Vauxhall corsa 1ltr- shocking MPG!

darich

newfoundglory wrote: »

So general rule to improve efficiency.... stay in 3rd when doing 30mph?

Depends on the car and conditions.
my car is fine in 5th at 30mph even though it's quite big because it has tons of power and torque. Going up hill though I'll usually need to change down.
In a smaller car more designed for economy eg 1.0L three cylinder corsa 3rd gear gets you closer to where the power is on the rfev counter. Because you're closer to the power you're less likely to need to change down a a gear when you climb a hill.

Every car is different and it takes a bit of experience to know which is the best gear for your car and the road.

cyclonebri1

darich wrote: »

Depends on the car and conditions.
my car is fine in 5th at 30mph even though it's quite big because it has tons of power and torque. Going up hill though I'll usually need to change down.
In a smaller car more designed for economy eg 1.0L three cylinder corsa 3rd gear gets you closer to where the power is on the rfev counter. Because you're closer to the power you're less likely to need to change down a a gear when you climb a hill.

Every car is different and it takes a bit of experience to know which is the best gear for your car and the road.

Methinks the biggest issue the op has is that she is driving around in 3rd gear all the time, that is nor even going to come close to the lowest fuel consumption guideline, ie, "under urban"? if youn see what i mean.

Hammyman

cyclonebri1 wrote: »

Methinks the biggest issue the op has is that she is driving around in 3rd gear all the time, that is nor even going to come close to the lowest fuel consumption guideline, ie, "under urban"? if youn see what i mean.

Rubbish. I do that and get >55MPG out of my Ford Mondeo. Perhaps you'd like to tell me why I exceed the manufacturers specs? The best I've had is 70MPG. That was travelling from YO25 to Hull Docks and back with the return journey being through rush hour.

Obviously you're completely unaware of ENGINE LOAD. An engine will be under a far higher load and have to work harder when trying to increase speed in 5th gear at 30MPH than in 3rd gear. What would require just a light press of the throttle in 3rd would need foot hard down in 5th. Guess which chucks more fuel into an engine? If you can't, an engine doing 3000 RPM with 10% throttle will use far less than one doing 100RPM with 100% throttle.

Revs are only a small part of the story, engine load is a massive part. The only time a higher gear is more economical than a lower one is if the change in engine load is negligble, such as driving on a fairly flat motorway at a constant speed.

i am so confident that driving in 3rd around town at normal speeds is more economical than fifth that I am happy to bet £100, drive the car of any of those pottering around in 5th and prove it.

In addition, 3rd gear gives you far more flexibility and allows you to be able to react to situations better where additional braking or acceleration would be required.

tomstickland

If you look at fuel consumption contour plots versus rpm and engine load then the peak mpg occurs roughtly in the same rev range for all loads. Somewhere around 2000-2500 rpm iirc.

Labouring an engine in 5th gear at 30mph is inefficient. It's alright if the car is just rolling along do virtually no work.

Back to the OP.

1) Measure the mpg using the brim method for several tanks.
2)If it really is only managing 25mpg then there's something wrong with it.

cyclonebri1

Hammyman wrote: »

Rubbish. I do that and get >55MPG out of my Ford Mondeo. Perhaps you'd like to tell me why I exceed the manufacturers specs? The best I've had is 70MPG. That was travelling from YO25 to Hull Docks and back with the return journey being through rush hour.

Obviously you're completely unaware of ENGINE LOAD. An engine will be under a far higher load and have to work harder when trying to increase speed in 5th gear at 30MPH than in 3rd gear. What would require just a light press of the throttle in 3rd would need foot hard down in 5th. Guess which chucks more fuel into an engine? If you can't, an engine doing 3000 RPM with 10% throttle will use far less than one doing 100RPM with 100% throttle.

Revs are only a small part of the story, engine load is a massive part. The only time a higher gear is more economical than a lower one is if the change in engine load is negligble, such as driving on a fairly flat motorway at a constant speed.

i am so confident that driving in 3rd around town at normal speeds is more economical than fifth that I am happy to bet £100, drive the car of any of those pottering around in 5th and prove it.

In addition, 3rd gear gives you far more flexibility and allows you to be able to react to situations better where additional braking or acceleration would be required.

Oh my God, Conors back, :rotfl::rotfl::rotfl::rotfl::rotfl:His Mondeo would go round the world and back and he'd have to take let the excess fuel out of the tank on his return.

Never read such a load of tripe in my life. Hammy I love you. 70mpg from a Mondeo under urban conditions, bet you were on the back of a tow truck, it being a Ford and all?

You seem to be unaware that if you are driving around in town in 3rd gear you are cruising, so cruise in a higher gear and save fuel.

If you want to post advice base it on what is actually achievable, to put it into context the OP can't get 25mpg from a Corsa, you can get 70 from a Mondeo :D:D, made my day.

cyclonebri1

tomstickland wrote: »

If you look at fuel consumption contour plots versus rpm and engine load then the peak mpg occurs roughtly in the same rev range for all loads. Somewhere around 2000-2500 rpm iirc.

Labouring an engine in 5th gear at 30mph is inefficient. It's alright if the car is just rolling along do virtually no work.

Back to the OP.

1) Measure the mpg using the brim method for several tanks.
2)If it really is only managing 25mpg then there's something wrong with it.

Tom, (are you the same TS off Migweb?) you are confusing fuel usage and mileage.

At the same engine load in 1st, 2nd 3rd etc etc the fuel consumpstion is roughly the same, forgetting wind resistance, rolling resistanace, tyre rolling radius and so on, the distance covered in the time the engine uses this amount of fuel is proportional to the loading.

To the other bit I highlighted, in town at 30 the engine is doing little work whether in 3rd or even 5th, go for the cheaper option.

HM in an earlier post suggested the lower gear is better to keep in control of the car, goos advice for the novice/new driver, but I have brakes and steering to take care of that, don't see the need for rapid acceleration in town to get me out of trouble, of course I just happen to have a gearbox as well.

Hammyman

cyclonebri1 wrote: »

Oh my God, Conors back,

Who?:huh::huh::huh::huh:

tomstickland

cyclonebri1 wrote: »

Tom, (are you the same TS off Migweb?) you are confusing fuel usage and mileage.

I'm not confusing anything, but your post is confusing me.

At the same engine load in 1st, 2nd 3rd etc etc the fuel consumpstion is roughly the same,

Take a look at a bsfc (brake specifice fuel consumption) contour plot and you'll find that for all loads the most efficient engine rpm is roughly the same.

forgetting wind resistance, rolling resistanace, tyre rolling radius and so on, the distance covered in the time the engine uses this amount of fuel is proportional to the loading.

The distance covered is proportional to the road speed of the vehicle. That is related to the loading, yes. You can select any of the gears and, going up the gears, the engine speed will decrease and the loading will increase for each successive gear.

The most efficient gear will be the one that gives a point on the contour plot that's the best. In most cases this will occur when the engine rpm is in the range 1500-2500rpm ish.

cyclonebri1

tomstickland wrote: »

Take a look at a bsfc (brake specifice fuel consumption) contour plot and you'll find that for all loads the most efficient engine rpm is roughly the same.


.

I'm not sure if that is correct, it probably is but you cannot rule speed, ie, distance travelled in a given time out of the equation. I am sure that on a flat road you get better fuel consumption at 2000rpm (or whatever figure you choose to use), in 6th gear than in 1st. If the op uses 3rd gear as a norm because there is no peformance available should she come to a hill, then she will get poor fuel consumption. It is, as I think we all agree, about being in the correct gear for the driving conditions. To gain max fuel economy it is sometimes beneficial to be in the next gear up and change down when neccessary. Maybe not highway cade stuff but we are talking economy.

Easy enough to prove if you have instantaneous fuel consumption facility on the cars computer, try it if in any doubt. :A

tomstickland

What it means is that the engine rpm should be contained to a fairly narrow range regardless of the load and road speed.
So if 30mph in 5th gear means 900 rpm then that's not enough rpm, and a gear that puts the rpm into the required zone should be selected.
2nd gear and 30mph might require 4000rpm and that's too high, so a higher gear should be selected.