New tyres and fuel consumption

[Roodosutaa]

Hi all

Got a set of Kumho KU-31s two weeks ago, have done well over 1000k's on them and been very happy with the wet/dry grip especially considering the price. Credit where it's due though the stock Conti's were the longest lasting tyres I've had, soft sidewall issues aside (blew two tyres as a result) - the front ones lasted 55000k's (been rotating them of course) - pretty amazing for a front driver.

What I have noticed however was a marked increase in fuel consumption - I filled up the same day I got the KU-31s - been commuting on the same work route, not hooning around just going with the flow - and the fuel consumption went up by 1L/100km!

Now I have considered these factors:

• Increased diameter of the new treads
• Possibly higher rolling resistance
• Higer grip levels on new tyres over worn tyres that have done 55000km

But I'm not convinced any/all of the above are significant enough to make that much of a difference. Engine seems grunty as ever (chipped). Any takes on this?

Cheers

[brad]

heavier.

having similar issues on my car at the moment - new wheels & tyres are 2kg per corner heavier & in variable speed traffic are causing increased fuel consumption. This is going from fuxored KU31 to supposedly "GreenX" Michelin.

I noticed that the Contis tend to run almost 1kg lighter than most other tyres (had them as OEM).

[seangti]

can't say I've noticed increase consumption with the 31's over the Conti's, wouldn't think there is much in for me. I've mostly stopped checking the consumption. For the price difference between the two tyres, and that my KU31's won't last long (track em), not too fazed. Would only recommend em for people that like a spirited run every now and than, or don't regularly put the car on the limit, the limit isn't that high. But otherwise, really can't complain for the price.

[brad]

The other thing of note is that I get the feeling that KU31s go "off" quite early. Nothing definate but I bought mine 2nd hand with 5mm tread remaining & have run them down to 3mm & the replacement (almost new 7mm/7.5mm) Michelin Premacy HP tyres are much better all round. This shouldn't be the case as the Premacy HP is a fairly basic touring tyre.

[noone]

How much are the Michelin's? I'd call the 31's a budget sports tyre, they are cheap, think I paid $110 a corner.

Can't comment on the fuel use, my TT wheels and bigger brakes added a heap of unworthy weight, one day I'll try and strip that back...

[brad]

How much are the Michelin's? I'd call the 31's a budget sports tyre, they are cheap, think I paid $110 a corner.

Can't comment on the fuel use, my TT wheels and bigger brakes added a heap of unworthy weight, one day I'll try and strip that back...

225/55x16 michelins are some stupid price - $300+ per tyre i believe. I bought mine with a set of mint 16x7.5 2010 model A4 rims off ebay for a bit less than a set of KU31s would have cost me.

I realise that KU31 are a budget sports tyre but by all accounts they are great when new but lose grip long before they are worn out. There is no way the Primacy, despite the higher price, should be a better tyre than the KU31.

What I'd really like now is some 16x7.5 or 8" ET45-40 forged rims that weigh about 6-7kg.

[Lomo]

Roodosutaa, the Contis are a low rolling resistence tyre made to provide a low fuel consumption. Also are your new Kumhos wider? As wider tyres provide better grip due to softer rubber which has a higher rolling resistance.

The energy needed to spin up the extra Kg at the outer diameter where it hurts the most would be insignificant, although times that by 4 and...

[brad]

Roodosutaa, the Contis are a low rolling resistence tyre made to provide a low fuel consumption.

Not as far as I know. Where'd you get that idea from (happy to be shown otherwise)?
Continentals LRR tyres are the Contiprocontact & the Contitouringcontact. The CSC2 has a high silica contact which helps fuel economy but doesn't make it an LRR tyre.

Also are your new Kumhos wider? As wider tyres provide better grip due to softer rubber which has a higher rolling resistance.

I agree a wider tyre will have a bit more drag but:
a) A wider tyre doesn'y necessarily have better grip
b) a wider tyre doesn't necessarily have softer rubber. In this case, KU31 are TW340 & CSC2 is 280. I know that numbers between mfrs aren't really comparable but the CSC2 does have softer rubber.

The energy needed to spin up the extra Kg at the outer diameter where it hurts the most would be insignificant, although times that by 4 and...

uh huh... so why bother to fit lightweight flywheels or spend a fortune on lightweight rims? Why, when I have fitted heavy chrome rims to customers cars (back when I sold wheels & tyres), did they come back complaining their cars felt gutless (nobody worried about fuel economy).

In order to accelerate, you need to overcome inertia. At the most basic level Force =Mass x acceleration. The higher the mass the more force you need to get the same acceleration. More to the point, you have to overcome inertia. I=m x radius squared, so the further out the mass is, the greater the force required to overcome the inertia. So 1kg on a 16" tyre is far more obvious than 1kg on the rim itself. Some worthwhile reading here.

Finally, it's unsprung weight. Reducing unsprung weight has a greater effect on a vehicle than reducing sprung weight. Figures vary - I've seen it quoted anywhere between 1.6x & 8x (eg 1kg saved unsprung weight is equivalent to pulling 8kg out of the body of the car). I have no idea what it really is but any weight saving is worthwhile.

So, IMO, I think an extra 1kg of tyre weight will effect fuel economy, acceleration, wear & tear on suspension components, steering response & handling.

[noone]

Its not that bad... I can notice a difference with my new rims and would love to slap something lighter on, but it all costs money and that's money I'd prefer to spend on power...

Interesting...

[DeanB]

Not wanting to go off topic wrt weight issues. But if you don't want to increase fuel usage surely it is commonly known that you don't start adding weight to the car - heavier wheels & tyres, bags of concrete in the boot, etc.