Changing to all 4 rear rims is the least expensive way to gain a smoother ride and less expensive tires.
I went with 195-55-15 General AltiMAX RT43and have been extremely happy with them! they ride much quieter, handle bumps in the road better, and seem to go a bit straighter in side winds. Not able to comment about winter driving yet, but that too will happen. Cheaper price than Kumhos or Conti's ($68.47) and a longer wear rating to boot.
http://www.tirerack.com/tires/tires.jsp?tireMake=General&tireModel=AltiMAX+RT43+%28H-+or+V-Speed+Rated%29&partnum=955VR5AMRT43&vehicleSearch=false&fromCompare1=yes
I also have four 195-55-15 General AltiMAX RT43s on 15x5.5 rims. Finding 2 reasonably priced used rims was the hard part. I ordered my tires at Discount Tires in Sequim and had them mount them. 32 psi in the front and 36 psi in the rear. The Discount Tires folks expressed their concern that I was ordering very much out-of-spec tires for my car. They also told me that I may have rubbing issues with the front tires. Duh!
@
mattwithcats : I originally had Discount tire mount the best rear OE 175-55-15 on one of my 15x4.5 rims - to be used as the spare tire. Discount tire told me that 175-55-15 was the largest size available for the 4.5 inch rim.
I have since bought a 3rd 15x5.5 rim and a 5th RT43 for use as a spare. Some time in the near future I will clean the 2 15x4.5 front rims and post them for sale.
Here is my smart with 195-55-15 General AltiMAX RT43 tires on all four corners. Note: when I had the Roadmaster baseplates and cross bar attached, the wheel well clearance was reduced by about a quarter of an inch. Do I care? Not at the moment. If I do/when I do, I'll buy four nice fender flares, have them painted to match, and have a body shop widen the front wheel wells and hide the work with the fender flares.
Some numbers (from my Discount Tire receipts):
four RT43 tires @ $76 each , lifetime balancing (includes the 175-55-15 spare tire), lifetime rotation, four TPMS rebuild kits, shop/disposal fees, sales tax = $462.18
RT43 spare tire @ $76, lifetime balancing, lifetime rotation, NO TPMS, shop/disposal fees, sales tax = $103.72