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· Registered
2004 FXT 5mt
275 Posts
3.4% is just slightly too much. Tire Rack recommend 2% or less in diameter difference. What you will find is that even at 3.4% difference, not only will the ODO and Speedo be off but you will also use more gas because you've effectively increased RPM due to the smaller diameter. I know this for a fact because I ran my WRX tires/rims on my FXT and the calculated difference was 4%. On the freeway the higher RPM being turned for a given speed was very noticeable. Over a long road trip, it would no doubt turn into a measurable increase in fuel consumption.

Every tire is different and manufacturers don't all measure the same, so calculation alone won't give you the actual numbers until you measure the exact tire you're considering. I'd give Tire Rack a call and propose this to them and see what they say.
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