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Discussion Starter · #1 ·
I recently discovered that my car registers about 1.5 mph faster than I'm actually going while testing using two different GPS units....

My 97 SAAB came equipted with a sports suspension package that included 16inch wheels on low profile tires..., and I have sinced then switched to 17inch tires that also have lower profile tires, but are approximately an inch and a quarter taller than the original wheels and tires.

If my suspicions are correct, than I've put on a lot of extra miles that are not true miles since 1998.

Does anyone know if or how the speedometers are adjusted to compensate.

regards
 

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I too went to the 17 inch wheels from 16 inch and use the 215 45-17 tires, but mine seem to be about 1/2 in. taller. I calc. that to be a little over 2 %. I thought the vehicle speed signal came from the ABS sytem to the ECU and the speedometer using one of the ABS wheel sensor. How to adjust ??? B e interesting if question was answered
 

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well all you should have to do to get your speedo calibrated is go to your local dealer and have them hook up to the tech2 and calibrate it for the wheels/tires that your running now. should be rather simple and i wouldnt even think that they would charge you, although i wouldnt be surprised if they did :roll: .

Paul
 

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Discussion Starter · #4 ·
I'll try that Paul.., but I didn't know it was that easy for them to do. .

I'll talk with them this morning.

thanks
 

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Discussion Starter · #8 ·
Dealership couldn't do it...apparently the MIM shows the car came equipted with 16" wheels for 97, and couldn't be changed...
 

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wait should having larger wheels/ tires make the car regeseter a going slower. ie. more wheel circumfrence means more distange for each rotation of the wheel :?:
 

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turbokick said:
wait should having larger wheels/ tires make the car regeseter a going slower. ie. more wheel circumfrence means more distange for each rotation of the wheel :?:
wheel size in itself doest affect the speed reading at all, its the size/circumference of the tire that matters, im assuming that with the larger wheels, a shorter than stock tire was most likely used.

Paul
 

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yeah it has to do with the circumference, i went from 15" to 17" and just used real skinny tires (40) and there is no problem because the overall circumference is the same as the 15s with the thicker tires
 

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Discussion Starter · #15 ·
PaulH said:
turbokick said:
wait should having larger wheels/ tires make the car regeseter a going slower. ie. more wheel circumfrence means more distange for each rotation of the wheel :?:
wheel size in itself doest affect the speed reading at all, its the size/circumference of the tire that matters, im assuming that with the larger wheels, a shorter than stock tire was most likely used.

Paul
My car came stock with low profile tires on 16inch wheels..205 /50R 16 now 215 on 17 inch wheels....both of the profiles are low relative to the tire size...and the circumfirance is larger on the 17 inch wheels with low profile tires, and actually looks closer to an inch difference in height .

Yes it does register faster speeds and more miles than you are actually going or have traveled. over time might create some issues if you recently upgraded..but since my car is over 200k miles now...I'm not going to worry about it anymore.

But it might be helpful to those of you out there that have recently or considering upgrading to larger wheel/tire sizes.
 
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