Gear ratio in differential

Asked by Mar 23, 2014 at 10:18 PM about the 1965 Pontiac Parisienne

Question type: Maintenance & Repair

I have a 65 Parisienne Convertible Custom Sport
I have installed a 4L80e transmission in place of the 2 speed power glide that it came
stock with.
I want to install lower gears to take advantage of the gears in the transmission.
The problem is identifying the differential I have, and the lowest gears I install in the gear
The only identification on the axel tube are MBC
None of the sites I've searched list MBC as a GM or Pontiac code.
Can you help with identifying the differential I'm working with?
Thank you!
Brent in Edmonton, Alberta Canada

6 Answers


What I could find is your rear end is a 3.90 to 1 ratio, that was common for the 2 speed automatic. I would leave it alone, 3.90 should be low enough for good out of the hole performance and still high enough for good mileage with the trans and top gear for highway speeds.

I wish I actually did have those gears, as I have a very sluggish out of the hole reaction ;-( From what I can tell from other sites I likely have 2.73's but perhaps the 3.90's are what I can fit in this differential? Can you recommend a source for gears? Thanks! Brent


Rockwell made a good set if you can still find them. If you do have 2.73 I don't think the carrier will handle the change to 3.90's. If you have a speed shop close to you they might be able to help with the configuration.


You can determine the gear ratio of the diff with one rear wheel off the ground. Put a mark on the driveshaft - tape, paint, whatever. Then turn one of the rear wheels 10 full turns, while you count the number of times the driveshaft turns. Divide by 10 and that's your gear ratio. (If you count 27, divide by 10, your ratio is 2.7). To come up with the "best" diff ratio for an overdrive trans you need to consider your tire size and intended RPM range. Go here: and scroll down to the RPM calculator. All you need is the MPH, Diff ratio, and tire diameter. Try a few gear ratios until you have something you can live with. Then check out Yukon or Auburn, they both make excellent limited-slip gear sets that will bolt into your housing. Good luck!


BTW: I believe that Canadian Pontiacs are built on Chevy frames and suspension - so check out the Chevy catalog before you give up on identifying your axle. It's important, because the BOP rear is NOT the same as a Chevy rear.

I had a '67 Parisienne 2+2, 283PG, it came stock w 3.36's, which I wold bet is what your '65 has. Are you sure you installed a 4L80? The '80 is a large heavy unit used exclusively in trucks. More likely you installed a 4L60, which is a re-named 700R4, 1st used in 1982. I have one in my 6.2 diesel Suburban and K5 Blazer. It was used in most Chevys, cars and lite trucks in the 80's and 90's. The 4L80 is a TH400 on steroids. Heavy, lots of internal drag, and way too much tranny for a 283. Google search, and read wikipedia, lots of good info there, and Google "pan ID", that will tell you what you really have. The 4L60 drops the rear axle ratio 30.5%, so a 3.90, (too steep for hi-way driving) into 2.70, a great hi-speed cruising gear, and 1st gear is lower, more around town oomph, so far better than a TH350 or 400. The 4L80 has a 25% overdrive, and the same 1st gear as a Th350 or 400, so clearly the 4L60 is a better choice for a conversion. Let me know what you really have

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