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Hello everyone new to The Forum Cobalt great little cars I bought an 06 that was wrecked with a good transmission and I also bought an 06 HHR with a bad transmission both vehicles have the 2.2 engine my question is as anybody swapped a transmission from a Cobalt into an HHR I have done a lot of reading and seeing that the gear ratios are different and it would work in my favor for the better fuel mileage on my HHR I know the transmission will bolt right up and everything will plug-in but my question is will it throw the speed sensors off enough to set a code or will my speedometer just be off just a little bit any help would be appreciated thank you. I could just go buy a used HHR transmission but since I already have this Cobalt one laying around that I know is good and I got for next to nothing and the gear ratio would work in my favor I figured be worth a shot
 

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Given they use the same part number, I believe you would have to the speedometer recalibrated in the tune to account for the taller final drive ratio. But yes, it would improve highway fuel economy at the expense of some low-end acceleration. Given the HHR really still isn't that heavy, I'd say it won't be a major deal. And since you have to pay someone to tune the computer (though, being a 2006 makes this much, much harder to do), you might as well pay for them to tune it as well.

But like I said, tuning an '05-06 Cobalt/HHR is much harder than '07+.
 

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The BCM takes the speed signal from the trans and sends it to the cluster in a way that the cluster knows the speed figure, no matter which cluster you have. However, the BCM would be expecting a 3.90:1 final drive, not the Cobalt's 3.63:1.

The TCM needs to be adjusted to send the correct signal.
 
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