So I stuck some 20" KR rims on the '08, using the 275/65-20 tires. Stockers were 275/70-18. I programmed the CTS with the 20" sizes and it reads 3% slower than the speedo. Programming the 18" tire gives a 5% slower reading. So I got to wondering if the speedo was off. I had a popo friend shoot me today, and the speedo is dead on with his laser gun, which was certified last week. I got screwed out of 5K miles on the 100K warranty
Installed CTS.
Speedo reads 10% faster than CTS when set for stock tires.
Installed 20" KR rims.
CTS reads 5% slower than speedo on corrected tire size for 20" rims.
Speedo reads correct by laser detector
Deduction: speedo is 10% faster than actual speed on stock 18" tires so warranty expired 10% quicker than it should have.
I hope that version makes sense. I'm in Italy sitting on the shore getting drunk. Maybe that helps:dunno::woot:
Woohoo! Well unless your CTS has a GPS function I'm sorry to say that using the tire size will not be accurate unless you get really lucky.
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