Different tire diameter, does it matter?
 

Just found a set of really cheap Bridgestone SO3, which I think some people here has given great feedback.
Sizes where not 100% correct though.
Rear 255/40/17 ( original size)
Front 215/45/17( smaller than current 205/50/17)
I suspect that the front tires will rotate at higher speed than the rear but will this mess anything with ABS or speedometer?

In a perfect world, yes the front tire rotation will not be matched to the rears. In the real world, its probably not enough to make a difference.

Some owners like to keep their cars "spec on" and others are willing to compromise a bit if its not going to create problems. Depends where you come in.

Actually, given the lower aspect ratio (sidewall profile), the overall circumference will be fairly close . Certainly not enough to effect the the ABS or speedometer that much .



Stock Tire - 205/50R17 Search Tires Tire 1 - 215/45R17 Search Tires
Section Width: 8.07 in 205 mm
Section Width: 8.46 in 215 mm
Rim Diameter: 17 in 431.8 mm
Rim Diameter: 17 in 431.8 mm
Rim Width Range: 5.5 - 7.5 in
Rim Width Range: 7 - 8 in
Overall Diameter: 25.07 in 636.77 mm
Overall Diameter: 24.61 in 625.09 mm
Sidewall Height: 4.03 in 102.36 mm
Sidewall Height: 3.80 in 96.52 mm
Radius: 12.53 in 318.26 mm
Radius: 12.30 in 312.42 mm
Circumference: 78.75 in 2000.2 mm
Circumference: 77.31 in 1963.6 mm
Revs per Mile: 829.6
Revs per Mile: 845.1
Actual Speed: 60 mph 100 km/h
Speedometer1: 61.1 mph 101. km/h
Speedometer Difference: - Speedometer Difference: 1.868% too fast
Diameter Difference: - Diameter Difference: 1.84%

You shouldn't have any problems with that size & you may like it better!