Does the manual say anything about what those adjustment numbers do, can you just experiment jumping different ones? I'm not familiar with those, is it like a small percentage multiplier for different size tires etc? Never heard of being able to adjust a stock speedo like that. If only you could figure out how the circuit actually converts the frequency into mechanical movement of the needle and set it off by a little bit with a resistor or something simple like that.
I was trying to convert a speedo once and couldn't for the life of me find signal multipliers that weren't in whole numbers, and your gauge apparently isn't off by a factor of a whole number. I have seen speedo adapters that are supposed to be universal though and can be programmed to make any gauge work just like you would calibrate an autometer, but the price of those probably defeats the purpose of using the maxima gauges in the first place... If you're lucky you can find a cog for the speed sensor that gets it close enough but that's a long shot.
Have you given up on the stock gauges? It sounds like your new speedo is in the same boat as your old one. If the tach doesn't read real low, it's probably not high enough input voltage. It couldn't just be off by a certain amount though, there has to be a ratio that it is off, which should tell you it's hooked up to the wrong signal source. Just thinking out loud.