Drivetrain > Differentials, Axles & Brakes
lower gearing choices
bbbbuick:
Hi all,
I've been debating whether to go OD trans or lower rear gear set to get better (lower) rpms on highway for quite some time, I'm currently running 3.55s. I've settled on rear end change as the method, mostly due to cost, but wonder how low to go. From what I've read from the forum it seems 3.08 or 2.93 is what would have come stock. JDRace is out of those but does have a set of 2.73s. Anyone running that low a set that could comment on drivablility? Thanks.
Elvin
gssizzler:
Are you running a nailhead with the stock st 300!? I know on my buddies skylark which is a 300 st 300 combo with a set of 2.78’s posi ,it is running 500-700 rpm less at highway speed! His car gets the best mileage and rolls down the road the best at 60-65!? If this helps!? So not fast off the line but super great cruising gear and not beating things up! Unfortunately they don’t make the 2.56 - 3.08 gears new that I know of and no posi units new under 2.93!? I believe so you have to find good used! I had to find a good set of 3.08’s for my convertible! Was a challenge! Put a want add up here and on V8!
WkillGS:
To put 2.78 gears into your car you will need a new carrier, the 3.55 carrier will not work with the 2.78's.
Does Jim @ JDRace have everything?
There are RPM/gear/speed calculators readily available to help you determine what the effect would be:
https://www.randysworldwide.com/calculators/rpm
Going from a 3.55 to a 2.78 would change your rpm about 600 rpm, or give you 15mph more at the same engine rpm.
Adding a GearVendors overdrive will give a similar result. The 0.78 OD ratio would make your 3.55 like a 2.77 when engaged. Cost is about $2600 for the setup:
https://www.gearvendors.com/hrgm3s.html
I have a 2.78 in my '66 Special with the 300 engine. I opted to put a Richmond 5 speed in it. I love it!
Jim:
Hi,had overdrive in our last three cars really miss running 70 at 2,300 rpm .jim
dsags:
--- Quote from: WkillGS on June 07, 2022, 06:23:34 PM ---
Adding a GearVendors overdrive will give a similar result. The 0.78 OD ratio would make your 3.55 like a 2.77 when engaged. Cost is about $2600 for the setup:
https://www.gearvendors.com/hrgm3s.html
I have a 2.78 in my '66 Special with the 300 engine. I opted to put a Richmond 5 speed in it. I love it!
--- End quote ---
Explored a gearvendors unit for a 65 Skylark. The owner said that the tunnel would have to be beat on quite a bit to make it fit.
Dan
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