Changing out the rear end gears in a SS

[UBB]

lightly boosted application, manual tranny. Who's swapped out the stock 3.45's. Strictly street and cruising uses, no track. Is it worth it? Pro's/Con's blah blah blah????

[pnwdan]

Originally Posted by UBB

lightly boosted application, manual tranny. Who's swapped out the stock 3.45's. Strictly street and cruising uses, no track. Is it worth it? Pro's/Con's blah blah blah????

Getting mine done Thursday. Haven't seen any cons with going to 3.91. Only Pros. Some say go 4.10 for big cams and I'm doing a cam install next but I'm sticking to the 3.91 because I may go forced induction later.

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[Taylortoons]

I have 3.91 gears with a big cam and FI, but still don't use 6th gear on the highway (rpms too low). If I did it over again, I would go 4.10.

[Batmanntexas]

Originally Posted by Taylortoons

I have 3.91 gears with a big cam and FI, but still don't use 6th gear on the highway (rpms too low). If I did it over again, I would go 4.10.

What do you consider to low? What size tire are you running in the rear?

Thanks,
Scott

[gringo]

Cons? Well, you won't be getting that fabulous mpg that we debated about way back when. lol!

[Black 6.2L SS]

Originally Posted by Batmanntexas

What do you consider to low? What size tire are you running in the rear?

Thanks,
Scott

Scott - Damn man, those HP and TQ numbers are incredible! I couldn’t imagine. A huge “Congrats” is an understatement.

Way back on my ‘95 LT1, I changed the 3.42 to a 4.10. I regretted it and paid to have the 3.42 re-installed. The 4.10 was too “short” and I was always out of tach (and traction). The 3.73 gear would have been perfect. .02

[Taylortoons]

Originally Posted by Batmanntexas

What do you consider to low? What size tire are you running in the rear?

Thanks,
Scott

At 75 mph in 6th gear I'm running about 2k rpm on stock ZL1 rims. Not a friendly big cam cruising rpm for any gear.

I haven't switched back to the 18" reps with MT ET R yet, but from what I recall last Summer, the rpms were still pretty low at highway speeds in top gear.

Either way, I only use 5th gear on the highway. It's ok, though, I still average about 7 mpg overall and can drive about 100 miles on a tank of E85. Didn't build her for gas mileage. LOL!

[InFiD3ViL]

Originally Posted by Taylortoons

...I still average about 7 mpg overall and can drive about 100 miles on a tank of E85.

HOLY CRAP!

My god, I bet that feels awesome. Someday I hope to be at that level with mine.

I love that you did this to a convertible. Man, I can only imagine how much fun this car is to drive.

[sixty9fordkiller]

too low of a differential gear for a boosted application on an M10 car will make 1st gear spin city unless you go with a tall tire. You already get out of the hole quick because of the torque from boost. If you wanna be fast, you dont need that much gear. Most FI guys do not run 3.91 gears. In fact, a lot of them go with taller gear


for a NA application, the lower the rear gear, the better! Not ideal for boost

[Taylortoons]

D3vil, she’s the big red drop top monster. LOL! No good reason to do it and few places to use it. But logic wasn’t the reason why.

69Ford, good point for a PD blower where the tq is immediate. On a centri, the curve is linear, so unless you’re dumping the clutch over 4K rpm, it’s mostly cam down low. But, that’s why I run sticky tires in the Summer ‘cause you never know....

[ZOOK]

I have 2010 2ss with a magi 2300 and cam from Jannetty.
650 + WRHP
I went with the eaton trutrac and 3,73 and drive it as a daily driver 80 K on it now.
mpg went from 30 to 26 mpg imperial ... 25 to 21.5 U. S.

Love the setup
Zook

[christianchevell]

Go 3:91 the price is much cheaper than the 4:10 and the 4:10s are prone to breakage with the small gear tooth size of 8.6 rear what has been documented on here by drag racers years ago...now if you had a 9.9" zl1 rear you most likely would go 3:73 as the gears being in production for models make the costs much lower.

Now if you want to go smaller gear teeth you have to realize it makes them more fragile. I recommend a LPW cover and new toes and trailing arms for most...… Good Luck

[Moto-Mojo]

Someone posted a nice chart yesterday on the 3:45 and 3:91 gears mathematically proving that torque and HP did not change but distance traveled did. And I can’t find the damn thread!

But my question is, if the 3:91 gears travel less distance at a given RPM and the TQ and HP don’t change from the 3:45 gears, what makes it such a popular mod??

[pnwdan]

Torque and HP are created by the engine. They won't change based on gearing. The gearing just gets the power to the road. 3.91 gearing is a 13% increase in how fast the power can be applied to move you forward. It's pretty simple. I just changed out to 3.91 gears a few days ago and already I'm pretty happy. It's not a giant difference but it's nice. My car doesn't buck at low speed creeping through parking lots and now I can use 6th gear a little sooner and at less speed.

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