Fuel Consumption and Tyre Weights

[Daved and Confused]

Following on from another thread I started, I thought I would post this as a cautionary tale.
This may be of no interest to most but someone may find it useful.
I have a Mitsubishi Shogun 3.2 Diesel, a few months ago I was needing tyres - 285/55x18
The tyre choice in this size is not the best and tyres range from about £170 to £200.
I was looking for a slightly chunkier tyre than the Pirelli Scorpions that were fitted as my I do drive over fields etc towing.
I had a look around on the net and came up with Cooper LTZ's, had good reviews and seemed to fit the bill but could only get 285/60 x18. So I ordered 4 at an also very appealing £133 each.
When they arrived I could not believe how heavy they were, but not knowing the weights of the tyres already on the car I had no comparison.
So when I fitted the new tyres I kept an old one to compare the weights. Now I know the tyre size is different but not a million miles away and the old tyres had 3mm and the new ones 12mm but the old tyre was 8.2kg and the new one was 21.9kg.
The result is a drop from 28/29mpg to 26mpg.
I know the rolling radius is larger so would effect acceleration but you can really feel the effect of the heavier tyres.

So if anyone is thinking of buying tyres this just shows the difference in tyre construction and the resulting weights.

[Southie]

Did you have the same PSI in all the tyres then before and after, also was your tracking spot on before and after??

[Daved and Confused]

Tyre pressures are spot on and tracking was out slightly before but sorted for new tyres going on.

[Southie]

I'm also confused, and called Dave lol

Maybe just more grippy so loosing MPG.

[Daved and Confused]

When your called Dave it doesn't take much lol

Yeah possibly but the weight of each wheel/ tyre now is ridiculous, I just hope I don't get a puncture as I may well be one of those pussies who calls the AA - not to change the wheel but to help me lift the punctured wheel onto the back door or I'll be leaving it on the grass verge.

[MK999]

I'm also confused, and called Dave lol

Maybe just more grippy so loosing MPG.

Or more like spinning up an extra 52 kilos on the outermost point of the wheels means a lot of his fuel is lost into momentum in the tyres and turned into more heat when his brakes work harder.

[Southie]

No matter what I say MK you've always got an answer :roll:

[MK999]

Point he's making is quite valid though, 52 kilos in the wheels, especially in tyres will make a massively noticeable difference in acceleration and make the engine work harder, hence the drop in MPG. 52 kilos is probably a good, very roughly, 20% or so increase to the drivetrain weight? And many more times that in inertia. So the fact his MPG has dropped by 10% isn't too surprising.

[Southie]

I mean EVERY thread I post in these days!

Appologies Mr Confused and Dave ;)

[MK999]

Well, I'm not following you. Post before 6PM more, I'm never around before 6 lol

[Southie]

PM on it's way!

[Daved and Confused]

No problem, joust away - it spices up my thread.

But, yes the unsprung weight is where we all strive to reduce weight and where we can help to improve handling.
This is confirmed by the, "feeling" from the car now - it just feels duller if you get my drift.
Also rolling acceleration is somewhat affected, as you know a shogun is no performance machine but previously at 70/80 you could accelerate noticeably to over a ton - now this is somewhat dampened. Due to the required effort needed to get the extra weight spinning.

[MK999]

Inertia is always more powerful on the outer edges, by definition, Hence pot type fly's being pots and not er, Hills? Domes? lol It will probably be pretty awful on more uneven surfaces now too if you do any offroading at any kind of speed, although also coming back to natural frequencies of the suspension system etc, the weight should make the natural frequency lower so in theory the faster you go the smoother it gets lol

[Daved and Confused]

Funny I was just saying that to my girlfriend the other night - the faster we go the smoother it gets, she didn't believe me but I'll show her this reply, with a told you so lol

[Southie]

Ben the boffin comes to mind (jesticles Mark) ;)

[MK999]

Funny I was just saying that to my girlfriend the other night - the faster we go the smoother it gets, she didn't believe me but I'll show her this reply, with a told you so lol

It's true, if you can prove where the natural frequency of the car is. Depending how much you want to win that argument I'm sure it's doable with a quick look around google at simple harmonic motion lol

[brainsnova]

Everyone always has a mate called Dave

[mowgli]

Try higher pressures..