more pressure through my right foot. It works for me. :D

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Yebut, are you trying to maintain a fixed speed or just get up the hill bl**dy quick? LOL

Hmmmmmmm, my head hurts :D

Access database anyone?

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Yebut, are you trying to maintain a fixed speed or just get up the hill bl**dy quick? LOL

Hmmmmmmm, my head hurts :D

Access database anyone?

Haha, good point. I'm just trying to get up the hill.

What hasn't been addressed here though, is the torque. More powerful cars will maintain speed better. My father had a nice Honda (at the time) and we were on the motorway and next to like a metro or something terrible, and every time the gradient went down, the metro would pass us, and then back up, we'd overtake the metro, but our speed was constant.

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Lol, thanks for all the comments guys :D

It's nice to know that it's just them being awkward and not just me - I know I probably could drive slightly more smoothly but I've only covered maybe 20k miles since I passed my test so I have a lot of experience to gain!

Also, I got 50MPG out-of it (429 miles before I filled up) so can't be that bad!

As for the whole physics discussion... I'm staying out of that one!

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This is an interesting discussion especially when you talk about the physics behind it :)

As long as you're not annoying other drivers by going from 70 to 60 to 80 then you probably don't need to worry about it. It can be difficult to keep a steady speed without cruise control unless you keep glancing at the speedo every couple of seconds, like I used to before I had a car with cruise control.

I always leave the cruise control on when going uphill, but it would be interesting to see how steep the hill would have to get before I can't get up it in 6th gear lol. I had that problem with my old Fiesta 1.4, when it was almost full of people and luggage it only just managed to stay at 70 going up some hills, even with my foot to the floor.

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AIUI -

If the aerodynamics remain constant, an uphill gradient will obviously use more fuel to maintain the constant speed of an object being dragged uphill.

"instaneous fuel consumption seen in climbing" is surely not instantaneous, it's a constant extra fuel delivery (for a linear gradient) for the length of the climb.

"So if you average 70 but drift between say 60 to 80, this will use more fuel than keeping a steady 70."

We're talking about trying to maintain a steady 70 but sometimes dropping to say 67. Obviously if we're drifting between say 67 and 73 (instead of 67 to 70) we'll hit more wind resistance at 73. So yes, more fuel.

I included aerodynamics simply because it is the most signficant user of fuel. Going up hill adds to engine load needed to overcome the aerodynamic load, going down reduces engine load and the number on the instaneous reading on the dashboard goes up and down to reflect it. Seeing low numbers on the display worries some people.

Using 60-80 as the drift was for illustration purposes, of course. (Though sometimes there are people that can't be far off doing that.)

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I included aerodynamics simply because it is the most signficant user of fuel.

I can definitely believe that. The forces involved in pushing all that air out of the way must be huge, and you can tell when you're in traffic with cars rushing past in the lane next to you, the displaced air will make your car move quite a bit.

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What hasn't been addressed here though, is the torque. More powerful cars will maintain speed better.

True. But cars are geared so they can hold 70mph on a motorway gradient in ideal conditions.

Obviously conditions are rarely ideal. This gearing means that if the driver doesn't notice a speed drop on reaching the gradient and responds late the car won't pick up again without a downshift. It is essential to look ahead and get ready. (Or have cruise control.)

Then there is the effect of slipstreaming and headwind. Two low powered cars reach a gradient; the slipstream means the driver of the second car has a bit of reserve as the gradient commences and pulls out to overtake. As this car gets level with the first it hits clear air and the tailback commences. I used to have a Primera 1.6 (quite heavy for the engine with tall top gear) and it was necessary to be very alert to this.

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If it's any consolation, I just tried maintaining 70 now, and fluctuated a fair bit (between 60 and 80 haa)

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It is interesting with cruise control how you see other cars fly past and x miles down the road you creep up with them again. You know u have maintained a steady speed and they obviously haven't.

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^ It's also interesting how you can sometimes tell when someone else is using cruise control. If you overtake them or they overtake you, and both cars are moving past each other at a perfectly steady speed.

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Drive how you want to drive, the problem is having a passenger who drives them self as although they never see the things they do wrong they'll be free to see the things you do and feel they should point them out.

I apparently shift up and release the clutch too quick that it hops slightly and a long term driving buddy of mine pointed that out... but he's a white van driver for his job and I won't go into the scarier things he does when driving so just take it with a pinch of salt, people will always not like the way aomeone else drives (we all do it), don't do anything stupid or dangerous and your fine :)

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I apparently shift up and release the clutch too quick that it hops slightly

I do this too, but I do it deliberately so that the gear change is quick enough that I don't slip the clutch. But I sometimes slow down my gear changes when I have passengers for their comfort, and go round corners more slowly. It depends on the passenger though, if I know they won't complain then I just drive as if they aren't there lol.

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