Strange HDi fuel consumption behaviour

[citroenxm]

Hdi 110bhp with Tuning Box.
255,500 k miles on the clock..
Well mechanically looked after. Always serviced etc on time
All original injectors.. second hand engine pump fitted

Experiances: While I had 195/60/15 the average fuel consumption was around 54mpg.

We went away not so long ago for a week in cornwall, before we went wanting to be all leagal and safe, I fitted a set of 4 205/60/15 tyres all round.

While we were away, I noticed a drop in performace, and fuel consumption.. felt like I had no turbo, and the consumption average fell to around 48mpg (I have a fuel computer fitted from a 406)

On our return it rose slightly to 50mpg, but I still felt this a bit low. I had decided to put this down to wider tyres and more rolling resistance.

However over the last day or so, I noticed an increase in performance.. and today on the way home from Manchester, I suddenly decided to flick to average MPG, and it has risen back to 55.3mpg.... when I got home and slowed to an average of 65mph it had risen to 57mpg...

Ive done NOTHING between the time we went away and this week... Do cars also suffer off days I wonder..

[Stewart(oily)]

Fuel quality perhaps ?

[citroenxm]

I only use two brands... Texaco, as I have a points rewards card, and shell, also because I have a points rewards card...

[myglaren]

Paul,
as you know I had a C5 estate with the same engine as yours. Over the several (6, I think) years I had it it would average 32mpg except on a long run (CXM when I would manage about 50-55mpg.
This was with 195/60-15's.
I did as you and changed to 205/65-15's. No difference whatsoever.

These are now on the replacement C5 (the size, not the same tyres) and average MPG is - 32mpg! Best mpg was achieved on the run to Kettering last month (National) 540 miles at 47mpg.

I think I may need the lead removing from my boots

Saw you in the distance at the national but never managed to catch up with you - sorry

[citroenxm]

Awww Steve, nice to hear from you as always.. We popped in for one day, BUT im making a full weekend of CXM in september this year... I guess you wont be there???

The trip to Cornwall was long enough and lighter on that trip..

The trip back from Manchester had ALL my Tools, etc in the back... A strange one.

Id expect a C5 to be slightly heavier on fuel then a C5 due to the engine being same CC and BHP as the xantia, but a heavier body..

[myglaren]

Hoping to be at the CXM but can't say for certain yet. May be a last minute thing.

[citroenxm]

That be great Steve!! Be good to see you..

[myglaren]

Thanks Paul, you too

[Peter.N.]

205/60/15s have a 4" larger overall diameter than 195/60/15 so the gearing has changed slightly, the consumption is probably the same but the mileometer is reading low, this would also account for reduce acceleration. Unless your original tyres were 65s not 60s

Peter

[citroenxm]

205/60 and 195/60 have the same rolling radius size peter. The only difference is the width of the tyre 195 to 205..

Both had 60 wall both 15" so makes no difference on the speedo reading. Which indcidentally is exactly 5mph fast..

[Peter.N.]

Are you sure of that Paul? The XM sizes are 195/65 or 205/60 which have the same rolling radius. The 60 or 65 is normally the percentage of the width that the height is - unless I've got it wrong again.

Peter

[citroenxm]

Yes im sure the 60 or 65 is the tyre wall size... so bith are 60 making them the same rolling radius..

[RichardW]

Peter is right, the 60 figure refers to a % of the width so a 205 60 has a wall height of 123mm whereas a 195 60 has a wall height of 117mm so the 195 60 is 12mm smaller in diameter. 205 circumference 1970mm 195 circ 1932mm so only 2% on the rolling length - not really enough to notice, but different all the same

[KP]

The 60/65 is a percentage thing not an actual measurement is the percentage of the width

For example range rovers have 3 sizes as standard,
255/60/18
255/55/19
255/50/20

The 205/60/15's are 1.1% smaller for their rolling radius or 7mm for the same tyres.

What are the tyre pressure like and tread/brand? I've found these can affect mpg a lot and worn tyres can be better on some roads and not on others

[Chris570]

Yes im sure the 60 or 65 is the tyre wall size... so bith are 60 making them the same rolling radius..

It's a % of the width so 60 is a different amount depending on the width of the tyre.

My HDi (228k) has it's off days and it has it's good days. I run BP Ultimate in mine and I do see a big improvement on power and mpg (depending on how much right foot is applied). I've run Texaco a few times and I've had poor mpg from it but that might just be local stockists.. I do also notice a difference on supermarket fuel so for the last 3k I've been on Ultimate, well my car has. I find it doesn't cost anymore with the savings on mpg. On normal fuel I get 200 miles out the first quarter of a tank but on Ultimate i get at least 250. I can drive from MK to manchester before the fuel gauge even moves from full.

My point is, fuel is the second biggest factor in MPG on mine (after right foot)