Differential change from 321 to 355

dillardgl · Oct 10, 2017

Hey has anyone changed their gear ratio from a 321 to a 355 and if so how much did it change your fuel mileage. I have a 2015 penastar .

KingTuna · Oct 11, 2017

You would probably spend enough on the gear swap that you wouldn't get your money back in gas mileage.

huntergreen · Oct 11, 2017

In my opinion, you need to go up to 3:92 to make this worthwhile. Yes it will affect mpg.

Wahrsuul · Oct 11, 2017

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.

RLJ10X · Oct 11, 2017

Spending a bunch of money on a gear change, then fretting over a few pennies on gas, makes no sence, neither does tuna's reply.

4.10s weren't a factory option in my truck either, but it was the best thing I did for my truck. If I had to drive 3.55s, I would trade the truck.

If I keep it under 60mph, and it's not windy, it still does pretty good gas.

You'd like 3.92s. A lot.

Wahrsuul · Oct 12, 2017

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.

shack · Oct 12, 2017

I think this is right

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.

Differential change from 321 to 355

dillardgl

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KingTuna

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huntergreen

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Wahrsuul

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RLJ10X

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Wahrsuul

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shack

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