wds928

Thinking of changing rear tires from 255/40x17 to 275/40x17.

Will the one-half inch diameter increase change my speedometer?

mark kibort

it will get you closer to actual speeds. im at 305s and its within 1mph now of actual vs indicated.
mk

SharkSkin

It will change it, but whether for better or worse depends on whether it reads fast or slow with the 255's. A taller tire will make the speedo read slower for the same ground speed than a smaller tire would.

heinrich

pretty sure speedo comes off front wheels on the 94

ew928

Bill,
I just went to 275/40-17.
Gotta remember that my speedo is probably much closer to the truth now.
So I can't go sneaking by at 72 in a 65 when I'm actually more than 5 over and that's
probably a very likely ticketable offense in VA.

Sneaking down to VA tomorrow. Giving work the big finger.
Ernest (NYC)

SharkSkin

DOH! Yeah, you're right!

dang newfangled... mumble... mumble...

928Quest

True, the speedo comes from the ABS sensors on the front. I think this is true for years 90+.

heinrich

Love the gif Dave

wds928

Heinrich, thanks for the info. I changed my front and rear's about two years and 18k miles ago. The fronts are 245/40 x 17 (24.7" tall) versus stock 225/45 x 17 (25 " tall). When I was clocked by Virginia's finest last year going to the Hershey Porsche Parade, my cruise controls was set at 78.......and so was the trooper's radar.

I'll go for the taller, bigger rears then. Thanks for the responses guys.

jakermc

Taller rears will also change your effective gearing, hurting your acceleration. If that does not bother you, go for it.

Alan

Mark are your 305's /40? or /35? /30? makes a big difference to the final radius.

Since the speedo is digital on the GTS (all digital dash) presumably it is not calibrated in a normal sense... I'd assume that any car with the same wheels & tire & tire wear would read the same... has anyone tested with new stock tires for accuracy? Would the nominal point be half way worn stocks in fact?

I'd assume Porsche made some provision for bigger/smaller wheels... didn't 17" wheels come in after the digital dash...? is there also some way to compensate for 18" or 19" wheels at least (what are those of you with them doing?) and does that provide an option for non-standartd tires also?

Alan

LaughaC

Any idea where the speedo sensor lives on an '87?

Randy V

This comes in handy:

http://www.miata.net/garage/tirecalc.html

Schocki

Put a hand held GPS in your car and calibrate....
Just an idea...

mark kibort

You must have missed the gearing class.

acceleration = power/(mass x velocity)

no where is gearing a factor in acceleration. Gearing can change your sweetspot range of where you can apply the most HP, but all gearing changes will do is shift the range. There are trade offs with gearing changes. .3" is a pretty small difference on the rear tire. I regularly change between a 26" diameter rear and a 27" diameter rear tire. see no difference in the dyno runs and no noticable differerence on the track.

Mk