speedo mph
Le Mans Master
Joined: Aug 2005
Posts: 5,375
Likes: 328
From: CA.
St. Jude Donor '13-'14-'15
It's not the size of the rim but the overall diameter of the tire when installed on the rim--A stock C5 has a rear diameter around 26.50" so if your 20's are close to that you'll be fine--If they are bigger (or smaller) then you'll need to get the computer adjusted to reflect the new size --The 20 will probably be bigger so your speedo will read slow--you'll be going faster than it reads--ticket time !!!! Also if you have an A4 it will affect the shift points of the trans--eventually creates a domino effect on most engine parameters as they are intertwined--wrong speedo reading will get you -----wrong shift points--wrong load sensing and wrong fueling--- hits wrong timig cells
Burning Brakes
Joined: May 2006
Posts: 1,049
Likes: 2
From: Houston TEXAS
It's not the size of the rim but the overall diameter of the tire when installed on the rim--A stock C5 has a rear diameter around 26.50" so if your 20's are close to that you'll be fine--If they are bigger (or smaller) then you'll need to get the computer adjusted to reflect the new size --The 20 will probably be bigger so your speedo will read slow--you'll be going faster than it reads--ticket time !!!! Also if you have an A4 it will affect the shift points of the trans--eventually creates a domino effect on most engine parameters as they are intertwined--wrong speedo reading will get you -----wrong shift points--wrong load sensing and wrong fueling--- hits wrong timig cells
Well said. All that...and you ABS system may become more aggressive too. I always try to advise my customers of potential side effects of increasing overall tire-wheel diameter. Kudos for taking the initative to do the research yourself.97C5
Discount Tire Asst Mgr
14 years
