Franks928s

Andrew - you were next on my response list...perfect timing. Is there a magical number (deviation tolerance) before the Speedometer is skewed? Also, what size wheels/tires are you running?

Frank

AO

Any deviation from the stock circumference will change the speedometer - obviously, the further you deviate, the more it'll be off. Don't know what the cuttoff is for the ABS, but I'd guess somewhere around 3-5%.

As for my tire sizes (18" rims), I'm running 235/40 on the 8" front and 295/35 on the 11" backs. Since the speed pickup is off the rear wheels, if i'm going 60MPH, my speedo will show 57.1MPH. If I had choosen the 295/30 tire, the speedo would be more accurate at 59.7, but it doesn't look right, so I'm sticking with the 295/35's.

You might want to look up my excel Wheel fittment Calculator. Sorry I don't have the link handy.

Chuck Z

No traction issues at all prolly b/c the only time the car ever sees rain is at SITM but this year we were parked when it rained......

I also ran the setup with an 18" front (for sale in another thread) wheel/tire and had no issues with the ABS either.

As far as ride quality; Perhaps I'm less sensitive to a more harsh ride but I have 19s on the GTS, 20s on a CLK and 21s on another car and don't think any of them ride very harsh (well except for the CLK but it would be harsh on 15s).

mark kibort

actually, speedo is off from the factory and needs a 26" tire to be accurate.

mark kibort

you got it a little backward below. if you are truely going 60mph, you need a near 26" tire to run 60mph indicated. if you run a smaller tire, at 60mph actual, you will be showing 64mph indicated. (running a 24.5" tire vs a 26" tire)

Sab

I have 19's on both of my 928's and while the handling is great, I would not go that route again on a street 928. The ride is just too hard and it makes even a solid 928 rattle on minor bumps in the road. The car is just not build for it.

Thoomas

I run 19's on my -90 GT.

Michelin PS2 N1 295/30-19 + 235/35-19
11x19 with ET 58 and 8,5x19 with ET52.

As Andrew, I had some rubbing issues on the inboard side in the rear and also on the fender on driver side.

First I bought 275:s for the rear but they were not wide enough for the 11'.

If I would do it again I would look for 10x19 and 265 tires with the same ET.

You can see some pics of mine here:

http://www.garaget.org/?car=69235

Thomas
1990 GT

Franks928s

THanks Thomas! Interesting car with the flat/matte black look...really stealth looking and those tires and wheels fit the look too! I'm surprised that the 275's were not wide enough for the 11"s. I found reference that stated 11" could go as low as 240, preferably be 280 and possibly as high as 320.

Frank

Franks928s

Thanks Sab! It seems like it might be a "Taste great...less filling" conclusion related to the ride. Those who are satisfied by it call it "firm"...those who dislike it call it "hard."

Frank

Franks928s

Mark...how did you learn of this? I'm curious as I have found a few reference posts that all spoke to the 25" standard. (And, I've been viewing my options with that calculation in mind). This could definitely change my game plan.

Frank

Franks928s

What do people think makes the 928 a "harder" ride with 19" wheels verses a smaller wheel base? I've also heard comments that the 928 is not as reliable on wet surfaces with the 19's too.My understanding is the newer porsches run a small aspect ratio and we don't hear those comments often from the new owners. SO - why the 928? Thoughts??


BTW - Andrew..what are your offsets?
Frank

AO

Frank -

Factory speedo's are generally "de-tuned" by about 2MPH - even hondas and toyotas. My calculator has the stock sizes and the calculated actual speeds if you have a different sized tire. If the tire you select is larger than stock, the faster you will be traveling relative to what the speedometer shows.

The reason the 19's are "harder" is there is less sidewall to absord the bumps on the road. It's not a bad thing (although I've never ridden on 19's) but it's different. Handeling will improve.

My offsets, 57 front, 51 rear (remember I run an 11" rear).

Glenn Evans

Nominal stock overall wheel diameter on 245/45R16 rears = 245 * 2 * 0.45 + 16 * 25.4 = 626.9mm

If the final drive ratio is 2.20:1, mph per 1000rpm in top gear = 626.9 * 3.142 * 60 /1000/1.609/2.20 = 33.4 mph per 1000rpm.

Andrew's nominal overall wheel diameter on 295/35R18 rears = 295 * 2 * 0.35 = 18 * 25.4 = 663.7mm

If the final drive ratio is 2.20:1, mph per 1000rpm in top gear = 663.7 * 3.142 * 60 /1000/1.609/2.20 = 35.3 mph per 1000rpm. At the speed at which the speedo read 60 mph on stock rears, it now will read 56.7 mph (nominally; this will vary between tyre types and with wear).

Frank, the stock S4 front wheel is 7 inches wide with a 65 mm offset. Your candidate front wheels are 8.5 inches wide with an offset of 56mm. This means that the outer edge of the rim will be 0.75 inch ((8.5 - 7)/2) plus 9 mm (28mm or 1.125 inch) closer to the guard.

Edgy01

For what it's worth, here's a data point.

When Porsche decided upon 19s for the 997 Carreras (2005 and up) they initially were testing the mules with the 996 suspensions. In time they discovered that the additional weight and stress imposed upon the 996 Carrera's suspension was so extreme that they were destroying those suspensions. They had been planning on retaining the 996 suspension parts through into the 997 version but they were found unsuitable. They were forced to go back in and reengineer the suspensions in their entirety for the 997s.

Having been an old 911 guy and then moved along into a 997S I am constantly in awe at the handling improvements afforded by the 19s. Of course, there is some of a ride comfort change but with their tailoring of the suspensions they made up for some of that stuff--the ride is quite nice. It's not too severe--for a street car.

I personally love the 19 inch wheel but not all suspensions are suitable for them.

Franks928s

Dan, Thanks. Now it makes sense as to why the ride description for the 928 is described differently then say a 997 with 19"s. So, net-net, are you saying the 928 suspension is not best suited for 19's?

Also...had a conversation with my mechanic who told me he saw a lot more problems with the wheel bearings on the older cars as they continued to size up in tires over the years due to weight and stress.

Frank