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Discussion Starter #1
Evening all

Last week my 9g civic 1.6d was 3 years old and so needed MOT. Also needed new front tyres, original Michelin Energy Savers having done 41,000ish miles (a record for me!).

Last week, averaging 64-66mpg (I-mid stats.), Friday swapped Michelin Energy savers for Goodyear Eff.Grip + MOT. This week, over 250 miles similar running I am struggling to average 57mpg. Can't see why MOT should change things so must be tyres - both makes are B rated on efficiency and the Goodyear's do seem quieter and feel a better ride.

Any thoughts or comments on this significant mpg reduction, should I have realised from my research?

Dave D
 

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Did you test in a lab, under exactly the same test conditions?

Same tyre pressures?

I think you'll need a longer test and more data before you can nail it down to just the tyres.

Although swapping from an Energy style tyre to a Grip orientated tyre could well be a likely cause.

At this point tho, I wouldn't rule out changes in traffic, weather, driving style, etc.

Instead of trying for max mpg, try to see how low you can get it. That'll be much more fun :grin2:
 

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I Hate tailgaters!
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You should always do a manual check for true mpg rather than trust the computers data. have you checked what air pressure they put into your new tyres, I find they seem to over inflate leaving me to correct.
May seem a silly suggestion but they did fit same size tyres?
 

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The difference in efficiency ratings is +- 7.5% from A to G from what I've found, so unless your wheels are made of glue you shouldn't experience such a large difference; 65 to 57 MPG is 13% :- Scientifically speaking, too much!

I imagine the new tyre reduction may factor into it, but other than that, It must be outside factors.

Is it possible that efficiency has been reduced by the recent weather? MPG Enthusiasts report an impact of ~10% due to wet weather, and it has been a bit worse recently; But to reiterate what Farqui was saying, there is a massive number of variables.
 

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Discussion Starter #6
Thanks for the comments guys

I appreciate there are a vast number of variables to consider but I, a spreadsheet geek competent in the art of Root Cause Analysis, have lived with (and liked) this car for 3 years, there is definitely something peculiar going on. I notice now that when I lift off the throttle on the same slight decline out of our village, the fuel consumption bar is more sluggish getting to spot reading of 100, it's a bit like what happens when you get a regen. but there is no extra heat.

I have checked for correct size and pressure, both ok.

I do regular runs from Exeter to Stafford and Exeter to Taunton, and usually get 68-70, best I've seen is 75 on a slowish run due to traffic and road works. I agree, wet (and windy) weather does make a difference but not as much as I'm seeing. Also noticed, even on a nice winter day, if temp is below about 10C you also see a noticeable difference.

The one thing I had forgotten was that the day before tyres and MOT I was forced to fill up with diesel at a supermarket, something I've only done twice before in 41,000miles. I wonder if this might have some impact. Also spoke to dealer on the off chance they had changed some setting during MOT, he said no changes but the car is left running for an HOUR during MOT, which I thought a bit odd. He did suggest the fuel might be a factor but said if it did not improve after refuelling I should take it in a check on their computer.

I have a final trick up sleeve - the rears are original Michelin Energy Savers with 3/4mm left, if all else fails I will swap to front to see if this makes a difference.

Thank you, Dave D
 

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If I use supermarket fuel in my 4x4 the mpg drops by appox 10% :(
 

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If I use supermarket fuel in my 4x4 the mpg drops by appox 10% :(
I never use supermarket fuel, it may be cheap, but there is a reason for that. I always use Vpower which I know some feel is overkill, but I want the best I can get for my car and have found over the years all the cars I have owned do run better on premium fuel whatever yhey are
 

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Although there are other variables I could never get any improvement when I back to back tested Asda vs Esso. Maybe there isn't enough of a difference between the fuels, would I need to go up another level to shell for example?
To be honest, any gain in MPG is negated by the fact that Asda has significantly cheaper fuel (I fill up at work in a different part fo the country to my home) and I dont run the derv as a performance vehicle :)
 

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I never use supermarket fuel, it may be cheap, but there is a reason for that. I always use Vpower which I know some feel is overkill, but I want the best I can get for my car and have found over the years all the cars I have owned do run better on premium fuel whatever yhey are
As a rule I never use supermarket fuel. Just done it a couple of times when caught short. :)
 

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Although there are other variables I could never get any improvement when I back to back tested Asda vs Esso. Maybe there isn't enough of a difference between the fuels, would I need to go up another level to shell for example?
To be honest, any gain in MPG is negated by the fact that Asda has significantly cheaper fuel (I fill up at work in a different part fo the country to my home) and I dont run the derv as a performance vehicle :)

I think a lot depends on the Vehicle. Both my Suzuki 4x4 and my Civic run fine on cheap fuel but do return lower mpg compared with my normal choice BP.
My old Mitsi Shogun was much more sensitive. Put supermarket fuel in it and the starting was poor, down on power and the mpg took a huge hit. :(
 

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Also spoke to dealer on the off chance they had changed some setting during MOT, he said no changes but the car is left running for an HOUR during MOT, which I thought a bit odd.
Surely that is you problem... An hour with the engine running burning fuel but going nowhere will soon knacker your mpg as the hour will count towards the average!!
 

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Discussion Starter #13
Surely that is you problem... An hour with the engine running burning fuel but going nowhere will soon knacker your mpg as the hour will count towards the average!!
Quite true, but surely once the trip is reset it takes that hour out of the equation. Now struggling to get much beyond 50mpg on the reset 'A trip' ( not doing much running about this last few days so still on supermarket fuel) while 'B trip' is still reading 64.6 over 8,400 miles. Going back to dealer later this week for a check.

Dave D
 

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Ex-Pat Geordie!
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I always use supermarket fuel and am getting back 73mpg on average? Have tried the brands and see no difference whatsoever?
 

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Discussion Starter #15
Thanks for the input, all interesting stuff. Thought I'd better post a quick update :smile3:

Let me recap

Car regularly averaging 62-66. Went for new tyres and MOT, swapped original Michelin energy Savers for Goodyear Eff.Grip. As I got to tyre fitter, which is adjacent to main dealer, low fuel light came on. Tyres changed, round to dealer & MOT done, which included running car stationary for about an hour. Left for home and due to proximity of fuel stations was forced to put in 3/4 tank of supermarket fuel. MOT day +2 did a regular run to Taunton, on way back reset trip and struggled to get beyond 57mpg. Over following week of 'pottering about' and resetting trip several times consumption deteriorated to 52/53ish, not happy as discussed above, intending to run tank down and fill with my 'normal' fuel + go back to dealer for checks.

MOT day +12, still on same tank of fuel, had to go down to Plymouth (from just South of Exeter). As I dropped onto A38 I did something I've never done in this car, I left it in 3rd and floored it, almost red lining for 1/2 mile or so. Slowly, over the rest of the return journey mpg crept up and by the time I got home computer was registering just over 66mpg, roughly pre MOT/new tyre levels. Yesterday did another 'regular' run to Taunton, slightly slower than normal due to traffic, and averaged 69.4 over the journey (only according to computer, I know).

Now here's my theory!! Low fuel level plus running for an hour during MOT picked up some 'crud' from tank and gummed something up, then my 'blast' down the A38 cleared it out again?? Anyway, if this is not the cause, and it might not be as car seemed to be running smoothly, I'm still a happy chappie :smile3:

Thanks for all your input. Dave D
 

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Honest John does recommend giving a diesel a good thrash once a month to help clear any possible muck in the injector. As you say possibly leaving engine running for a hour with a low fuel level might have introduced something that caused the problem, though you would think the fuel filter would have trapped any contamination. But if its solved your problem. Happy days.
 

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Supemarket Diesel I would say. Previous diesels I have had have suffered poor MPG after using supermarket fuel. I have a 1.6 iDTEC and love it and the MPG, have only ever used SM fuel when I had no option and no more than a couple of times in 2 years and easily saw 10% plus drop in MPG. On previous cars I have also suffered poor starting and excessive waxing.
 

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Supemarket Diesel I would say. Previous diesels I have had have suffered poor MPG after using supermarket fuel. I have a 1.6 iDTEC and love it and the MPG, have only ever used SM fuel when I had no option and no more than a couple of times in 2 years and easily saw 10% plus drop in MPG. On previous cars I have also suffered poor starting and excessive waxing.
It's all in the mind :grin2:
 

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New to Old Tyres is about 2% difference in speedo reading, so that accounts for a little. As will higher rolling resistance.

Supermarket fuel is a nonsense, they all come out the same refinery, and minus a few bespoke add ons, they're the same fuel, same octane rating, and same efficiency. Placebo effect for those thinking it's worse.
 

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Thats intersting about the MPG efficiency returning after the revving blast down the motorway

I just got a 2014 Diesel civic but just before I parted with my Mazda 6 - I had a problem with the DPF (Diesel Particulate) filter and one of the suggestions with that was to drive at high revs, I did this and it was better for a few days and then went again - but presume it is to burn off some of the crud and therefore make it run better
 
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