Car and Driver : Impacts of upsizing tires ...

[UG_Passat]

Effects of Upsized Wheels and Tires Tested

Plus-Size Models: Testing the performance effects of upsizing wheels and tires.

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From 4/9/10 , doubt much has changed .

On the interwebz forums and car meets... bigger is always better, since people are over-compensating for something else.

On my old VW, I kept the same 18" wheel and tire size, but went lighter, with Flow-Formed wheels

 

[930.engineering]

Except that a 235/40R18 tire is not rated down to a 7.5" wheel width... Which you have, assuming you didn't change the wheels.

Yes, however the OE Mk7 GTI wheels use to have an effective width of
7.75". I remember someone posting pictures auf the Austin wheel fitted
with 235/40 PSS and the sidewalls looked pretty square, not bulged at all.
That said the current 235/40 R18 DT1 PS4S isn't even much wider than its
225 brother, but of course still a tad taller. That's why I decided to pass on
these 235s so far.
In the colder season I'm running the smallest diameter wheels that fit over
the brakes. That's 16" on the Mini and 17" on the GTI. One inch bigger for
summer wheels though, just because proper UHP tires are only available in
certain sizes, so this Mini kind of 'needs' 205/45 R17 and the GTI 'requires'
225/40 R18 or bigger. I have the 18" Pretoria wheels, which aren't exactly
light weight but still less than the standard factory wheels (Austin, Milton
Keynes). 18" Pretorias are same weight as 17" Conical Spoke Mini wheels.
Weight is important both regarding ride and acceleration. Remember this
also applies to the tire, perhaps even more so.
My Porsche is sitting on its original 16" forged Fuchs wheels. Stupid enough
proper tires actually available in stock sizes 205/55 and 245/45 R16 (PS4) . . .
.

 

[KS-Ryan]

I've got a little over 12k on my Michelins. They're not wearing funny and look great on the wheels. I don't drive the car hard and can't come up with one negative thing about the setup. I didn't upsize for a performance reason, it was because the wider tire looks better on the car, rides better and it fills a little of the fender gap. I'm not interested in lowering it. It seems to be a pretty popular setup when doing my research on them.

 

[Jimmy_Russells]

Not every tire and wheel is exactly the same. Maybe he installed a "narrow" 235

Find me a 235/40R18 tire that is approved for use on a 7.5" wheel width. I'll wait

I mean, what do I know. I owned a Mk6 GTI for eight years and worked in the tire industry for over a decade. People can either follow manufacturer recommendation, or not. Just don't complain about it if they develop a problem.

 

[andyd]

I'm sure my Camry could do a 100mph. I usually cruise at 75ish, traffic depending. Since gas cost 4.40 a gal for e-10, I'm driving slower on the highway and try maintain a steady 60 in the right lane until I over take somebody even cheaper than me moseying along at 50. Check the mirrors and launch into the left lane, blowing the MPG ,maybe hit 85 tops for an instant. I have tried a few fast exit ramps and it does get a little floaty. The tires are 205 16 Firestones that were on the car when we bought it at 16K miles. The car has 55K on it. That's the extent of my needs for a tire

 

[billt460]

That’s how my truck is. I don’t feel quite as bad hitting a pothole with them lolView attachment 94254

I love tires with tread on the sidewall. You never know when you'll have to 2 wheel it around a tight corner.

 

[930.engineering]

Find me a 235/40R18 tire that is approved for use on a 7.5" wheel width. I'll wait

Remember E36 M3 came with factory fitted 7.5" wheels and 235/45 R17 tires.
Audi A4 B6 and B7 came with factory fitted 7.5" wheels and 235/45 R17 tires.
Granted, 17", not 18", however you see it can't be that off. And again, many
OE VW/Audi wheels of nominal 18x7.5" size are actually 7.75" wide and could
also be sold as 18x8" rims. Not saying ETRTO is nonsense, but in this case that
wouldn't prevent me from putting 235s on my GTI.
.

 

[Jimmy_Russells]

Remember E36 M3 came with factory fitted 7.5" wheels and 235/45 R17 tires.
Audi A4 B6 and B7 came with factory fitted 7.5" wheels and 235/45 R17 tires.
Granted, 17", not 18", however you see it can't be that off. And again, many
OE VW/Audi wheels of nominal 18x7.5" size are actually 7.75" wide and could
also be sold as 18x8" rims. Not saying ETRTO is nonsense, but in this case that
wouldn't prevent me from putting 235s on my GTI.
.

I asked for A and you give me Z?

I'm sure you understand the difference between a 45 series tire and a 40 series tire. There are dozens, perhaps hundreds of cars that came with 7.5" wheels and 235/45R17 tires. I'm sure you'll also find that if you look at a 235/50 or a 235/55 or 235/60 that the recommended wheel widths get narrower as the sidewall height increases.

No 235/40 examples though right? That is because that tire size is not recommended for 7.5" wheels by virtually every (maybe every) tire manufacturer, and vehicles that do come equipped with that size from the factory have 8" wide wheels.

 

[930.engineering]

.
Absolutely, I'm aware of that (otherwise I wouldn't have pointed it's 17"),
and I admittedly don't know a single example. I haven't been searching
for them though and I'm not even interested in doing that. Like many
others I wish a GTI would come with 8" wheels and 235/40s as standard.
However, both Audi and BMW likely didn't make these decisions on the
edge and there's perhaps a bit of margin left. Ideal or not, whether it's
conform to ETRTO or not - what I've seen fitted to OE GTI wheels looked
fully appropriate to my eyes. Do tire manufacturers even take 7.75" wide
wheels into account? Please don't ignore the rest of what I've said. I'm
not even interested in convincing anyone in fitting 235s to 7.5" wheels,
just saying I think it wouldn't be that bad.
.

 

[Jimmy_Russells]

.
Absolutely, I'm aware of that (otherwise I wouldn't have pointed it's 17"),
and I admittedly don't know a single example. I haven't been searching
for them though and I'm not even interested in doing that. Like many
others I wish a GTI would come with 8" wheels and 235/40s as standard.
However, both Audi and BMW likely didn't make these decisions on the
edge and there's perhaps a bit of margin left. Ideal or not, whether it's
conform to ETRTO or not - what I've seen fitted to OE GTI wheels looked
fully appropriate to my eyes. Do tire manufacturers even take 7.75" wide
wheels into account? Please don't ignore the rest of what I've said. I'm
not even interested in convincing anyone in fitting 235s to 7.5" wheels,
just saying I think it wouldn't be that bad.
.

It's not that bad, no. But it's not technically correct, and tire manufacturers absolutely will deny warranty for things like this. How it "looks" doesn't mean a whole lot. I would love to put a 265/30 onto the front of my car, both for handling purposes and to better match the rear tire diameter. However with an 8.5" wheel, I'm not going to do it.

In terms of the wheels being 7.75", they are typically made by BBS or Ronal and say 7.5" on the back of a spoke. So I am going to disagree with that point. Rim width is measured bead to bead, not the entire width of the wheel.

FWIW, in 19" the Mk7 chassis did some with 235/35R19 tires and 8" wheels. Mainly because the "correct" upsize, a 225/35R19 does not have the proper load capacity.

 

[930.engineering]

.
BMW E36 M3 as well as Audi A4 B6 and B7 came with 7.5" wheels and 235/45 R17 tires.
Golf Mk7R, 8R, GTI Clubsport S & TCR came/come with 8.0" wheels and 235/35 R19 tires.
Wouldn't one conclude that 7.75" wheels should match 235/40 R18 tires as nicely then?
Isn't it the golden mean, the happy medium?
Hope you don't want to tell me how rim width is measured. 'Looks' can actually mean a lot.
If sidewalls are sitting straight, vertical, square - it more than just seemingly is a proper fit-
ment.
I remember some guys on Mk5/Mk6/Mk7 forums running 255s on stock 7.5" wheels for
autocross (because rules mandate using stock wheels) pruposes. Not that I would do that,
but it's definitely done a couple of times. That must be a horror vision for you, Jimmy.

Both Mk6 GTI and Mk7 GTI came with 7.5" wheels and 225/35 R19 tires as a factory option.
Only GTI Clubsport, GTI TCR, Golf R (both optional) and Clubsport S (standard) came with
8.0" + 235/35 R19. I'm aware some of this wasn't even available in NA.

That said, I'm under the impresion we should get back on track > back on topic and that's
impacts of upsizing tires.
.

 

[Jimmy_Russells]

.Wouldn't one conclude that 7.75" wheels should match 235/40 R18 tires as nicely then?

.

You keep going on about this mystery 7.75" wheel. They are 7.5" wheels dude. And it doesn't even matter because a 235/40R18 is rated for 8", not 7.75"

I am surprised a guy with engineering is his name is ok with doing things not so by the book. Whatever floats your boat.

 

[930.engineering]

.
It's me being surprised. You keep on ignoring half of what's being said.
.

 

[Jimmy_Russells]

.
It's me being surprised. You keep on ignoring half of what's being said.
.

You mean this?

.
BMW E36 M3 as well as Audi A4 B6 and B7 came with 7.5" wheels and 235/45 R17 tires.
Golf Mk7R, 8R, GTI Clubsport S & TCR came/come with 8.0" wheels and 235/35 R19 tires.
Wouldn't one conclude that 7.75" wheels should match 235/40 R18 tires as nicely then?
Isn't it the golden mean, the happy medium?

That your theoretical fitment extrapolated from two completely different fitments somehow overrides the actual recommended fitment?

Or, the fact that some autocross guys use a 255 on a 7.5" wheel? That has nothing to do with what is engineered to be correct. People also run 235's on 10" wheels, can't say I would do that either.

I have never seen a 225/35 equipped on a VW from the factory, but I also live in North America.

The OEM virtually always uses the narrowest wheel possible for the tire size they want to run, other than very high performance vehicles.

 

[CapriRacer]

Allow me to clear this up! The following is straight out of the ETRTO yearbook.

And for those who don't know ETRTO = European Tyre and Rim Technical Organization, the tire standardizing body in Europe. Both vehicle manufacturers and tire manufacturers reference these yearbooks.

A 235/45R17 has a measuring rim width of 8", with an allowable range of 7 1/2" to 9".

A 235/40R18 has a measuring rim of 8 1/2" with an allowable rim width range of 8" to 9 1/2".

A 235/35R19 has a measuring rim of 8 1/2" with an allowable width range of 8" to 9 1/2".

So, No!! a 235/40R18 does NOT have a "golden mean" of 7.75". Just an FYI, for passenger car tires, rims come in 1/2" increments, so a 7.75" rim shouldn't exist. And the "golden" should be the measuring rim, not the minimum!

 

[930.engineering]

The following is straight out of the ETRTO yearbook.
......

A 235/45R17 has a measuring rim width of 8", with an allowable range of 7 1/2" to 9".

A 235/40R18 has a measuring rim of 8 1/2" with an allowable rim width range of 8" to 9 1/2".

A 235/35R19 has a measuring rim of 8 1/2" with an allowable width range of 8" to 9 1/2".

I think there's a consensus just because it's fact. Nobody denied it.

So, No!! a 235/40R18 does NOT have a "golden mean" of 7.75".

You know I didn't say it that way. I was giving two/three real-world
examples and was suggesting one conclusion.

Just an FYI, for passenger car tires, rims come in 1/2" increments, so a 7.75" rim shouldn't exist. And the "golden" should be the measuring rim, not the minimum!

While 1/2" increments are common there are still 1/4" increments
existing, even for stock wheels. For example previous model Renault
Megane RS came with 8.25" wide wheels from factory. I do remember
that just because it is that special.
However, I've been talking about a different thing. OE VW wheels of
18x7.5" nominal size but actually 7.75" wide. ETRTO allows for certain
tolerances and VW seemingly made use of them.
Granted, it's still not correct on paper, however it may be ok if you do
this on your own car. Again, I didn't ever recommend anything. I don't
even do that myself.
.

 

[A_Harman]

I listened to a video a couple of months ago about the upsized wheels on the new F1 cars. They went from 13" to 18" wheels, and the tech expert being interviewed said that they expected lap times to INCREASE by 2-3 seconds per lap due to the larger wheels. Higher moment of inertia adds a lot of effective mass to the car, making it harder to accelerate and decelerate. Also the shorter sidewall on the tire was forcing a complete retuning of the suspension springs and dampers. On previous F1 cars, the lower spring rate of the taller side walls was a significant part of the spring rate in the suspension.

Another example of going too large on the wheel would be GM's 5th-Gen Camaro. When first released the performance wheel option was 20", but when the max-effort track car Z28 was developed, GM went down to 19" wheels.

Wheels should only be as large as necessary to fit the brakes you need to get whoa'd at the end of the straight. On my Camaro, that meant 17" wheels and C5 Corvette brakes. If I added power, I'd have to put bigger brakes on, then increase to 18" wheels, which are more expensive. Speed costs money, how fast do you want to go?

 

[HangFire]

I have to thank the +1/+2/+3 crowd. Whenever I need a replacement wheel for one of my cars, OEM wheels are listed for sale everywhere.

 

[Cdn17Sport6MT]

The bigger the rim, and the clearer it is by way of spokes, the more it highlights underachiever sized brakes, often evident in Japanese cars, sometimes Domestics too, not so much the German cars.

 

[Cdn17Sport6MT]

For the above reason I'll be the last person on earth to paint my fist calipers (on my Mazda) with anything other than a dull silver that attempts to emulate the un-rusted, as-delivered rough casting look. H*ll, I don't even think that fist calipers (as opposed to four-or-more opposed piston calipers) are worth celebrating (in bright red or fly-yellow) .