Print

Please login or register.
1 Hour 1 Day 1 Week 1 Month Forever
Login with username, password and session length

[UrbanFox]

getting around 25-28mpg from my 3.0 at 60mph - this seems very low to me?  Any ideas what could be causing it?

[TheBoy]

Auto or manual?

[UrbanFox]

Auto.

My last 3.0 Auto would do low to mid 30's...

[05omegav6]

I would suggest that no 2 cars are the same. I had an auto 2 litre Granada Scorpio that struggled to do more than 27 mpg. Replaced it with a Granada Cosworth auto that managed 31mpg in spite of being caned. Which is about what I got from a manual 2.8i mk2 Granada. :-/

Could perhaps try full load tyre pressures and make sure that all the levels are spot on. A good service can also make a difference on some cars.

[tunnie]

Sounds about right for an auto. 60mph is wrong speed really, I find mine is most economical 70-80mph.

[Darth Loo-knee]

Is the timing set properly??
Mrs Loo-knees 3.0 estate sitting at 75mph did 34.7

[Kneepad]

Sounds about right for an auto. 60mph is wrong speed really, I find mine is most economical 70-80mph.

I am quite sure that tunnie is in no way suggesting that should break the law.   

[geoffr70]

Is the timing set properly??
Mrs Loo-knees 3.0 estate sitting at 75mph did 34.7

Yeah me aswell, I manage this on runs at 70-80mph!

[Omega_Dan]

Whats the condition of the air filter element? could be knocking off a few mpg by it being overly dirty/clogged.

[Kevin Wood]

Sounds about right for an auto. 60mph is wrong speed really, I find mine is most economical 70-80mph.

Not sure I'd agree with that, on both counts.

From comparing notes with other members there appears to be the consensus that the 3.0 auto is slightly better on fuel than the 3.2 auto. Having said that, my 3.2, on petrol, will return mid-30's (36 MPG on the MID, about 35 estimated at the pumps) at a constant 60 MPH. I mean constant, though. Over a 100 mile motorway run at 60 on cruise control, looking ahead and changing lanes to avoid having to change speed.

I find fuel economy steadily declines with higher speed. At 80MPH mine is probably hovering around 30MPG. Best economy probably occurs about as slow as you can go but remain in top gear with the TC locked up.

This is exactly as we'd expect. The main factor at motorway speeds is overcoming aerodynamic drag (proportional to speed cubed ) with a small contribution from rolling resistance (proportional to speed), so the difference between 60 MPH and 80 MPH, in terms of power required from the engine (and hence fuel consumed by it), is significant.

Assuming all our losses are aerodynamic the 33% increase in speed from 60 to 80 MPH requires an increase in engine output of 1.33³ = 2.37 times as much power.

Unless the engine is operating at an extremely inefficient speed at 60MPH it can't fail to be more economical.

Kevin

[tunnie]

when i was doing 500+ miles a week i did experiment with cruise at different settings, over my 75 mile run from London to Midlands i honestly found cruise set at just below 80 actually gave me more mpg than 60mph.

As the M40 has quite a lot of hills, i found at 60 it just lacked momentum to get up the incline, i could really feel the cruise opening the throttle up more to keep it at 60. Where as at 80 it needed less adjustment

[Darth Loo-knee]

Kev you sound like a mad scientist with your pye squared minus the theory of relativity. . .   


I think that cruise control uses more fuel. . . Also at 75 mph I seem to get better economy. . .  But that is by the MID   :-?

[Kevin Wood]

when i was doing 500+ miles a week i did experiment with cruise at different settings, over my 75 mile run from London to Midlands i honestly found cruise set at just below 80 actually gave me more mpg than 60mph.

As the M40 has quite a lot of hills, i found at 60 it just lacked momentum to get up the incline, i could really feel the cruise opening the throttle up more to keep it at 60. Where as at 80 it needed less adjustment

How many revs are you doing at 60? I can only assume that your engine is not that efficient at that speed. IIRC the 4 pots are quite peaky so that might well be the case.

Kevin

[feeutfo]

Who on EARTH sits at 60 on a motorway? I'd nod off.

[Kevin Wood]

Kev you sound like a mad scientist with your pye squared minus the theory of relativity. . .   

 

I think that cruise control uses more fuel. . . Also at 75 mph I seem to get better economy. . .  But that is by the MID   :-?

The issue with cruise control is that it can't anticipate hills, etc, so will pile on a load more power, kick down, etc. when it gets caught out by a hill whereas the driver would probably avoid this.

At the end of the day it's the same engine and car, etc. just something else propping the throttle open so there shouldn't be a lot of difference. I find my foot progressively gets heavier if I don't use cruise, though.

Kevin