Parents bought this car with 30K miles on it, and about 2k miles later, the speedometer locked up at 40, no matter what speed we were going, and 20 in reverse. The car now has over 170k miles on it, and is in perfect running order... except that damn speedometer. it still sticks at forty, and the only time it works, is when it's over 90 degrees out. And I don't want to go driving then. If someone else has had this problem, and knows how to fix it without spending TOO much money, please let me know. It doesn't matter that much, I already have an excuse if I get pulled over, but my drivers license test is coming up, and I need a car with a working speedometer.

Any help is greatly appreciated. I'm a die-hard Subaru fan, I don't want a broken speedometer killing it for me.