Wheels, Rims, Big boy shoes

[Pommes-T]

I've just come back from a trip to a small town near Frankfurt where my parents are from. That's a 2 times 600 km trip. 2 weeks ago I came back from Italy where I all in all drove 3000km in my dad's A6.

Thing is: To Italy I drove on our winter tyres. Ugly 195/15 inch tyres. My topspeed was 250km/h on the speedo, GPS said 235 km/h. On friggin winter tyres. Last saturday/sunday the A6 had the (once...) good looking 225/17 inch tyres on that are said to be more sportive. Well, of course roadholding is really better. Especially in the Kasseler Hills, where you have high speed corners on the Autobahn you can really feel a difference. BUT, and that's a big BUT: My absolute topspeed that took a looooong time to reach was 235 km/h (speedo). And the fuel consumption was a well 1,5l/100km more.

My post might not bring any news to you, but I really got a new opinion upon tyres because of this. I always had the feeling tyres needed to be the bigger the better (of course I am not talking about 24 inch wheels or anything, but in ordinary sizes). But now I think the perfect size for our A6 should be 205/16. Our neighbour i.e. has an Avantime with 245/18 tyres on a 167hp car. Now I gotta say that this is pretty pointless.

Yeah. Now you now what's on my mind. 

[S204STi]

True, bigger is not always better!

[Vinsanity]

I got a set of little 16"s that I'd be more than happy to get off my hands and sell to you.

Don't know if you want to pay for them to be shipped to Germany, though

[Gotta-Qik-C7]

True, bigger is not always better!

WUT U SAY FOOL? U MUST BE CRAZY!!!! THROW SUM 40S ON THAT BITCH!!!!!!!!

[AutobahnSHO]

WUT U SAY FOOL? U MUST BE CRAZY!!!! THROW SUM 40S ON THAT BITCH!!!!!!!!

lawlz

The dudez looking are thinking "cool!" even though it probably drives like Krap and you have to jump to get in, not to mention the wheels look like the MB logo...

[Vinsanity]

lawlz

The dudez looking are thinking "cool!" even though it probably drives like Krap and you have to jump to get in, not to mention the wheels look like the MB logo...

They should've just put them on an actual Mercedes

[Laconian]

lawlz

The dudez looking are thinking "cool!" even though it probably drives like Krap and you have to jump to get in, not to mention the wheels look like the MB logo...

A loaded freight train would have a shorter stopping distance than that monstrosity...

[giant_mtb]

I think the explanation is relatively simple.  The skinnier tire has less surface contact with the road which means it has less friction to overcome than the wider tire.  Hence, it gets better gas mileage and gives a potentially higher top speed.



Also, were both sets of tires on the same car?  If so, comparing the top speed between them is irrelevant (unless you used the GPS for both readings) because a smaller diameter (15 inch) tire will give a higher speedometer reading than the 17 inch diameter tire.

[Pommes-T]

I think the explanation is relatively simple.  The skinnier tire has less surface contact with the road which means it has less friction to overcome than the wider tire.  Hence, it gets better gas mileage and gives a potentially higher top speed.
Yeah, I know that!

Also, were both sets of tires on the same car?  If so, comparing the top speed between them is irrelevant (unless you used the GPS for both readings) because a smaller diameter (15 inch) tire will give a higher speedometer reading than the 17 inch diameter tire.
This is something I didn't think about, actually. One should also consider that the difference is a bit smaller, because the 15 inch tyres add more rubber to the total diameter than the 17 inch wheels. All in all, they are of course smaller. I didn't check the GPS topspeed this time, but the last time I maxxed out the car on summer tyres was 224km/h, which is still a good 10km/h less than on winter tyres.

[giant_mtb]

Perhaps the difference in top speed is a little of both a friction difference and a diameter difference.

[S204STi]

I think it has more to do with the weight of each setup.

[Pommes-T]

I think it has more to do with the weight of each setup.

I don't think weight plays a major role here. Don't have any specific physical laws on my mind to backup this, but the weight difference isn't so very huge (of course this is unsprung masses, but anyways). You also feel the size difference in corners or on wheel ruts, so I think the area of contact of wider tyres is more important here.

[AltinD]

Pommes, apart the size and the weight issues, the speedometer on German cars are more accurate with bigger wheels on.

[Autobahn]

I think it's more a result of the friction.

What exact size where the tires? Not only the diameter of the rims but the "height" of the rubber is important (I don't have any clue what this is in English).
The overall diameter of the wheels is robably more or less the same, depending on the wear of the tires. So I guess this is not the main denominator. If the difference would be significant (i.e. the result of the diameter difference would be so large that it would greatly affect the speedo-readings) you wouldn't be allowed to use that size of tires.

[NomisR]

Well, typically, bigger wheels would mean more weight unless you're comparing ultra light weight wheels vs steelies.  So just that in itself would make some difference.  Just going smaller on wheel size while keeping the same overall diameter and width would allow you to shave some time off your lap time.. in majoirty of the case