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All other things being equal a 45 series tyre will produce slower lap times than a 60 series tyre. Note I'm only talking about lap times, not steering feel, which does not make a car faster.
There have been tests, TopGear and other magazines, etc. and all cars are faster with the smaller wheel and higher profile option that a manufacturer sells their cars with. Ferraris, Alfa Romeos down to Holden Commodores.
Can you provide a link to these tests, showing smaller wheels and high profile tires have better lap times, because that is not my understanding.

In this test, in a magazine called Grass Roots Motorsports, they tested a 16x7.5 a 17x8-inch and a 18x8-inch, the 17x8 wheel produced slightly better lap times

"We tested three sets of tires and wheels starting with the Plus Zero upgrade of 16x7.5-inch wheels and 205/55R16 tires. We then jumped to 225/45R17 tires on 17x8-inch wheels followed by 225/40R18 tires on 18x8-inch wheels."

Wheel size: 16x7.5 inches
Tire size: 205/55R16
Mean lap time: 43.353 sec.

Wheel size: 17x8 inches
Tire size: 225/45R17
Mean lap time: 43.155 sec.

Wheel size: 18x8 inches
Tire size: 225/40R18
Mean lap time: 43.339 sec.


I do understand that the greater mass of bigger wheels & tires can hurt acceleration (especially off the line) and braking times.

Personally I like 15in wheels.
 

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I cannot find a good video to post, but I read a lot about low profile tyres making driving fast more predictable.

I think in your test they are all what I call low profile tyres, but in some ways it proves my point as the 18" wheel was slower, but yeah the 16" wasn't faster ... oops, but also it was thinner!, so not really a fair test (?)

If I find a quality reference, I'll return here and link it
Pete
 
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1986 Alfa Romeo GTV6
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Discussion Starter #63
It depends on a lot of factors.
I do believe the gtv6 was tested when 190's were considered wide.
The relation between width and hight, that's the secret.
F tires consider 5 sizes.
Height, width and rim like us and the hight to the start of the thread and thread width.
The total width of the tire does not allow the play a regular tire would.
Also the total hight of the tire helps with the track imperfections.
Going back to my tire I don't think my 225/45 is such a low profile.
After all its almost half the width.
It's 101.25mm against the 114mm from the original 190/60 tires.
 

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@sekal 1978 if you like you can come to my house and put on the wheels/tires from my gtv6, Bridgestone re-71r 205/50-15 (about the same sidewall height as your 225/45-16) on OEM campy 15x6 wheels and see if you have the same instability. I use 205/50 because there are not extreme performance tires available in 205/55. I had a set of Dunlop Direzza DZ-102 205/55 that were nice and comfy but lacked the traction I was looking for. It's a bit of work to do the swap without jack stands and an air wrench.

101mm versus 114mm is a pretty big difference. But plenty of folks have run 16's and 215 or 225 with no problem. That said, its the whole package that makes the difference and 101mm is not like a rubber band. Tire slip angles, camber changes and roll center changes in cornering, sticky tires, comparing race cars to street cars at the limit of adhesion, and even more comparing the forces generated by the speed, downforce and power of an F1 car are interesting theory but in actuality? I would find it hard to imagine much of a difference for the same car with the same tire model between 195/60-15, 205/55-15 and 205/50-15, but a difference at 225/50-15 or 225/45-16. I have run 225/50-15 on alfettas and gtv6, with clearly more traction at the limit and no stability issues..

My own take on this situation is that since this car has been maintained by an experienced shop, an alignment adjustment is the first place to go, cheap and easy, nothing new to buy. Second thought is that your particular tires are not well matched to an essentially stock gtv6 suspension, and wider+extreme performance tires tend to magnify issues that a more compliant tire shrugs off regardless of size. Third thought is that the perhaps a tire is slightly damaged or imperfect, although a shimmy or drift is not present based on what has been said. Your car is not jacked up in the front, granted it doesn't have great 100mph+ aero. Likewise your suspension has been checked and checked over with no glaring issues front or back. So that takes me back to alignment and/or your specific tire.

BTW the best street tire I ever had on a GTV6 was Michelin Pilot Exalto PE2 in 225/50-15 on 15x7 wheels, unfortunately that size is no longer available, and I had a set long ago of Pirelli P700z in 205/50 that were also great on stock 15x6 wheels (originally had TRX tires, then I went to the P700z and then to the Michelin PE2).

Just my perspective, not dismissing any of the other good comments and others experience on tire sizes, suspension issues, aero effects...

Good luck, Peter
 

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1986 Alfa Romeo GTV6
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Discussion Starter #65
Thank you for the offer @Peter A . Will eventually meet to grab a beer or something alike. Will not bother you in this heat to put 2 cars on stands (only have 2 stands ), and swap wheels. Will probably leave that for Paul at EuroTech. Although, the the test drive we could do up and down 78, or any other, that would be fun. With the new virus and the almost bankruptcy of the states safes, Cops now have automatic pens that write tickets at the speed of light.
third thought is that the perhaps a tire is slightly damaged or imperfect,
I thought of that. Being a super soft tire and rolling once a week, or every other week, could it be getting "square" because they stand in the same spot for so long?
 
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