Hey guys! I'm a new second gen owner that I'm currently restoring & one of the few issues I have with the truck is the speedometer isn't calibrated. I have 265/70/16 tires on the truck and the speedometer is way off. I changed the transmission speed sensor with part number SC105 and It's reading the same. I have never had a car/truck do this. I don't know the oem tire size as I replaced the tires with the same size that were on the truck when I got it. The speedometer is saying I'm going 25 mph but with a GPS app I'm doing 37mph, on the highway with the speedometer reading 60mph, but I'm doing 90mph lol... It's a pain. Any ideas on what could be the fix for it?

 

There is a pinion gear in the transmission output housing that can be anywhere from 30 to 39 teeth depending on gearing and tire size. However the tires you have now are not that much larger in diameter than what came from the factory back in the day (less than 5%), certainly not enough to read off by 50%. The other possibility is that either the transmission or the rear axle was swapped and now the speedo gear doesn't match the axle ratio. But 39 vs 30 teeth is only 30% difference not 50%. It could be that the speedometer itself is bad but that seems unlikely. To test it you would need either an oscilloscope or a signal generator, 55 mph should be 118.8 pulses.

 

You will need to know your tire diameter and rear end gear ratio to determine what speedometer pinion gear should be in your transmission to render the correct speedometer reading. You can calculate this using the link below:

https://www.tciauto.com/speedometer-gear-calculator

 

Is there a transmission difference between a 97 and 98 5.2 2wd? I don't think the rear end was changed at all & it has that 3.55 rear ratio.

Edit: I now remember I had to get the torque converter for a '98 not a '97 but the filter kit for the '97 was right.

 

There are some significant differences between 97 and 98 when it comes to the speedometer. In 97 the speedo was driven from a vehicle speed sensor at the transmission, in 98 the speedo was driven from the PCM by way of the rear axle speed sensor that is also used for the anti-lock brakes. There is still a speed sensor in the transmission but it's in a different location and functionally different.

 

I used part number converter from O'Reilly, #CR91, and it worked. Based off that number it only fits 1996 and 1998, rams, not a 97(lol) truck drives and accelerates fine but I can't get a good speed reading after changing the input sensor. What would you say I do from here? I have no idea how to test cluster gauges. Do I change the abs sensor in the rear too? I got the dorman output sensor too that I haven't changed yet.

 

I have a '96 and am trying to figure the same issue out. I talked to a local transmission shop and there is something with 4 settings. I have yet to figure out where it's at though. On the interstate, GPS said I was going 73.2 mph but the speedometer was showing 80. When I get done with the body and paint work, I plan to see if my odometer is off too. That will be easy. The mile markers on the interstate are set one mile apart. Some of them are even fractions of a mile.

 

That's likely a misprint, as the 98's don't have the VSS in the trans/t-case.