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I've just done a run up to Liverpool and back home to Cardiff and noticed a huge discrepancy in mpg for the two legs.

On the way up, started with a full tank and used just over a quarter of a tank, so roughly 38 mpg. On the way back, same tank of fuel and average driving speed, the car drained the tank, so I estimate roughly 22 mpg.

Has anyone got any thoughts as to why the mpg should be so different for the same journey?
 

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I find it makes a big difference how busy the motorway is. When it busy I get better fuel economy which I put down to the lower driving speeds and that fact that I am part of a column of air all moving in the same direction...sort of slip streaming. I was stuck on a bit of country road about a month ago at 55mph in a que of cars that just wasn't worth getting past. The comp' had me averaging 43mpg (I know the computer is horribly inaccurate, but I was just using it as a comparison to my normal 32mpg on the same journey)
Kev

Kev
 

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Welcome to Saabscene RichS

As Kevster explained a lot can depend on traffic, driving style, air con and to a lesser extent weather conditions.

On my 9000 I see a quite a difference between cold and hot days - (44 down to 38pmh) and even more when the AC is on.
 

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What did the computer say??

I never trust the fuel gauge, whenever I drive along way the fuel gauge barely moves but come out to the car next day and shes well down, I should have thought an average of at least 30+mpg is easy on a motorway, I get 26mpg just bumbling about in my 2.3T.

Cruise control is a killer aswell, it just seems to boost when it needs it, although good fun watching the little bar going up and down!!

Dulux
 

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Welcome to Saabscene, RichS


I concur with the variability of "apparent" readings. In particular, the linearity of the fuel gauge is rather poor. So, alhtough it might have appeared that you were 3/4 full, after the first leg, I'd place money on the fact that in reality you'd used nearly half a tank. Assume you got about the same again, and that's another half tank or so. If you do a crude average, then you got 30mpg, which might not be too far from the truth. The only sure way is to do a brim to brim measurement. Although consistent, the EDU readout is not necessarily accurate. It can be re-calibrated though.

Another little quirk to be aware of... the 9000 fuel gauge is optimistic when going uphill, and pessimistic going down.
 

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I can totally agree with the others, it appears that I can get 200miles from half a tank from full according to the gauge, however the second half never manages more than about 130miles !

I averaged 33mpg on a 326mile trip to the lakes, speed was around 75mph on the motorway, with very little use of full throttle
 

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Just a snippet on fuel gauge calibration (not that it affects mpg though).
Car designers (at least those at my company) deliberately tune the gauge reading away from linearity, so don't be fooled by the fuel gauge readings.
At the top end the needle stays "full" for ages, to fool you into thinking that you're getting good economy. Then it speeds up and goes to "empty" well before you are really empty, to stop scrooges like me from running out before the petrol station.
Empty on the gauge typically means you've got something like 10 litres left.
 

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Originally posted by Dave225
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Empty on the gauge typically means you've got something like 10 litres left.
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Ssshhh, I told the wife its running on fumes down there & 'empty' is where I always leave it when I know she wants to borrow it for shopping (her Polo boot is tiny). The 6 mile round trip into Whitby = £20 in my tank because she's too embarrassed to put any less in

Out of interest, what's left at the point where the fuel warning light first comes on?
 

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Well, there's no hard and fast rule (and it's a long time since I worked with fuel systems). The general theory was that if you had the needle reading correctly, people would cut it too fine and get stranded. So, they had to build in a safety factor. If (and I stress IF)I remember rightly, when the light came on you had about 30km left.

So 10 litres is about 2 gallons - about 60 miles? I'm not so sure about that 10 litres now!! I did say it was some time ago!
 

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RichS's findings are a bit extreme so perhaps traffic density and driving smoothness played a part but ...

The effort needed for the car to displace air is proportional to the cube of the apparent wind speed. For a fixed throttle setting giving 70mph in still air this means that -
with a 15mph tail wind the car should go significantly faster than 85mph
with a 15mph nose wind it should go significantly less than 55mph
(ignoring other friction and drag factors)

The quality of the air plays a part too - humid high pressure air is denser and needs more effort to displace (as sailors of nimble dinghys will have noticed).

I've certainly noticed that my 9000 flys more gracefully downwind.
 

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When the light comes on the amount of fuel remaining is "approximately 2 imperial gallons" (to quote from the manual, if memory serves me correctly) so yes, about 60 miles assuming an average mpg of somewhere around 30.

cheers,

Ben.
 

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fuel remaining is approximately 2 imperial gallons (to quote from the manual)
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oops, thanks Ben - I'll have to re-read it & see what else I missed
 

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Not exactly scientific but when the red low fuel light came on I filled up (to brim) and set the trip.

Put in 56.48 litres or 12.55 gallons.

Checked again when the red low fuel light came on. Got 461 miles from one tank on a mixture of town and country driving.

Average works out at 36.7 mpg. I'm happy
 

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Originally posted by Mark E:
[qb]The gear ratios for that box were originally specified for a 2l NA engine. [/qb][/b]
No. The 2.0i has an even lower final drive than the 2.0LPT, which in turn is indeed lower than that of the 2.3T.
 

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Originally posted by Naranto:
[qb]Not exactly scientific but when the red low fuel light came on I filled up (to brim) and set the trip.

Put in 56.48 litres or 12.55 gallons.

[/qb][/b]
Which equates quite nicely to the "approx 2 imperial gallons remaining" given the tank is 65 litres (or is it 66? I can't remember) and 2 gallons is close to 10 litres.
 

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The book says: 8.5 l/100 km (about 33 mi/imp. gallon) on a highway/road mix and 14 l/100 km (about 20-21 mi/imp. gallon) in town. This is about what I get on a '92 9000 CS non-turbo, non-DI. My computer is optimistic by about 2 mpg (1 l/100 km). The best way to figure out what it really consumes is:
- fill up to the brim
- reset trip
- after 400-600 km (250-375 mi) refill to the brim
- take your receipt (stating quantity as well as cost)
- calculate actual consumption (divide miles by gallons to get miles per gallon or divide litres by km/100, e.g. if you have driven 550 km, you divide the number of litres by 5.5 to get litres per 100 km).
I do not think 36 mpg is achievable on a car this heavy (unless you are cruising at about 90-100 kmh (55-60 mph) on a relatively straight highway, which is no fun at all).
 

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Originally posted by Ludmil:
[qb]I do not think 36 mpg is achievable on a car this heavy (unless you are cruising at about 90-100 kmh (55-60 mph) on a relatively straight highway, which is no fun at all). [/qb][/b]
Welcome Ludmil

I have had an indicated current consumption of 37 mpg in my 9000 Aero (manual) with cruise control on. Unfortunately here in the UK, our main roads are that crowded that 60-75 mph cruising is all too often the norm.

But on the back roads - 25-29 mpg is more common
 
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