Cruising Speeds And Economy

[monya]

Drove up to Northampton from London using the M1 this morning - I live within a mile of it.Set the cruising speed to 55 mph as there was no hurry. Averaged 56 mpg.

Returned at 65mph and averaged 61 mpg.

My theory is that a lower speed can cause higher engine revs to get up the slopes than a higher cruising speed that has more momentum meaning the engine has to work less hard on the 'ups' Traffic conditions were similar both ways including a 10 mile 50 mph speed zone.

I never used to care about economy to this extent - must be getting sad .

Any support to my theory or am I just chattin sh** here?

[Jan&tone]

Over 2.5 years of a Gen2 and 6 months of a Gen3 I've come to the conclusion that 65 is a pretty fair speed to set the cruise control at if you can. It gives a fair journey time and a pretty good consumption too....

- an added bonus is that who cares about that police car you just passed!!

[dwilson]

Drove up to Northampton from London using the M1 this morning - I live within a mile of it.Set the cruising speed to 55 mph as there was no hurry. Averaged 56 mpg.

Returned at 65mph and averaged 61 mpg.

My theory is that a lower speed can cause higher engine revs to get up the slopes than a higher cruising speed that has more momentum meaning the engine has to work less hard on the 'ups' Traffic conditions were similar both ways including a 10 mile 50 mph speed zone.

I never used to care about economy to this extent - must be getting sad .

Any support to my theory or am I just chattin sh** here?

It would be interesting to see if you could repeat these figures and if you had the same results if you went at 65 mph and returned at 55 mph.

David

[DrCez]

Drove up to Northampton from London using the M1 this morning - I live within a mile of it.Set the cruising speed to 55 mph as there was no hurry. Averaged 56 mpg.

Returned at 65mph and averaged 61 mpg.

My theory is that a lower speed can cause higher engine revs to get up the slopes than a higher cruising speed that has more momentum meaning the engine has to work less hard on the 'ups' Traffic conditions were similar both ways including a 10 mile 50 mph speed zone.

I never used to care about economy to this extent - must be getting sad .

Any support to my theory or am I just chattin sh** here?

I think it's important to consider wind resistance (as an occasional cyclist I speak from bitter experience!). Theoretically air drag is proportional to velocity squared, so there should be a significant difference between 55 and 65 mph, even for an aerodynamic car. This obviously does not fit with what you've seen, but there was a moderate N/NNE wind today (around 10 mph) which could explain the difference. Cars with manual transmission will be more efficient at certain road speeds which match an optimum gear ratio but I can't see how this would be the case for the Prius with its HSD: it can rev the engine at pretty much optimum rate for any road speed.

Quite how this balances out with the effects of hills and using the momentum of the car I don't know, but getting better economy at higher speeds seems to violate the laws of physics (whatever you do, don't tell Jeremy Clarkson: he'd have a field day ;) ).

[monya]

Agree that in theory greater speed should increase fuel consumption and I'm sure this holds true on a flat surface but maybe less so on normal roads.

[Gmac70]

Also got to remember Nottingham is up hill from London, and return journey was downhill - lol

[Phoenix999]

Also got to remember Nottingham is up hill from London, and return journey was downhill - lol

Yes a good observation

Thats why us sane folk from down south tend not to frequent the northern cultural wilderness.... it is a serious uphill drag ;)

[Jan&tone]

Also got to remember Nottingham is up hill from London, and return journey was downhill - lol

Yes a good observation

Thats why us sane folk from down south tend not to frequent the northern cultural wilderness.... it is a serious uphill drag ;)

Yeah - and dont forget if we havent got our passport with us they wont let us in

[Carltona123]

I actually drove down to London from the "cultural wilderness"(Birmingham)on Sunday. Fuel consumption on the way home was much worse than going. Must have been the weight of the silverware in the boot ha ha!!

And before anyone asks, no we didn't have to stop at IKEA to buy a cabinet to put it in.

[Cyker]

As well as air resistance, the engine's peak efficiency has to be taken into account.

I could drive my Yaris at 55mph but at 60mph the engine's just entered that flat-peak of max torque between 1800 and 3000rpm where it's running at its max efficiency.

It also will go up the steeper hills a lot more willingly in 5th at 60-ish than at 55-ish :)

[flow]

Another thing i've noticed is wet weather - this seems to hit my economy, due to the added resistance on the tyres.

[Aimhigh]

There are definitely sweet spots (speeds that give better economy than others) with the Prius. I found this to be the case with my old Gen2 and I am still working out the best speeds with Gen3.

At speeds faster than 65mph the relationship is not linear in my experience. For example sitting at 70/71 might be worse than sitting at 75mph.

[hertsnminds]

Way back in the dawn of time, just before Euro rules and regulations the best fuel economy was always quoted at 56mph (now known as extra urban). If you bear in mind the 10% error your speedo is allowed you should find the best mpg in any car will be 56mph actual or up to an indicated 62mph. For whatever reason the gearing ratios always give the best MPG at this speed it doesn't matter how many cogs (if any) the gearbox (drivetrain) has.

[capnbirdseye]

In theory what you might gain going one way you nearly always lose coming back or visa versa.

If you climb a half mile hill which is then followed by a 4 mile steady descent with practically no fuel being used then coming back you have a four mile steady incline with the engine on all the time only to get a half mile glide the other side.

I can easily get 65mpg+ on a nearby 11 mile trip to Stratford on Avon which mimics the above scenario but if I reset before coming back it's on 56 when I get home

[sjrainsford]

In terms of ideal speeds, I've found an indicated 60mph about right on mine for decent fuel consumption. Go above or below that by 10mph and it suffers.

Funnily enough though, the difference between 75 and 90mph appears much smaller on mine. Not that I ever drive at 90mph officer! But it really does cruise well at 90mph which has surprised me.

[kithmo]

In terms of ideal speeds, I've found an indicated 60mph about right on mine for decent fuel consumption. Go above or below that by 10mph and it suffers.

Funnily enough though, the difference between 75 and 90mph appears much smaller on mine. Not that I ever drive at 90mph officer! But it really does cruise well at 90mph which has surprised me.

That's because you've just gone through the 88mph speed that induces the flux capacitor.

[capnbirdseye]

In terms of ideal speeds, I've found an indicated 60mph about right on mine for decent fuel consumption. Go above or below that by 10mph and it suffers.

Funnily enough though, the difference between 75 and 90mph appears much smaller on mine. Not that I ever drive at 90mph officer! But it really does cruise well at 90mph which has surprised me.

That's because you've just gone through the 88mph speed that induces the flux capacitor.

Say hello to Marty McFly for me

[johalareewi]

Not having cruise control I find I get the best mpg when the HSD is just below the PWR band. When the speed gets too much, I can switch to glide mode (HSD in the left hand section but not in CHG zone) until I am going too slow. My take on pulse and glide.

If I keep the HSD just to right of the half way line, and keep constant speed, the instant mpg reading is good but the overall mpg for the journey is lower vs the pulse and glide technique.

Must be something to do with what cyker said.

[sjrainsford]

In terms of ideal speeds, I've found an indicated 60mph about right on mine for decent fuel consumption. Go above or below that by 10mph and it suffers.

Funnily enough though, the difference between 75 and 90mph appears much smaller on mine. Not that I ever drive at 90mph officer! But it really does cruise well at 90mph which has surprised me.

That's because you've just gone through the 88mph speed that induces the flux capacitor.

Ah, that would explain why when I reach that speed I see some serious sh*t!

Hmmm... wonder if I could fit a Mr Fusion to the back?