Whats the MPG now on your D

[Richard_Jackson]

Hi guys i'm looking to trade in my old car (MG ZT) CDTI for a Mini Cooper D i was just wondering what kind of MPG are you getting out of all your mini's? i've read through a few and seen alot of 58-60 mpg? is that driving like a grandad on the motorway? i would like to know because i commute 45 miles each way, on dual carriage ways,

Regards,
Richard

[timp]

Kinda depends on the commute really. I find that the few places where I can overtake and which hammers the consumption is balanced by the places where I can only trundle along in slower traffic. The rare occasions where I can have fun (sensibly) for most of the journey still return mid to high 50s mpg because of the 30mph limits at either end of the journey, plus the 50mph bit in the middle. Mines a late 09 Dooper, so not the Beemer motor.

[Richard_Jackson]

I'm seeking to buy a 08 Dooper, and most of my journey will be dual carriageways doing 70mph, however by points not by choice i have to religiously stick to my speed, so therefore i was thinking that it would return quite good numbers, i would be happy with high 50s - low 60s

[ScoobyChris]

I have the BMW-engined Dooper and my car gets a hammering yet regularly returns me around 50mpg (currently at 18k miles). The PSA engine is more economical than the BMW one so I'd expect you to see the figures you're after without too much difficulty (and even more with a remap).

One thing that's also worth considering is wheel size - the standard car comes with 15" wheels so I'd expect the book figures to be generated with these, but quite a few of the cars on the second hand market have bigger wheels which, while aesthetically more appealing, will have a negative impact on economy. I opted for 16"s for mine and reckon it probably costs me 3-5mpg.

Let us know how you get on

Chris

[Angib]

In a petrol Cooper, I reckoned switching from 15s to 16s cost me 2mpg - this is using the average over, say, ten tankfuls but even then it's hard to be definitive. Of that change, about half is accounted for by the change in tyre diameter, so that is not really a loss in economy, just a change in measurement.

So that leaves about 1mpg loss at 40mpg in switching from 15s to 16s - though I think the important thing for loss of economy is the tyre width. So maybe that would have been 1.5mpg loss on a diesel doing nearly 60mpg.

[timp]

Wont you see more than that sort of difference based on the temp during the run? I can get into the low 80s in summer but struggle to get into the 70s in winter. Got to drive like Miss Daisy for the 80s too!

[ScoobyChris]

Quote: Originally Posted by Angib (original)

In a petrol Cooper, I reckoned switching from 15s to 16s cost me 2mpg - this is using the average over, say, ten tankfuls but even then it's hard to be definitive. Of that change, about half is accounted for by the change in tyre diameter, so that is not really a loss in economy, just a change in measurement.

What tyre size did you change from and to? Some rough back-of-envelope calculations for the standard sizes suggest around 1/2" difference in diameter which is about 1% between the 15" and 16" wheels so I'd be surprised if that altered the calculations by a significant amount?

Quote: Originally Posted by Angib (original)

So that leaves about 1mpg loss at 40mpg in switching from 15s to 16s - though I think the important thing for loss of economy is the tyre width. So maybe that would have been 1.5mpg loss on a diesel doing nearly 60mpg.

Tied in with that is also the type of tyre on the rim and I've noticed on previous cars ditching "economy" tyres for something that provides more grip also increases the economy. Unsprung weight is also a consideration as bigger wheels (from the same manufacturer) generally weigh more and this will affect acceleration and potentially how much fuel the car uses maintaining speed.

Lots of factors

Chris

[Angib]

I went from standard 175/65-15s to standard 195/55-16s, so (nominal) diameter went up from 608.5mm to 620.9mm (see excellent wheel FAQ on this site). That's an increase of dead on 2% so with no other changes I would expect the mpg to drop by 2%, or 0.8mpg at the 39-40mpg I get. This is not an actual loss of economy, just a change in how it's measured.

I don't believe wheel or tyre weight makes any difference to fuel economy - aero drag is by far the dominant component, so wider tyres hurt that but extra weight makes little difference except maybe to purely urban consumption.

I also don't believe wheel or tyre weight makes any difference to acceleration - 16s definitely feel slower but I think that's down to the 2% higher gearing.

What wheel and tyre (unsprung) weight does make a huge difference to is grip over bumps as the suspension can't keep heavier wheels/tyres pressed to the road as well as lighter ones.

Chris, you might like to look at this Ecomodder.com thread on tire testing - interesting that the spacesaver tyre gave the worst results. Data on this forum should be treated with a very long pole - there's some superb stuff mixed in with complete nonsense.

[ScoobyChris]

An interesting read there and it would seem my back-of-envelope calculations aren't as good as I'd like

My Physics is also a tad rusty, but Newton's 2nd Law of motion states F=ma and so for a constant force, as the mass increases, the acceleration must decrease. This also applies to accelerating a body around a fixed axis. Which makes sense really, it's harder to overcome the inertia of something heavier. I seem to remember reading that for every 5kg of unsprung weight, the car "loses" 1 bhp so the effect is probably more marked on cars with lower power engines.

From an economy perspective, it could probably be ignored on a perfectly flat surface travelling at constant speed as there will be no acceleration, but it's rare we are ever in that situation, imho. I suppose we could also consider the effects of the increased weight on momentum which may mean we can be off the throttle for longer ... possibly

We need to find someone who has changed from steel wheels to lighter, same-size alloy wheels, with the same tyres and see what effect it has.

Chris

[Angib]

Quote: Originally Posted by ScoobyChris (original)

I seem to remember reading that for every 5kg of unsprung weight, the car "loses" 1 bhp so the effect is probably more marked on cars with lower power engines.

Here beginneth today's nerd-fest....

'Unsprung' weight is irrelevant - that's a measure for suspension performance. However wheel/tyre weight that is rotating is relevant and is one part of unsprung weight, so we'll let that one scrape in.

I've crunched some numbers. Did you know that a Mini doing a steady 60mph has 440 kJ (kiloJoules) of kinetic energy? That's the same as 0.12 kWh (kiloWatt.hours) - or the energy your boiler at home puts out in 15 seconds.

The wheels/tyres rotate, so as well as linear kinetic energy (because we're doing 60mph), they also have rotational kinetic energy (because they are going round at 860rpm). The rotational energy is 17% on top of the linear energy, so heavier wheels do matter a little bit more than regular weight, but not by much.

If you do an extreme swap from 15s to 17s adding, say, 20kg for a set of four, then that is the same as carrying 24kg of luggage. That's a big weekly shop-up - but would you notice that difference accelerating (if you didn't already know it was there)?

Incidentally, I weighed one of my summer 195/55x16 X-lites to give me data and the bathroom scales say one wheel/tyre is 16.1kg. This forum's utterly superb wheel FAQs says an X-lite on its own is 8.0kg, so that means the tyre (and air!) is 8.1kg. So looking for lightweight wheels may be a bit of a wild goose chase if we then put heavy tyres on them.

And this starts me thinking that 15s, 16s, 17s and 18s all have about the same outside diameter so as the wheel size and weight increases, does the tyre weight decrease? Maybe we should start getting all forum members to weight one wheel/tyre and get a list of weights going?

Oh, and adding 5kg of wheel/tyre weight has the same effect as adding 6kg of regular weight. That's 0.5% of any Mini's weight, so it's the same as a One losing 0.5hp or a JCW losing 1hp. So there's your 5kg = 1hp!

[timp]

so a lard arse with 15s equals a skinny person with 18s????

or was that the red wine talking?

[Angib]

Quote: Originally Posted by timp (original)

so a lard arse with 15s equals a skinny person with 18s????

Spot on. Though the lard arse is probably a bigger effect.