Why so many short tires?

[AndreasK]

I´m not talking to all you guys doing AutoX and such. But the rest of us.

I recently got my 1st 911. (A 1977 911S, with SC flairs) The car has 18" cup replica rims and I´m looking for something more period correct. 15" and 16" tires are few and far between. But there are some possibilities. But I se most guys running 24" tall tires. 245/45-16 or 255/40-17 are common and an option for me. My car now has 245/40-18. A 25" tall tire. I think it looks far better and fills the wheel well nicer. I´ll be using the car out on the open road and perhaps one or two track days each year. I´m not to worried about the reduced acceleration. Are there other reasons not to get a 25" tall tire? If 245/50-16 was a common size to get hold of, I´d be all over it.

Euromeister and Rota Fox´es are likely choices as far as rims go. Does anyone run 245/45-17´s in the back. How´s that working for you?

[darrin]

This might help you with tire height calculations. https://tiresize.com/height-calculator/

Keep in mind that tire height has a direct correlation to a car's ultimate gearing. The taller the tire, the taller the car's ultimate gearing gets. This comes at the expense of performance -- the taller the tire, the higher the gear, the slower the acceleration from stop.

My carrera came stock with 205/55-16 fronts (24.9" tall) and 225/50-16 rears (24.9" tall) 245/45-17s would be nearly an inch larger in diameter than stock -- suspect this would have a noticeable effect on acceleration -- FWIW, there's a great selection of performance tires in this stock size -- running 16x7 fuchs up front and 16x8 fuchs in on the back with these tires (on an SC or Carrera) fills wheel wells nicely and provides (imo) a great balance

[Bill Verburg]

The less torque output from the motor the more these cars rely on gearing and rpm for motivation.

given a stockish motor and stock gearing a '77S was designed around a tire that had ~25.4" OD, in practice the tire isn't round and actual has a loaded/running radius less than that. The tire height is part of the gearing equation.

For better gearing a shorter tire is used.

w/ SC flares you can use an 8 to 9.5" tire comfortably. 15 to 18" each have attractive possible tire sets

You do want a balanced set of tires f/r, but for the moment lets concentrate on the back of the car, here you really want to keep OD < ~25", wheel 8ET10.6 to 9.5ET19

At the short end 225/45x15 OD 23.9 to the tall end 285/30 18 OD24.9" you want to keep the rear tire heights at or under stock for best performance

the wheel width and ET is the biggest determinant of tire width and fitment along w/ chassis setup and available tires.

8ET10.6 x15 w/ 225/50 is a common short rear fitment, 225/45 is a bit shorter, wider wheels allow wider tires to be fitted up to 285/40 x15(OD23.9") which wants a 9.5ET19 wheel, there are a # of in between tires that want 8 to 9.5" wheels w/ custom ET.

8ET10.6 x16 w/ 225/50 is a common fitment here, OD 24.9, the widest is 245/45 on a 9ET15, OD 23.9

8ET10.6 x17 w/225/40 is a common fitment OD 23.8, most common is 255/40 on 9ET15 OD24.7, widest is 275/35 on 9.5ET19 OD 24.7

8ET10.6 x18 w/ 225/35 is possible OD24.1, common is 255/35 on 9ET15 OD 24.9, widest is 285/30 on 9.5ET19 OD 24.9

[wayner]

Also keep in mind the some people run different ride heights on one car compared to another.

a lowered car looks better with 24" vs 25" than one that has not been.

[T77911S]

my 930 has 18's which look good and fit well but make the car sit too hi.
it also slows it down.

[GH85Carrera]

Some of it I think is the Chip Foose phenomenon or that look. It has pervaded almost every car and builder.

They all want to build a hot rod with giant rims and rubber band tires. I am so sick of that look. It has become the standard for all cars, new and old.

I wanted a tire that was logical to use on long road trips, and city driving. I drove to Maine and Canada last summer for a 5,500 mile road trip. I have driven my Porsche in over 40 states and plan to drive my Porsche in all the lower 48 states. A tire that fits a 15 inch rim and can be safely driven in the rain and on long trips and still be used at an autocross is just not available anymore.

I gave up and bought the Euromiester 17 inch rims.




With the 17s

These still look good on a 911 but are summer performance tires. They have orders of magnitude more grip than the last set of tires I had on the 15 inch rims that were mud and snow rated. The do great in rain on the highway and my only downside is I can't drive them in low temperatures.





The old 15s.

[Bill Verburg]

Besides gearing the other main reasons for shorter tire is to get shorter sidewalls which have a better performance envelope, + allowing the car to run at a lower ride height which also enhances the performance envelope.

Optics are probably in there somewhere, but it's not something I've ever though about.

the key is to get the tire OD f/r, tire width f/r, wheel width f/r, stagger, rake and the fitment to the chassis right

[Bill Verburg]

Here's a visual showing some nice tire sets for SC/Carrera, ranging from 15 to 1, max OD is 25" max width is 10.6"

[AndreasK]

Tanks for a lot of good input! I'm focusing on rear for now. Front will follow. My car now has 245/40-18, and for aesthetic reasons I want more sidewall. To me the car drives great and it's low, perhaps to low! 17 is clearly the way to go to get a good selection of tires. 245-255/40-17 is the most common tire for 9" rims. But that's roughly the same sidewall, as I already have. Will there be a significant improvement in performance?

[darrin]

if you want more sidewall, go with 225/50-16 on 16x8 inch wheels - LOTS of high performance summer tire options in that size at tire rack

Or, while the bridgestone re-11 is the only real street option in the 245/45-16 tire size that will also work on a 16x8 inch rim, it's a great tire and is also available in the 205/55-16 size needed for the front.

Seems that if you're looking for more sidewall, going with 16 (vs 17) is the way to go, as this will allow you to maintain faster gearing AND get you a taller sidewall

[Tippy]

I run 26.1" tall tires in the rear, and would prefer even taller since I am turbo and have tons of torque to fill out the fender wells. I did notice a loss of torque off-boost going coming from 25", but I needed the extra "gearing" for drag racing and not hitting rev limiter going through the traps at nearly 130 MPH.

I think even the 26.1" looks short:

[Bill Verburg]

Originally Posted by AndreasK

Tanks for a lot of good input! I'm focusing on rear for now. Front will follow. My car now has 245/40-18, and for aesthetic reasons I want more sidewall. To me the car drives great and it's low, perhaps to low! 17 is clearly the way to go to get a good selection of tires. 245-255/40-17 is the most common tire for 9" rims. But that's roughly the same sidewall, as I already have. Will there be a significant improvement in performance?

245/40 x18 is ~25.7" tall,too much OD

the same wheel could probably use 245/35 @~24.7" but then the short side wall isn't your cup of tea.

So if changing wheels, in back use 8 to 9.5, wider is better for performance narrower for comfort

in 17 a sweet spot is 225/45 & 255/40, @25" OD, you'd want 8ET25 to 30 & 9ET12-15 or 9.5 ET19 wheels.`

for taller sidewalls go to 16 or 15, still using the above o/s, the lower the ET the more potential of issues at the lips and the more restrictions on chassis setup.

[AndreasK]

Originally Posted by Bill Verburg

245/40 x18 is ~25.7" tall,too much OD
.

Too much for what? It fits and I can't say I'm missing performance. But I have very limmited experience. Will I be surprised if I go to a shorter tire?

[rattlsnak]

245/45/16s are a nice and period correct look. Dont go with a 245/50. It looks like a NASCAR tire!!

[AndreasK]

Originally Posted by rattlsnak

245/45/16s are a nice and period correct look. Dont go with a 245/50. It looks like a NASCAR tire!!

Haha, I think it looks awesome (not my car, mine will be lower)


Like I´ve said, I´m new to this, so I´m sorry for rambling on.

My 911 S has the original, rebuilt, 2,7, with SSI headers, 964 cams and a 2 in 2 out muffler. And what you are all saying is I will get real world, noticeably increased performance with a shorter tire?

[tobluforu]

What some people fail to realize is that when you go from say a 205/50 to a 225/45 is that the OD of the tire increases as you go wider. So a 225/45 will "almost" look the same as a 205/50. You also get the benefit of being able to squeeze this tire under an earlier car, the car will handle better, and best of all, it looks cooler with a fatter tire.

[Driven97]

Originally Posted by AndreasK

And what you are all saying is I will get real world, noticeably increased performance with a shorter tire?

Kinda applicable. I AX, and I'm not allowed to directly change gearing in my class. Tire diameter, however, is free. I currently run a 255/40-17 @ 25" tall. There's a new 245/40-15 @ 22.7" tall. Very short and looks kinda goofy on a 911, but I ran a thrust chart to see what the difference would be.



Now this isn't perfect for various reasons, but it's close enough for comparison sake. Basically I take a torque plot from a dyno chart and multiply it out by gearing and finally by tire radius to get theoretical thrust. The chart is 2nd gear only as that's all I care about on course.

So not only does the shorter tire give me ~10% more thrust across the board, it compresses the power delivery to make things happen at lower speeds. Double win. The car would get on cam in the 40mph range, where I have to wait until 50mph on the 17s. On the 15s the car is making more thrust at 40mph than it peaks at on the 17s. AX has a lot of digs from the 30-50 range, so the car would have significantly more poke on course with the stubby tires. Unfortunately, they're on backorder so I can't try them yet.

Of course, there's no free lunch. On the stubby tires I run out of gear at 64mph, where on the correct sized tires 2nd goes to 72. Horses for courses and all that jazz. That and the short tires are just miserable on the freeway - turning a lot of rpm just to keep up with traffic.

My example is a pretty wild swing in diameters, but hopefully it illustrates how a diameter change affects performance.

[AndreasK]

Originally Posted by Driven97

Kinda applicable. I AX, and I'm not allowed to directly change gearing in my class. Tire diameter, however, is free. I currently run a 255/40-17 @ 25" tall. There's a new 245/40-15 @ 22.7" tall. Very short and looks kinda goofy on a 911, but I ran a thrust chart to see what the difference would be.



Now this isn't perfect for various reasons, but it's close enough for comparison sake. Basically I take a torque plot from a dyno chart and multiply it out by gearing and finally by tire radius to get theoretical thrust. The chart is 2nd gear only as that's all I care about on course.

So not only does the shorter tire give me ~10% more thrust across the board, it compresses the power delivery to make things happen at lower speeds. Double win. The car would get on cam in the 40mph range, where I have to wait until 50mph on the 17s. On the 15s the car is making more thrust at 40mph than it peaks at on the 17s. AX has a lot of digs from the 30-50 range, so the car would have significantly more poke on course with the stubby tires. Unfortunately, they're on backorder so I can't try them yet.

Of course, there's no free lunch. On the stubby tires I run out of gear at 64mph, where on the correct sized tires 2nd goes to 72. Horses for courses and all that jazz. That and the short tires are just miserable on the freeway - turning a lot of rpm just to keep up with traffic.

My example is a pretty wild swing in diameters, but hopefully it illustrates how a diameter change affects performance.

That´s really interesting to see! And obviously a significant change for AX. But that change from a 25" to a 22,7" tire is huge. And I´m not going to be running a 22,7" on my street car A rough estimate from your chart indicates a change from a 25" tire to a 24" tire would result in perhaps a 3-4% thrust increase.

The tire I have now at 245/40-18 is 25,7" tall, the "ideal" tire for a 17" rim is 255/40-17, at 25". That would be a change of roughly 2% in trust. I somehow don´t think I would really notice. Or am I missing something?