porschefig

I haven't tested the accuracy of the speedometer in my Turbo, yet.
However, I do know that on a trip that was 167 miles (according to the map) my odometer gained 185+ miles

According to Yokohoma, my tires are the exact same radius as the originals. So, I'd assume that my 117,XXX odometer reading should actually be ~103,XXX ! Anyone else notice this??

I can't remember, is the odometer/speedometer still read off the front/left axle on the Turbos?

toddk911

What is stock sizes again???

Also, how do you compute if rear wheels are bigger than front? Same diameter just different profile.

billindenver

No, it's read off a sensor in the tranny.

turboted

Ha. I forget stock tire sizes, too. But I think they are listed on a sticker on the inside of the gas cap door.

Compare stock to aftermarket sizes for both front and rear tires to determine differences in speedo readings, then average the difference. That calculator is also helpful in determining which size aftermarket wheels/tires to buy in order to keep your speedo readings close to factory specs.

toddk911

Stock Tire - 215/60R15

porschefig

I have this written down here:
Stock
F 205/55 VR 16 Width 6.5 Diameter 25
R 225/50 VR 16 Width 7 Diameter 25

I have stock sizes now.. but wanted to go to this

F 225/50 ZR 16 Width 7 Diameter 25
R 245/45 ZR 16 Width 8 Diameter 24.6

My goal was to have the same tire "measured rim width" as my wheels, and the same diameter front tire. BUT, looks like I should try to keep the 25" (or bigger) rear to correct my problem. Darn things will look like truck tires, not gonna happen, I guess my car will continue to put on more "miles" than reality.....

Fluidplay

Stock tires from factory for 86 turbo: front 205/55/16 rear 225/50/16.

ausgeflippt951

Cars '86 and prior read off a sensor off of one of the wheels (think it's front left). In '87 they moved the sensor to read off the tranny.

billthe3

About stock tire sizes and whatnot, remember that tire pressue could also affect the speedo reading. If your tires are over or under inflated, they'll increase or decrease the radius of the tire and mess with the speedo reading. Obviously not as much as changing tire sizes, but it could still affect it.

Fluidplay

Turboted - checked out the tire size calc... cool tool. What's the largest wheel/tire dia. we can safely run on our cars? Looked at a 265/18/35 and that came out to approx. 25.5"

2bridges

both 86 951 trannies I have = spedo pickup in trans NOT front wheel

Fluidplay

Aren't there 2 sensors located there?

Scootin159

Same here (86 951 transmission in my 87 944). I believe the speedo change was in 85.5 with the new dash.

turboted

I have 265/35/18's on the rear (10" wheel) and 235/40/18's up front (8" wheel). I've had no problems with this setup, and my speedo only reads about 2% slower than stock. (note: my speedo may still be innacurate if Porsche originally designed it to reflect higher than actual speeds)

Technically, you can run any tire/wheel size combo that you can squeeze into the wheel wells, though super-large sizes will more dramatically affect speedo readings and may require you to roll your fenders. I've seen 951's here with 19" wheels, so I know that can be done. I've never seen 20" wheels on a 951, but I assume it's possible. The issue of safety has more to do with clearance than anything else (which is also affected by suspension travel to some degree).

Also keep in mind...nearly all wheel upgrades on our cars will require the use of spacers (if adding wider wheels). I don't recommend running wider than 8 or 8.5" wheels up front, as anything wider may cause the front end to get steering happy. Just my preference...

toddk911

I'd love to put some 10's on the back but mine is lowered about 2 inches so not sure if there would be clearance issues.

You ever get to Orlando???