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Strangely, on the V6/8spd, the towing difference is huge. With the 3.21, you're limited to just under 4500lbs, With the 3.55 you're up to around 7000lbs. When I was looking, these were the limits on a 2013 Pentastar. There was no option for a 3.92, and with that limit, I don't think a 3.92 would be necessary.
The OP didn't say it was a deal breaker, just looking for info on the possible ramifications of the change. it also depends on what he's looking to get from the change. If I had bought the '13 I referred to, the change to the 3.55 would have been perfect for my needs. The 3.21 with the 8spd was actually very nice to drive around town, I just wanted some more towing capacity.
My '09 5.7 has the 3.55, and I think the 3.92 might be better for my daily driving, but it's a different engine/trans combo.
Personally, I don't think the change to a 3.55 will make much of a difference in MPG.
So engine RPM I think is a good measure here, now I'm not 100% sure, but I'm figuring off 20" rims and factory tire size ~32" diameter. I think the 17" rims end up around the same total diameter right? I'm not claiming to be an expert, and full transparency I used this website to get these #'s: Engine RPM Calculator
I sourced the gear ratio from the Wiki page on the ZF 8-speed.
So 3.21's cruising at 70mph in 8th gear (.67:1) would be turning at: 1581 RPM
So 3.55's cruising at 70mph in 8th gear (.67:1) would be turning at: 1748 RPM
So your motor will turn about 200 RPM more down the highway in eight, so I'd have to imagine you're not going to see bad mileage at all.
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