Fuel Effeciency

[Misho]

It is known that the fuel effeciency of car varies according to a big variety of factors. It is also safe to say, that most cars reach their maximum fuel effeciency at around 90-120 km/h. My question is, why is it that maximum fuel effeciency is reached at these speeds ?? Why would driving at 60km/h and 160km/h consume more fuel than driving at 110km/h ??

Another question is, assume you drive at a straight, level road for 5km at 60km/h. And then you drive under the exact same conditions, but this time at a 5% incline. Which would comsume more fuel, and more importantly why ??

[Niko_Fx]

Originally Posted by Misho

It is known that the fuel effeciency of car varies according to a big variety of factors. It is also safe to say, that most cars reach their maximum fuel effeciency at around 90-120 km/h. My question is, why is it that maximum fuel effeciency is reached at these speeds ?? Why would driving at 60km/h and 160km/h consume more fuel than driving at 110km/h ??

Another question is, assume you drive at a straight, level road for 5km at 60km/h. And then you drive under the exact same conditions, but this time at a 5% incline. Which would comsume more fuel, and more importantly why ??

To mantain 60km/h with a 5% inclination you have to keep the car a little bit more accelerated. If you don't, your speed will start to decline. With a 5% inclination you would consume more gas. Why? because your rpm's are higher.


About the 90-120 km/h I don't know, it sounds weird. Whenever I reach more than 65 km/h (40mp/h) I shift to 5th gear to spend little gas. By logic going at 65km/h in 5th gear I should spend less gas than at 90-120 km/h.

[Misho]

Originally Posted by Niko_Fx

About the 90-120 km/h I don't know, it sounds weird. Whenever I reach more than 65 km/h (40mp/h) I shift to 5th gear to spend little gas. By logic going at 65km/h in 5th gear I should spend less gas than at 90-120 km/h.

i never really saw anyone shifting into 5th at 65km/h !!!

dont forget that going 100km/h will take you less time than going at 65 would. so although the actual rate of consumption might be higher, the less time could overcome that and you end up using less gas at 100 than at 65.

[Egg Nog]

Theoretically, a car should get better distance-covered-per-amount-of-fuel efficiency the faster it goes. At least I think so, anyway. Not entireley sure. hell, you're the engineer! Anyway, the aerodynamics of the average vehicle above around 120km/h are absolutely horrendous, so they massively counteract the force of the tires on the road.

It also has a lot to do with gearing. Obviously you won't be in top gear if you're mulling around at 30km/h, and if you're not in top gear, you're not getting the best possible fuel efficiency. It brings up a few more questions... for example, if I were to drive my car in fifth at 60km/h for 1 flat kilometre, would I use just as much gas as the same distance at 80km/h?

I remember going for a two-hour drive with my buddy in an Infinity G20. Getting there, we cruised at about 90km/h the whole way... on the way back, we spent one long stretch getting all the way up to 190km/h, and in the process of those 2 or so minutes, used up more than we did on the entire way there!

[Misho]

logically speaking, the faster a car goes, the higher the rate of fuel comsumption. i can understand that, for the same route, going at a 100 will have a higher rate of consumption, but it will take much less time than going at 60 (which has a lesser rate of comsumption) so you end up using less fuel at 100 than you would at 60. but using that same logic, shouldn't 160 use even less fuel than 100 ? or would the rpm and gearing not make that possible ?

[Egg Nog]

Originally Posted by Misho

logically speaking, the faster a car goes, the higher the rate of fuel comsumption. i can understand that, for the same route, going at a 100 will have a higher rate of consumption, but it will take much less time than going at 60 (which has a lesser rate of comsumption) so you end up using less fuel at 100 than you would at 60. but using that same logic, shouldn't 160 use even less fuel than 100 ? or would the rpm and gearing not make that possible ?

You'd have to press more onto the accelerator to counteract the huge aerodynamic losses. This is where the increased consumption comes from.

[Misho]

Originally Posted by Egg Nog

I remember going for a two-hour drive with my buddy in an Infinity G20. Getting there, we cruised at about 90km/h the whole way... on the way back, we spent one long stretch getting all the way up to 190km/h, and in the process of those 2 or so minutes, used up more than we did on the entire way there!

are you sure ?? coz that mean that you used less fuel to cover 180 kms than u did for those 6 kms !!

[Misho]

Originally Posted by Egg Nog

You'd have to press more onto the accelerator to counteract the huge aerodynamic losses. This is where the increased consumption comes from.

isn't that also the case when comparing 60 to 100 km/h ?

[Niko_Fx]

Originally Posted by Misho

i never really saw anyone shifting into 5th at 65km/h !!!

dont forget that going 100km/h will take you less time than going at 65 would. so although the actual rate of consumption might be higher, the less time could overcome that and you end up using less gas at 100 than at 65.

The car doesn't accelerate AT ALL when it is in 5th at 65km/h I just do it when I have a long road in front of me and the speed limit it's about 70km/h and I don't feel like wasting gas. The rpm's are like 2000 at that speed, or less I would have to check.

The gas is going UP UP UP here in the states

I miss that about Vzla a lot, you could fill your SUV's empty tank with about 3 US Dollars or less.

[fpv_gtho]

this whole issues been prought up recently hear because the governments been going crazy lowering all the speed limits, like it used to be 60km/h in suburbia but now its 50 etc. research has come forward saying the average car is most efficient between 70-90km/h based on thats when cars are most efficient using top gear. with a high percentage of people now buying auto's, top gear's being used as much as possible.

[eyebrows]

ok one thing... ur going at 60km/h in 5th ur using less fuel but ur also covering less ground and at 160km/h ur using more fuel but u r also covering more ground so shouldn't they use about the same amount of fuel for distance? and i have no point im lost im tired goodnight...

[Matra et Alpine]

Originally Posted by eyebrows

ok one thing... ur going at 60km/h in 5th ur using less fuel but ur also covering less ground and at 160km/h ur using more fuel but u r also covering more ground so shouldn't they use about the same amount of fuel for distance? and i have no point im lost im tired goodnight...

It's been mentioned numerous times, so this repeat I'll make it bold ...

AERODYNAMICS

[Niko_Fx]

Originally Posted by eyebrows

ok one thing... ur going at 60km/h in 5th ur using less fuel but ur also covering less ground and at 160km/h ur using more fuel but u r also covering more ground so shouldn't they use about the same amount of fuel for distance? and i have no point im lost im tired goodnight...

I'd love to go at 160km/h. I would arrive anywhere pretty quick, but here there is something called "Speed limit" If I have to go slow I might as well spend the less gas possible.

And well, if you are going at 160km/h your rpm's are going to be let's say 5000 but if you go at 70km/h their going to be 1000-2000. It's all relative..

I don't know, I guess I'm just discussing here without knowing. I'm going to back off.

[NAZCA C2]

If you are going up a hill (even a slight one) your car will use more fuel because it has to do more work. The car can't overcome gravity.

[Misho]

Originally Posted by NAZCA C2

If you are going up a hill (even a slight one) your car will use more fuel because it has to do more work. The car can't overcome gravity.

is that regardless of gearing ? lets say i go up the hill doing 60 on second gear with 3000 rpm. and then i go up the same hill again doing 60 but this time on third gear with 5000 rpm.
would the first case use less fuel because of the lower rpms ?