Figuring gear ratios for dummies

Hey guys,

I found a guy selling a limited slip rear differential for a Delica. I would love to have one for my van. However I need to make sure it's the proper ratio. I asked the seller to count the number of teeth on the gears. He emailed me with 40 on the crown gear and 28 on the pinion gear. How do you determine the ratio. Math is involved I'm sure of that. I was thinking divide one by the other, but neither of those gives me a number that seems to make any sense. I found on the Delica forum that the L300 came in only 4 different ratios. 5.285, 4.875, 4.625, and 4.222. My math shows 4.285 possibly but, I was never good at math anyway. My van has 4.875 ratio. Is it possible this really isn't for a Delica, or he miss-counted? Can someone help this dummy (me) out please?

First off, don't tell him what ratio you're looking for

Have it tell him: Tell him to put a mark on the ring gear and count how many time he has to turn the pinion shaft for the mark to come back around to the same point. He'll have to estimate the fraction of the last rotation but it should be close enough. Best of all would be for him to use a camcorder as he does it and sending the video to you.

I'm guessing he miscounted or made a typo. 28 would make for a very large pinion gear in what I assume is a very small rear end. 4.88 is (usually) 39 on the ring gear, 8 on the pinion, IIRC.

40/28 would be a 1.43, which is miles away from what you're looking for. More than likely that 2 snuck onto the front of the 8 and he has a 5.00 (40/8)

Here's a calculator to help you along.

I was thinking 28 was way too much as well, and my math came up with 1.4285. Again I am bad with math and thought maybe just move the decimal point and that is where I got the 4.285 but again I'm talking out of my arse.

The funny thing is he bought it for his delica before he took delivery and then realized his van already had the limited slip. I asked if he even thought about buying the correct ratio. He had no idea what I was talking about, or rather the language barrier (he's in Quebec) maybe didn't translate. Again 28 seems way too many. I think the similar differential I pulled out of my REPU only had 9 or 10 teeth on the pinion.

A lot of times the gears will be marked. This came out of a r200. The pinion has 14 teeth and the ring 47. 47 divided by 14 gives you 3.357 for the gear ratio. When I did the counting method above I got about 3 1/3 turns.

Can't you just change the differential using your existing gears ?

I'd guess he miscounted and mistyped and has a 39/8 4.875 gearset.

iceracer wrote: Can't you just change the differential using your existing gears ?

Holy E36 M3 I never thought about that! I was over thinking it like I usually do. Of course I would have to remesh the gears which is something I've never done. I guess I was just hoping to do the easy button and pull one differential out and install the other.

Most Mitsubishi's I've seen have the ratio printed on a sticker on the case.

He doesn't have the case. I don't see any sort of markings on my existing differential chunk, or the housing either. The vin badge under the drivers seat lists the axle ratio. That's the only way I know what my van is equipped with. I could count the turns as well, but I'm going by the data tag.

I wonder if he's counting the spider gears?

Okay, so I just got to looking at his email on my computer at home. On my phone it didn't show any pictures, but on the computer it did include pics. He is counting the splines for the axle shaft for the 28 number. I tried counting the ring gear teeth from the picture, but I can't see all of them of course.

If you can see 1/4 or half you can count that and do the math and be close.

Have you considered getting the air locker from a montero and swapping that in? I think they're generally the same mechanically.

From my research the air locker will not work in the Delica housing.

Seller got back to me. Says pinion has 8 teeth. Says the pinion spins 4.8 times for one rotation of the ring gear. Didn't say anything about the number of teeth on the ring aside from saying 40 in an earlier email. I think it probably has 39, judging by the number of rotations. What say you guys?

Chris

Is it as simple as I think it should be in replacing the non-limited slip diff with the limited slip diff? I was thinking it should be as simple as removing the axles shafts(not simple, but not terrible), unbolting the diff from the housing, removal, and replace with LS. Replace fluid with some that has special friction modifier?

??

Profit

Mazdax605 wrote: Seller got back to me. Says pinion has 8 teeth. Says the pinion spins 4.8 times for one rotation of the ring gear. Didn't say anything about the number of teeth on the ring aside from saying 40 in an earlier email. I think it probably has 39, judging by the number of rotations. What say you guys? So that makes the ratio 4.8 to 1. QED Chris

The ratio goes to an earlier post. 4.875

If you just replace the differential. Unbolt Your ring gear and install it on the replacement, assuming it fits. Much easier than changing gears. You will need to check the backlash. Adjustment can be easy or a pain depending on the design. Check to make sure your axles fit. Same spline.
Kind of like buying a pig in a poke.

.

Yes, changing a "front loading" diff out is that easy. That's part of why the drag guys like the Ford 9", you can keep a spare 3rd member on hand and swap it quick when you blow one up.