Speedo is out, what do I change?

[linuxgurupodboy]

Hey, I bought my Supra just recently, the speedometer is running about 35% lower than it should be (eg. when it says I'm doing 80km/h, I'm actually doing 110km/k). The last owner did the conversion and says that I shouldn't change the diff to a higher ratio, (besides the diff I was looking at was selling for NZ$695 second hand at a wrecker. Approx US$550). So where do I go from here? I looked here http://www.mkiiitech.com/forums/viewtopic.php?t=40 and I wondered should I get the "'87 - '88 MA70 turbo MKIIIs have a 3.91 clutch-pack LSD differential" instead of the " '89+ MA70 turbo MKIIIs have a 3.73 clutch-pack LSD differential".

But i did the math and worked out that my speedo would end up reading 115 km/h when I'm doing 110 km/h. And then I could do that AND change to bigger wheels without lowering the profile of the tyre.