I had started getting about 35 mpg, even though most of my driving is very short trips, very stop start, with frequent car park visits and a mind numbing amount of having to give way to oncoming vehicles on narrow roads.
Then I decided to check my tyres and I found that they were about 8-10 PSI too slow.
Ah, I thought, I'll pump 'em up and get even better fuel economy - so I reset the trip computer.
I've done about 80 miles since and I'm getting 25 mpg (albeit that this is all according to the computer).
Could inflating them have impacted the fuel economy?
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>> Could inflating them have impacted the fuel economy?
Increase in tyre diameter? Wouldn't have thought it would make that much difference though. (Even having trouble working out whether increase diameter on its own increases or decreases the mpg...)
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I wouldn't have thought it'd change to the tune of 10mpg.
Does the engine still pull ok? Sound different? Exhaust smell normal? If it's easy access maybe pull the air filter and see if it's manky or clogged up with leaves or something like that. Engine getting up to temp ok, not dropping off?
Tyre pressure on it's own might cause a couple of mpg but normally in the other direction - less sidewalk flex = larger diameter wheel = longer gearing effectively = less revs.
Engines not bogging down a lot wanting a lower gear?
Codes read might be worthwhile if it's a concerning difference. For the £15 for a basic scanner I reckon they're a worthwhile investment at that price.
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>> Does the engine still pull ok? Sound different? Exhaust smell normal? If it's easy access
>> maybe pull the air filter and see if it's manky or clogged up with leaves
>> or something like that. Engine getting up to temp ok, not dropping off?
>> Engines not bogging down a lot wanting a lower gear?
>>
>> Codes read might be worthwhile if it's a concerning difference. For the £15 for a
>> basic scanner I reckon they're a worthwhile investment at that price.
Nothing seems different and I am really only comparing the last 10 days or so, with the previous 10 days or so (I had reset the computer about 10 days before inflating the tyres).
It is due to go in for a service in a few weeks, so I can get them to look at it but, again, everything seems the same, apart for the tyre pressure.
At this rate, it'll be cheaper to sell the car and be carried around in a gold carriage drawn by unicorns :(
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Soft tyres would overstate the distance travelled, which is calculated from wheel revolutions at an assumed normal circumference. More m, same g, so more apparent mpg.
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Theres no' much gets by you ;-) Green thumb well earned.
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Yes, but nearly 30% difference? Perhaps 2%?
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Try using hydrogen gas - it's lighter than air.
hth.
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You're too kind, Sk. But it wouldn't be enough to explain the observed difference. I'd want to know what information, and what time or distance window, the computer is using for its calculations.
Last edited by: WillDeBeest on Wed 2 Feb 11 at 09:42
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My rather rudimentary computer gives range on fuel remaining and is always 429 when I brim the tank which suggests to me that it is not taking account of the mpg of my recent driving. I also get instantaneous mpg and average mpg from when I last reset the trip meter. All I can reasonably establish is that I get fewer mpg in Winter than the Summer.
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IIRC recent discussion on tyre pressure concluded that under-inflation has little effect on economy on modern, lower profile tyres.
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If the computer works by tank level measures and you fill the tank to the brim, it will undestate fule used due to the non recording of fuel in the filler neck.
But I cannot see that changing that much.
Of course, if the last driving has all been in very cold weather...
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The 2.0TDCi engine in the OP's car gulps fuel on short runs, so 25mpg is not unreasonable if it is literally going no more than a couple of miles at a time.
A common fault on these, which will cause high fuel consumption, is loose turbo intercooler pipes.
This fault also causes lack of turbo boost, but that is only apparent at 40/50mph and above, so the OP - who lives in London - may not have noticed.
Another clue is a spray of diesel over the engine.
The intercooler pipes sound complicated, but they are simple enough to check.
Around the engine there are a few pipes about the diameter of your forearm.
Check if the hose clips at the various joins are tight, I think there are around six.
You may need to yank the plastic dustbin lid-type cover off the top of the engine to gain access.
Several of the clips are easy to reach to tighten, but some are low down and are best done with a long extension.
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You said "I had started getting about 35 mpg".
Do you mean that was the consumption you got when you first aquired the car, or do you mean it used to do a different figure but had recently started doing 35?
If the latter, I'd suspect some error in the computer which has now corrected itself, and the tyre pressure change is a coincidence.
You could test the theory by dropping the pressure again and seeing what happens.
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>> You said "I had started getting about 35 mpg".
>>
>> Do you mean that was the consumption you got when you first aquired the car,
>> or do you mean it used to do a different figure but had recently started
>> doing 35?
>> If the latter, I'd suspect some error in the computer which has now corrected itself,
>> and the tyre pressure change is a coincidence.
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>> You could test the theory by dropping the pressure again and seeing what happens.
>>
Well, I hadn't reset the computer for a while (over 1,000 miles), and I can't remember what the mpg, but I had been doing a few longer trips, so probably not a good comparison.
So, yes, the 35 mpg was over those first 10 days or so, and the 25 mpg was over the next 10 days or so.
This morning I reset it again. I've only done 5 miles since, but that probably covers about 1 "lap" of my normal routes. It is currently reading 31 mpg.
The thing is, I set-up the display to so the current mpg, which I do keep an eye on, so perhaps that keeps my driving a bit more focused on efficiency.
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>> Try using hydrogen gas - it's lighter than air.
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>> hth.
>>
Inflated ego.
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Fuel economy - Focusless
