New tyres and fuel consumption

[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.

[philthy]

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.

Haha... fair enough.. I may just have to take some time out to have a crack at doing the calcs in a bit more detail when I'm supposed to be working tomorrow.

[noone]

My TT's are 25 pounds a piece. Brutal... But they look good

[instigator]

i'm searching for light tyres that provide enough grip.
i know potenza's are verry good in grip levels, but they have a hard tyrewall on the side wich makes them heavy.
not ideal foy Superleggera's :-s

there are many things that have influence on the car etc, with only tyres.
thats why searching it out is not easy, not much info can be fund on these things.
my best fuel economy is 5.0L/100Km with my GTI with the use of cruise control and constant speed.

[brad]

Have a look at ContiSportContact3. The sidewalls are quite thin. All the others are about the same weight AFAIK.

or, go down to the tyre shop with some scales

[kangfu]

Im definitely noticing a difference going from the stock 16" wheels and 205/45/16 Toyo T1-R tyres, to 17" wheels with 215/40/17 Bridgestone RE001's.

MUCH better grip though, the extra width puts power down much better i feel. Out of corners ESP isnt going off at me like a nutter anymore.

Prefer it, even with the slightly higher fuel economy.