2000 Isuzu FRR box truck


New Recruit
Hello, I have few questions on a recently purchased 2000 Isuzu FRR truck. There isn't a lot of information on the internet on the problems that I have encountered so I'm hoping you guys would point me in the right direction.

1. My speedometer in reading 7mph over the actual speed that I am driving it with. So for example when I'm doing 75mph it shows 68. Do you know how can I get that adjusted?
2. Related to the first problem, how can I increase the speed limit that is set for the vehicle? I think on the speedometer I can only do 75mph and as I said I actually go with 68, so I was wondering how could I increase that limit?
3. Is it possible to get a truck manual form you guys, when I purchased the truck it didn't come with one?
4. My fuel gauge is reading wrong. On the fuel tank it says it has 42 gal capacity and it seems to be reading fine until I use about 20 gal. After I use those 20 gal the gauge reads fine to the mid point and shortly after maybe 5 more gallons used it goes to empty and stays there, any idea where the problem might be and how to fix it?

Would really appreciate some help with my truck, thank you!