Hi, I recently put a cat back exhaust on my Silverado HD. I never noticed this before, but now that it's a little louder I have been finding my tranny isn't shifting into OD for a long time after a cold start. I live in Western NY and my truck is in a mildly heated garage. After I leave and get to around 55-60 mph, the truck shifts to OD and the r's drop to 2000. Then within a mile the r's bump up to 2300 and stay there until I hit around 12-15 miles. At that time the tranny shifts to od and r's drop to 2000 again. The only thing I noticed when it finally shifts is that the trans temp needle has just barely moved. Does anyone know if this is normal for this transmission? Thanks.

Jason