How far off is my speedometer going to be since I changed to 205-55-15 tires.

Unfortunately, there's no perfect formula as there is quite a difference in rolling radius from tire manufacturer to manufacturer. If you know your old tire's circumference and the new tire's circumference, you could easily apply a correction factor, but the speedos were never calibrated that accurately in my opinion. They often tend to be optomistic!

You might want to find a buddy with the iPhone app that gives you a speed while you're driving (obviously using GPS), and compare that with your indicated speed. Otherwise, find an interstate and time yourself while driving 60mph for a measured mile (using their mile markers). Either way should give you a pretty good estimation.

I believe that if you need to make corrections, North Hollywood Speedometer can probably do the calibration work. I've been happy with their services in the past.

Cllick on the "914 Info" tab at the top right of the page.
There's a tire size calculator that will give you a pretty close estimate.
Mike

I got a GPS for xmas and just went out for a spin a few minutes ago. With my 205x55x15 tires the speedometer reads about 4-5mph fast (meaning speedo says 40, gps says 35)

I have used them to do two of my speedos over 25 years and they do a great job but not cheep either!

Thanks all for the responses.