i have been looking online for a speed calculator and have found a few. the problem i have is that my friend threw larger tires on his truck and his speedo reads higher then his real speed. the calculators i have used are sayin the opposite of that.

i was hoping anyone that went from 31s to 33s can tell me the speed they are really doin compared to what the speedo reads.

 

In general a speedo will read slower than actual with larger tires (and no gear change). At least assuming a speedo that is driven off the transmission or transfer case. Why? It in a sense if just reading the driveshaft RPM and inferring the vehicle speed from that. Thus driveshaft-RPM / axle-gear-ratio = axle-RPM. And axle-RPM * tire-diameter (* some contstant) is equal to MPH. Take a larger tire and spin it the same RPM as a smaller tire and the larger tire will have a higher surface speed than the smaller one. From 31" to 33" you should expect to see the speedo reading about 6% slower than actual. Might be the speedo was reading fast before and now it is may still read fast but less so. What gears does the truck have?

 

the stock gears for now 4.10 it is a 88 v6