FWIW, I don't believe for a minute that either petrol engine will do anywhere near 37mpg at an actual road speed of 90mph.

pearce_jj said:

FWIW, I don't believe for a minute that either petrol engine will do anywhere near 37mpg at an actual road speed of 90mph.

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Not to be funny, but we spent most of our time cruising along at 85 and got 47 out of our 500

Whenever i do a long journey on motorways sitting between 75 and 85mph my mpg always goes up to around 42. Thats even after half a tank around town at only 35-37mpg. So higher speeds must be more economical.

The best ive had is 44 but thats a very lucky 50 mile round trip at 75-85mph from a full tank. But the next day around town at 25-35mph brought it back down to 39mpg.

mjsmke said:

Whenever i do a long journey on motorways sitting between 75 and 85mph my mpg always goes up to around 42. Thats even after half a tank around town at only 35-37mpg. So higher speeds must be more economical.

The best ive had is 44 but thats a very lucky 50 mile round trip at 75-85mph from a full tank. But the next day around town at 25-35mph brought it back down to 39mpg.

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:facepalm: Higher speeds are NOT more economical. The reason why you get better economy on the motorway is that you can drive along and never touch your brakes, the brakes take the kinetic energy that the car has and converts it to heat which is wasted. Drive slower on the motorway (ie 55-60mph) and you will see a significant increase in your economy.

I've started a new temp job this week with a commute of 40 miles each way and keeping to a steady 55-60 the car has done an indicated 57mpg over 420 miles because I've kept my speed down and there's very little braking to be done. On my commute last year which was 20 miles each way and the only time I got that was with narrower winter tyres. This is partly down to the fact that you car probably takes 10 miles to get properly up to temperature (never mind the temperature gauge) and I was only doing 10 miles with the engine up to temp and now I'm doing 30 miles with it up to temp plus I don't do as much braking now.

mjsmke said:

Whenever i do a long journey on motorways sitting between 75 and 85mph my mpg always goes up to around 42. Thats even after half a tank around town at only 35-37mpg. So higher speeds must be more economical.

The best ive had is 44 but thats a very lucky 50 mile round trip at 75-85mph from a full tank. But the next day around town at 25-35mph brought it back down to 39mpg.

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I can identify with what Mike is saying here. Given the thirst of the 1.4 it seems happy at higher speeds with it coming on cam - it's like its 'sweet spot'. On one of those trips that I posted when I averaged 40mpg it was 37mpg at the mid way point and the return journey was 43mpg at a more sensible pace. I know what the speedo was reading when I was getting 37mpg.

306maxi said:

:facepalm: Higher speeds are NOT more economical. The reason why you get better economy on the motorway is that you can drive along and never touch your brakes, the brakes take the kinetic energy that the car has and converts it to heat which is wasted. Drive slower on the motorway (ie 55-60mph) and you will see a significant increase in your economy.

I've started a new temp job this week with a commute of 40 miles each way and keeping to a steady 55-60 the car has done an indicated 57mpg over 420 miles because I've kept my speed down and there's very little braking to be done. On my commute last year which was 20 miles each way and the only time I got that was with narrower winter tyres. This is partly down to the fact that you car probably takes 10 miles to get properly up to temperature (never mind the temperature gauge) and I was only doing 10 miles with the engine up to temp and now I'm doing 30 miles with it up to temp plus I don't do as much braking now.

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I meant driving at 75-85mph is more economical than driving at 25-35mph. (in the 1.4 anyway)

mjsmke said:

I meant driving at 75-85mph is more economical than driving at 25-35mph. (in the 1.4 anyway)

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That is simply not true. How often do you travel 25-35mph for long distances without braking as you do on a motorway? Next time you're driving in your car turn the instant consumption on and try travelling along in different gears at 3k rpm on a flat surface and you'll see that the higher the speed the lower the fuel economy.

306maxi said:

That is simply not true. How often do you travel 25-35mph for long distances without braking as you do on a motorway? Next time you're driving in your car turn the instant consumption on and try travelling along in different gears at 3k rpm on a flat surface and you'll see that the higher the speed the lower the fuel economy.

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Theres an 8 mile road here, just under half is 30mph and the other is 50mph. The 50 mph stretch is always more economical for me. Both sections have roundabouts and both are flat.

I know its a very short distance but as 'loveshandbags' said, there seems to be a sweet spot at higher speeds with this engine.

mjsmke said:

I meant driving at 75-85mph is more economical than driving at 25-35mph. (in the 1.4 anyway)

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My interpretation is the following...

25-35mph. Humping around the locality. Stop start. You're doing well to get 35mpg in a Euro 4.
75-85mph. Averaging around 40-41mpg.
85-90 (Speedo). 37mpg

When I thought about Pearce's input that a petrol 1.4 couldn't do 37mpg at a 90 he was probably right. The speedo 'is out more' at the higher speeds. So if the Speedo read 90 it was probably 85.

If you look at some of the threads on the Panda 100HP most of them were getting around the same mpg and the Panda's drag co-efficient is 0.40

cd is not everything though, cd times by frontal area is what you want to know for a total drag figure, and the 500 is wider in terms of track and therefore probably has more frontal area which will even things out.

mjsmke said:

Theres an 8 mile road here, just under half is 30mph and the other is 50mph. The 50 mph stretch is always more economical for me. Both sections have roundabouts and both are flat.

I know its a very short distance but as 'loveshandbags' said, there seems to be a sweet spot at higher speeds with this engine.

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Please listen!!!!!!

Go do the test tomorrow. On the 50mph stretch do the same revs in 3 different gears and check the instant consumption. Tell me which one shows the best fuel consumption. There are any number of reasons why your car could be more economical on the 50mph stretch, less traffic, less braking, warmer engine etc etc etc. Like I said, do the test in 3 different gears at the SAME revs and have the instant consumption showing and you will see that as a rule, the faster you're going the more fuel you're be burning because of increased drag.

306maxi said:

cd is not everything though, cd times by frontal area is what you want to know for a total drag figure, and the 500 is wider in terms of track and therefore probably has more frontal area which will even things out.

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I have the following figures but no frontal area for the 500 so I cann't dispute your deduction.
http://www.mayfco.com/dragcd~1.htm
Panda is quoted as 0.40-0.42 Frontal area of 1.70sq.m whilst the
500 is 0.325

Even if I had the figure I might need a bit of help with the maths.

http://www.carinf.com/en/99e0418474.html

Drag coefficient (Cd/Cx/Cw)
Quantifies the resistance (drag) of the vehicle, while moving through the air. Contemporary automobiles achieve a drag coefficient from 0.30 to 0.35. Cd is also known as Cx in France and Cw in Germany

Drag area (CdA)
Expresses the aerodynamic efficiency of the vehicle and is measured by multiplying the drag coefficient (Cd) and the frontal surface area (A). The lower the drag area is the more efficient aerodynamically the vehicle is.


EDIT: Using the height and width as way of calculating Frontal area these are the figures that I have ended up with...
http://www.parkers.co.uk/cars/specs/Detail.aspx?deriv=49867
Panda 1.2
Width 1578
Height 1540
Frontal area = 2.43sq.m.

http://www.parkers.co.uk/cars/specs/Detail.aspx?deriv=39086
Fiat 500 1.2
Width 1627
Height 1488
Frontal area = 2.42sq.m.

co-efficient x frontal area = drag

Panda = 0.40 x 2.43 = 0.972
Fiat 500 = 0.325 x 2.42 = 0.7865

http://blogs.insideline.com/roadtests/2011/06/2012-fiat-500-thoughts-on-fuel-economy.html
64.1in -> 1628
59.8in -> 1519 US 2012 (31mm extra off the ground – extra suspension travel ?)

Fiat 500 US = 0.325 x 2.47 = 0.8027

Not size, but frontal size.
-----------------------500------Fiesta
Length (in.)-------139.6-----153.3
Width (in.)----------64.1-----66.7
Height (in.)---------59.8-----58.1

mjsmke said:

Theres an 8 mile road here, just under half is 30mph and the other is 50mph. The 50 mph stretch is always more economical for me. Both sections have roundabouts and both are flat.

I know its a very short distance but as 'loveshandbags' said, there seems to be a sweet spot at higher speeds with this engine.

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Just to clarify the sweet spot. This is when the car is coming on cam and and it seems to be going at its own momentum. It will still be using up more fuel at the higher speed but the difference in mpg loss isn't that much. I hope I haven't contradicted myself here.

Hopefully someone else with a 1.4 might come online with better English than me.

@Maxi, a bit sweeping statement there - best economy always in the highest gear at lowest revs, declining as you say with speed. In other words 30mph in 5th (in the MJ) goes furthest.

pearce_jj said:

@Maxi, a bit sweeping statement there - best economy always in the highest gear at lowest revs, declining as you say with speed. In other words 30mph in 5th (in the MJ) goes furthest.

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That's what I'm saying 30mph in 5th will be more economical than 60mph in 5th

Ah yes ok, well it is late.

Anyway the point is drag in every sense increases with the square of speed. This is offset with petrol engines to a degree because the engine operates at peak efficiency only at full throttle (at slight openings there is very little compression).

pearce_jj said:

Anyway the point is drag in every sense increases with the square of speed. This is offset with petrol engines to a degree because the engine operates at peak efficiency only at full throttle (at slight openings there is very little compression).

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Looking at the peak efficiency for the 1.4 petrol this would be at 4,250rpm - its torque peak - this is its lowest fuel consumption/unit power.
http://www.viragotech.com/fixit/FuelEconomyEngineEfficiencyPower.html

From a previous thread I plugged in the gear ratios for the 1.4 and 1.2 in a spreadsheet provided by Knitrip plotting the graphs. (note: I used the same Final drive ratio on the 1.2 from the 1.4).

I have done a very rough plot of the fuel consumption figures for the 1.4 using Jnoiles 25mpg at 110mph from the above thread as a 'marker'.

It can also be seen that at a true 90mpg the 1.4 is at it's peak torque with a 'drag figure' of 0.7865 and it's quite possible that you could get 37mpg at that speed.

As you approach the maximum speed of a 1.2 - the 1.4 could be more economical at only 4,500rpm.

Re the subject of 'sweet spots' - when I googled '16 valve sweet spot' I got back the following ...
http://rideanddrive.co.za/reviews/driven/new-fiat-500c-in-500-words/
The six speed box spreads the power evenly and assists the motor by holding it in the sweet spot.

P.S.
http://www.autocar.co.uk/CarReviews/FirstDrives/Fiat-500-0.9-Twin-Air/251295/
Twin Air - At an even 90mph (measured by sat-nav) the engine was pulling just 4000rpm in fifth