Got a question: I just ordered some Vredesteins Sessanta's for the rear 275/35/18 and noticed according to the specs they are about an inch and an 1/8 smaller in diameter than the runcraps that went flat from sitting over the winter (Dry rot) - so I am wondering what that translates into differences in the speedometer readout? What is involved in re-calibrating to read correctly? Posted this in the general forum also, but didn't want you guys left out.

 

None. The computer doesn't compute speed by revolution but by actual wheel speed.

 

Are you sure about 1 1/8" smaller in dia.? Thats a lot and I think your ABS won't like it.

 

Your rear tires should be a little larger in diameter than the front tires and certainly no smaller.

 

Its the same size as many other forum members have and I don't recall any of them having an issue. As I don't have them yet in hand, all I have to go by are the specs provided for the Vredensteins and the other specs available for other tires on Tire Rack. Its kinda interesting to see the diameter variation between manufacturers even when the tire size xxx/xx/xx is the same.

 

Are you replacing rears only? With the price of those tires I would certainly get a full set. More than 1" diff in OD between tires of identical sizes is pretty significant.

Edit: Can I ask from where you are buying and getting the specs on the Vredesteins?

 

The tires AREN"T identical size - they don't offer it. The diameter for the ones I am buying is 25.56" The original size 275/40/18 Goodyear are listed as 26.7". I bought new Kumho's last year for the front as I didn't anticipate having to buy new rears this year. I changed rims twice last year after I bought the car, once because I bought a full set with one bent one (yes from a forum member who refused to work with me on it) and I couldn't live with it. Learned my lesson and bought new ones (C6). Had the tires re-mounted again and now I need rear tires. Long story short I am tired of changing tires/wheels etc. and storing the others. If I didn't already have practically new fronts I would definetely be buying all new tires. Found a great deal east of the bay on that place you can't mention here. Free shipping too. A couple of side notes: I wasn't about to give Goodyear anymore of my business for $450 tire that can't hold air (with plenty of tread) from sitting in heated storage. All I've heard has been good things about these tires - I'm sure unless my rear end is dragging on the ground these are going to be much better than what they are replacing, especially in the rain. Since I do not race and seldom drive it hard other than acceleration, I doubt I will notice any negative effects from a size difference. If I do and its significant, then I will change them.

 

Sorry, I missed that fact that you were going to 275/35, so that makes more sense. Did you check the OD of the Kumhos you have on the front? Others have reported issues with AH/TC if front OD>rear OD.

 

You may be thinking about gear changes, which have no affect on the C5 speedo, since the speed sensor is on the output side of the differential. However, smaller tires will affect the speedo. As mentioned, the new rear tire is over 1" smaller, which will probably also cause ABS/TC/AH problems.

 

The correct size tire for the rear is 295/35-18. The 275/35-18 can be used in the front along with the rear 295's. The 275 is too small for the rear.

 

275 is fine, but it should be taller.

 

An FYI, after posting the original message and getting some quick responses, I was able to change my order in a nick of time to 295/35/18, which are taller and hopefully alleviate any concerns. I should have them next week. Thanks