New owner here - '03 GT with a question about tire size

connolla

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Hi there, new member here! I recently picked up an 03 GT with a 5 speed that I'm really enjoying so far. I just have a question about tire size (apologies if this has been asked a million times before, I tried searching and didn't really see anything). From what I can find online the stock tires on the car would've been 245/45R17s, but the previous owner put on 225/50R17s in both front and rear. The speedometer is off by about 10 MPH and I'm wondering if that might be why? I was told that the car still had original gear ratios. Would I need to get the speedo recalibrated regardless or would putting stock size tires back on correct that?

Thanks!
 

92wastheyear

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I have 245-45-R17s on my 96. I would love to help you out, but I am out of town right now or I would measure from the ground to the top if the tire and you could compare that measurement to what you get when doing the same.
If they are different heights, then there is the cause of your speedo issue.
If you are still looking for an answer when I get home (about a week), I will happy to measure mine and let you know
 

07GtS197

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The height is different and could contribute to the difference. Is it a constant 10 mph or does it go up gradually with the speed? You’ll need a tuner to adjust it but I’d get rid of those tires myself. Yes it’s expensive but you have a smaller contact patch with those tires and more sidewall.
 

connolla

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Thanks for the help guys. Yeah, it's a pretty constant 10MPH difference, and the tire height looks to be 25". Is there a certain benefit to running smaller tires? I'm just trying to understand why the owner before me would downsize.
 

connolla

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Gotcha, thanks again. Guess I'll look into 245s - I'll have to recalibrate the speedo after putting those on anyway, right?
 

92wastheyear

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Gotcha, thanks again. Guess I'll look into 245s - I'll have to recalibrate the speedo after putting those on anyway, right?
I wouldn't think so. If they are the stock size for your car, the speedo should work correctly
 

92wastheyear

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Thanks for the help guys. Yeah, it's a pretty constant 10MPH difference, and the tire height looks to be 25". Is there a certain benefit to running smaller tires? I'm just trying to understand why the owner before me would downsize.

Hmm...I finally got back from vacation and had chance to measure my tires. Also 25" top to bottom for my 245/45 R17. The tire size may not be the issue with the speedo.
A constant 10mph difference sounds odd for a tire size issue (upon futher thought). I had bigger back tires on my old '66 coupe back in the day. I recall getting pulled over on the freeway one time. Cop asked me what speed I thought I was going. I was honest and told him 70mph. He said he clocked me doing 78. He was the one that pointed out that I had oversized tires and then let me off with a warning...but not before telling me that it was a nice car.
Point is (after rambling), when tire size causes a speedo issue, it always off by a percentage rather than a fixed number. When my speedo said 70 it was really 78. However, when my speedo said 35 it was really 39. (When the speedo said I was going half the speed, it was off by half as much)

Hope that made sense
 
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connolla

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Interesting, thanks for the follow up! I'll have to keep looking into it
 

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