I understand from what I read the the 4T45E was used in a few different models including HHR’s….my question is…can a different GM model transmission be put in an HHR and function correctly or is there different gear ratios and or programming?

 

You can swap 4T45E transmission, there’s a compatibility list on line, yes there are different gear ratios

 

What would you like different? The 4T45 is overkill for your car, even used in some GM 6-cylinder cars.

I've wondered if changing the ratio would freak out the ESC. Maybe someone here knows. Seems you can change manual transmission ratios without trouble.

 

The 4T45E is the automatic transmission in every automatic HHR. The final drive ratios aren’t that different to confuse the TCM

 

no change….just extra transmission possibly