Franks928s

Thanks Glenn for the lesson on computing the overall speed. That, along with Andrew's note, helps me understand better. Regarding the net effect on closeness to the Guard, I am challenged. I recently bought the MY87 as a project car and I'm not certain the wheels currently on it are stock.. After all of the posts so far, I'm concluding that I now understand how to calculate the effect of the offset but what I really need to know is:
1. What's the physical distance between the wheel hub and the guard
2. What's the physical distance between the face of the wheel hub and the inner fender liner.

Am I correct that if I know those two answers, I can determine if a certain tire coupled with any offset will rub either on the outer guard or the inner liner (Or, what the max tire width can be for a given offset)?

Frank

Dean_Fuller

The ride was MUCH more harsh. Of course I did go with 275 on the rear 10" wheel so there is a lot more rubber on the road.

IIRC the wheels I replaced were 17" with a 9" rear.

orangecrush

Consensus on this forum would say that the rear can go 18x10 or 18x11 with no problems? The front at 18x8 or 18x8.5 as well? That is what I am hearing. 19" seems to be pushing the car to the limit of design and could open up more $ for repairs.

I would like a little side wall to help with the odd pothole down here in AZ. Luckily there are not many of those here. I don't see the point in getting to wide on the front tires. Isn't 10" at the front overkill? It seems it would make the car track funny or is it the other way around? I would like 11" at the rear. The less wheel spin and means less money for tires especially if increasing hp/tq in the future.

AO

Depends on the offests, but a 10" rear is certainly no issue, an 11" wheel will fit, but tire choice (in particular the 295/350 can be the difference between rubbing and not - even with the ideal offset.

Fronts need to have an offset greater than 50mm on the 8" and 53mm with an 8.5". The Carrera III wheels (very popular now) have an 8" front with 57mm offset and the 10" or 11" rears with aa 51mm offset ont he 11" - sorry I don't know what it is on the 10", but I know they fit.

orangecrush

Andrew, I like your setup. Seems to work best with the car if you have a supercharger and want to keep the rear end planted. 19" spread that rear sidewall a little thin, something I am a little leery about for everyday driving. Especially at night when you cannot see the pavement far enough out in front of you.

Thoomas

Thanks!

The 275:s were also Michelin PS2 and there was a hugh difference between the 275 and 295 in width. And when I come to think of it when I measured my rims they were actually wider than 11.

Thomas
1990 GT Matteblack

Franks928s

I came across this file from one of Auzivision's posts and found it to be VERY informative about how offsets work, acceptable tire ranges etc. It's basically a compilation of a lot of smart 928 people and their guidance.

Note within the thread why the offset appears to be very important on the front wheel due to engineering design.

mark kibort

I had a cop friend radar me at 60 and 80mph and then correlated it to my tire sizes. I always thought I was going faster than I really was.

mark kibort

generally.

But, in our case, the 928 seems to be off a lot more. a 305x35x18 rear tire seems to give very accurate speeds, based on radar.

you cant even count on the RPM gauge to be accurate. my new one in the black racer redlines out at indicated 6200rpm, where the holbert gauge was a true 6600, calabrated and confirmed with the dyno as well. (all wiring and ecus are still the same)

AO

Agreed the stock gauges are not very accurate. My point was that if you deviate to a larger tire than what you're runnung now, when you drive at 60MPH per your speedometer, your actual speed will be faster than with the previous tires.

BTW, my head unit has a GPS built in and will display GPS calculated speed. Actual speed tends to me about 1MPH more than the digital readout and 2MPH mor than the analog gauge. Running 295/35r18's

mark kibort

get a radar test and get back to me, just to check. GPS depends on slope too. I think I have seen some variances in GPS too, but Ill admit, dont know much about them. hoewever, what I do know is radar is very accurate and have been tested. why dont you test yours? it would be interesting. by the way, give us a RPM vs speed of your car too. see if that is mapped correctly.
compare that to the diameter of the tire too.

curious

auzivision

I can't take credit for creating this (this was Phil's work), but I have reposted it a few times. It's about as complete as it come and really helped me understand sizing... especially about how offsets work.

I understand Tire sizing when I study it and forget how it works shortly there after. I don’t know if they could have devised a more convoluted system… especially since the mix of English (wheels) and Metric (rubber except for radius) systems.

AO

Well the last time I did a radar check, my speedo said 76, and the cop said I was going 75. I got a slip of paper to prove it too. I'm not due for another radar test for another year.

Franks928s

Can anyone help answer these questions for an 87? Kurt? Others??

Franks928s

well that's one way to follow the methodology of "trust but validate"

Frank