I've finally wore my original rear tires down to slicks and I'm looking for some replacements. I've just about decided on the Khumho ecsta spt as it seems to be a pretty good value and I don't compete just a daily driver. I was wondering if the differenig treds would present a problem?

 

At the limits it can be a problem. There is no way to know how two different tires are going to handle together until you get them on the car. It would not hurt the car to mix them but handling is the best with the same tire on all four corners.
If I can help let me know.

 

I am wondering about different tires for the rear as well. This car is my daily driver with 10 miles round trip on city streets, it rarely driven on highway. Since I live on the coast of So. Cal. rain is rare and only in winter. There are 3 tires that I may choose from to replace the RE050 before winter: Avon Tech M500, Kumho ECSTA SPT and Fuzion ZRi.

If you go with Kumho ECSTA SPT, please post your impressions about the tire.

Section widths, tread widths and overall diameters of OEM RE050 and other tires in size 245/40-17, according to TireRack:

Bridgestone RE050: 9.6" - 9.0" - 24.6"; $200
Avon Tech M500: 9.8" - 9.0" - 24.7"; $115
Kumho ECSTA SPT: 9.8" - N.A. - 24.7"; $105
Fuzion ZRi: 9.8" - 8.5" - 24.7"; $97