So I installed P237/75R15 tires and my speedometer is reading 4-5mph, is there a way to calibrate the speedometer without the tools to do so I've seen a video of a guy doing it by holding the needle in place but I don't want to take the chance on messing it up, I can't afford a tool to calibrate my speedometer, any advice?

 

What year truck ?

 

1996 Ford ranger XLT extended cab 3.0L

 

The gears are color coded. Find an explorer with 16 inch wheels stock and pull the speed sensor from the differential for your ranger. That should get you closer to actual speed displayed on the gauge.
Your gauge is only off a couple of percent though so it might go too far the other way.