Re: rrv sproket size
Pretty sure OEM sprockets are 16/42.
It's odd that your speedo reads to slow. Typically it's the opposite, anywhere from 3% to 5% too fast. And that's with OEM sprockets. Most people reduce ratio and do the -1+2 or +3. Or simply a -1, in your case a 15 tooth. Your speedo will display a higher error, as much as 10% to 20% depending on your sprockets and tire size. And it's always to fast - not slower than actual speed.
A speedohealer will solve your problems. I use one, it's easy. Set it and forget it.
You may have other problems with your speedo.
You're talking about an error of around 30% too slow.
That's probably not sprockets. Count the teeth on your sprockets to be sure.