Width is not so much a factor in throwing off the Speedo as is aspect ratio. Width of the tire just means more rubber is meeting the road allowing you to grip a bit more of the road at the cost of some cornering performance.
It'd be nice to know what size rims you currently have. Let's assume you have a base model 4th Gen. That would put you with a 245/50R16 and 16x8 rims. Now, you can easily put some 255/50R16 on a 16x8, but I wouldn't go any higher than that. (255mm width with a 50% height aspect ratio) You really need to be more concerned with how tall your tire/rim combo is as opposed to how much rubber meets the road. Here's a nice link for you to do the math, I'm not going to do the math for you. Be wary about going plus size, you may need to space the rim from the hub so you aren't hitting the caliper and putting on spacers isn't really something you really want to do. (It'd be better to do a hub conversion really than to use more than .5in in spacer. IMHO)