You can also google GM VSSB and there are websites out there that will walk you through the steps of modifying the vssb that you have...... do you want to spend time or money....... (you have to be comfortable soldering on a circuit board to do this)
For my tahoe, I went cheap and did the calculations and moved the jumpers around. (did not even spend the money on buying dipswitches) ..... of course I did not get it perfect the first time around so I had to go in and adjust it again. But at that time I had more time than money.
You must have some SERIOUSLY deep gearing in that thing for 35" tires to be reading 80 on the speedo when actual ground speed is 65. The normal (if gearing isn't touched) is that with oversized tires the speedo will read slower than the actual ground speed ("But Officer, the speedometer said I was going 65 in the 65 zone." "Sorry, Sir, but I clocked you at 74." "Must be these new monster swamper mudders I put on here." "That's no excuse, Sir. Sign here, this is not an admission of guilt but a promise that you will appear in court. Have a nice day and drive safely.").