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.

[brad]

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...

removing weight is like adding power. ref Lotus

[kaanage]

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).

Additionally, more unsprung weight makes it harder for the suspension to do it's job since the extra mass increases the energy that must be absorbed by the dampers and springs. This makes for a harsher ride and if the spring and damping rates are not increased, can lead to lower roadholding on bumpy roads. Since most times that people fit wider and heavier wheels and tyres, they are seeking improved grip, the required increase in spring and damping rates makes for an even more harsh ride.

And the increase in additional rotational inertia adds to braking loads as well as reducing acceleration.

This + the fuel economy issue is why I posted that I thought the Polo was overtyred in the stickied tyre thread.

If you want a really good improvement in this area, then you need to invest in a set of 3 piece wheels with magnesium centres and spun rims. Expensive, though.

Fantastic post, brad.

[Roodosutaa]

Thanks folks - yes I've never thought about the weight of the tyre itself could've made that much difference or the fact that street tyres could differ in weight by as much as a kilo between brands! Thanks Brad for pointing out the unsprung weight would bog the car down more than the equivalent sprung weight, I'm aware of its effect on suspension movements due to inertia etc but didn't realise it could possibly translate to higher fuel use as well. And as far as I've been aware the old Conti's weren't known to be low rolling resistance tyres - which was why I was surprised by the higher fuel consumption of the Kumhos.

Obviously I'm not losing sleep over a few bucks a week but was hoping it wasn't anything more sinister than just tyres.

And the new tyres are stock size - obviously I'm aware of the additional drag that could be caused by wider tyres, bit of a no brainer really.

[philthy]

Have you checked the pressure? Lower pressure = higher consumption. Tyre shops sometimes do strange things.

The rotational inertia of the tyre would be SFA compared to the force required to move the car under acceleration from rest. Just think about when the car is jacked up and park brake is off. You can start the wheel spinning with your pinky... but when the car is on the ground how easy is it to move it by spinning the wheel by hand?? No way there's a 10% increase there.

[brad]

Have you checked the pressure? Lower pressure = higher consumption. Tyre shops sometimes do strange things.

The rotational inertia of the tyre would be SFA compared to the force required to move the car under acceleration from rest. Just think about when the car is jacked up and park brake is off. You can start the wheel spinning with your pinky... but when the car is on the ground how easy is it to move it by spinning the wheel by hand?? No way there's a 10% increase there.

I agree, tyre pressures could be way out. You are right, tyre fitters get some weird ideas.

but

F=ma
When you are spinning the wheel with your finger, how fast are you accelerating the wheel? ie: push it slowly & it's quite easy, now try & push it really fast.

I'm not just talking theory on this. My latest wheel tyre combo is 2kg heavier & in variable speed traffic it has really hit my economy. If I can maintain aconstant speed on the run home, I can manage <6L/100 but if it's varying between 60-120 then I'm up near 7L/100. It's also taken the edge off my acceleration.