Do taller rear tires mess up the speedo? 94 GTS
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Do taller rear tires mess up the speedo? 94 GTS
Thinking of changing rear tires from 255/40x17 to 275/40x17.
Will the one-half inch diameter increase change my speedometer?
Will the one-half inch diameter increase change my speedometer?
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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.
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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)
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)
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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.
I'll go for the taller, bigger rears then. Thanks for the responses guys.
Last edited by wds928; 05-31-2006 at 09:02 AM. Reason: spelling
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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
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
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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
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
Originally Posted by jakermc
Taller rears will also change your effective gearing, hurting your acceleration. If that does not bother you, go for it.