33s, tuner, speedo out of whack

[Travis Crawford]

Shortly after getting the truck I put a lift and 33s on it. When comparing the speedometer to Google maps my mph was spot on but my shift points going into overdrive was terrible. 

 

Just got a superchips flashcal updated the tire size to 33s, and the speedo is now 3mph higher than Google maps. Shifting is better. 

 

How do I correct both?

[m3n00b]

Did you measure the actial height of your tires?  33s are usually a bit smaller than 33".  

[Travis Crawford]

It only let's me select full sizes. 

[CamGTP]

Try selecting the next size down. If that doesn't fix it, you should contact Superchips directly to see if it's an issue with their software.

[mikeyk101]

I ran into the speedo error when I used the Rough Country speedo correction. I have 275/60R20 tires which are basically 33". I did start using 40-42psi in the tires but when I selected the 33" tires with the Rough Country correction, it actually showed slower on the speedo than I was travelling. I went down to 32.25 (IIRC) and the speedo was dead on. In the past, many speedos would usually read a slightly faster than your actual speed just to keep you from getting caught speeding. I prefer to have my speedo read the exact speed I'm moving at.

[Travis Crawford]

Calculated a 3mph difference to be 1.5" smaller. So I set my tire size to 31.5 and my speedometer is now correct. 

 

But still ~80% of the time if I set the cruise to 62mph. It stays in 5th gear at ~2k rpms. If I adjust my cruise down 2mph to 60mph it will then shift into overdrive and be fine. Or if I hit a downhill slope it will also shift. 

 

Considering replacing the speed sensor on the transfer case. Just trying to avoid wasting more money. 

 

 

[CamGTP]

Replacing the speed sensor will be pointless in my opinion.

 

It needs specific transmission tuning to make it do what you want. That is something the superchips just can't do. Fixing the speedo is one thing, changing the whole shifting schedule is another.