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I have an 09 Ford Focus with 205/50-16. I'd like to increase diameter to 205/55-16 to gain better ride, less noise and maybe gain a little mileage. Besides affecting my speedometer is there any other negatives?  

Discount Tire doesn't list that size as fitting the Focus.  I think that means there is interference somewhere.  They do list a 215/50R16 though.

Oh, and changing tire size has very little affect on fuel economy and noise.  Those 2 properties are more affected by the tire itself.  

i totally agree with capriracer..

Oh, and changing tire size has very little affect on fuel economy and noise.  Those 2 properties are more affected by the tire itself.  

it depends upon where the sweet spot in the RPM range is... and on an 09 model.. trying to improve what ford engineers spent months or years calculating..  with fully instrumented test mules...  you are not going to be able to improve it with the seat of your pants.. not saying its not possible.. but just improbable..

cars that were produced when the national speed limit was 55 MPH can improve SOMETIMES from increasing tire diameter to reduce engine speeds to keep up with traffic..    if you live in montana.. or some wide open highway location..    using the dash board gauge to calculate the sweet spot in fuel economy.. might help.. .

changing driving habits to hold your right foot as still as possible when driving.. so you are NOT continually depressing and releasing then depressing the gas pedal again and again will greatly improve your fuel economy..

you can pick up displays that plug into the OBD2 port.. give you info on what the fuel control system is doing.. so you can learn to optimize your throttle habits..