Rear end confusion

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Rear end confusion - 5/12/2008 1:10:15 PM    Comma Posts: 22 Joined: 4/6/2008 Status: offline I've got some rear end confusion on my hands... I always knew my speedo read innacurately but recently figured out that it was reading +15% (read ~46 while going ~40), also knowing that the rear on my truck is not stock (replaced by a previous almost-owner, long story short) I logically concluded that I must have a different final drive ratio than stock. Here's where it gets confusing: I jacked up my rear right wheel and turned the tire 10 times while counting the driveshaft revolutions, I got 10 tire and 17&1/3 drive shaft. Now, IIRC you divide your driveshaft revs by 1/2 the tire revs... Resulting in a 3.46:1 final drive ratio... That's what confused me, AFAIK that's not a stock MOPAR ratio so I don't think this is a MOPAR rear (then again, I don't know rears by sight like some do)... Hell, I've never even heard of a 3.46:1 rear end! I want to correct my speedo but I'm not sure what gear to buy now. I'm probably going to find a junkyard rear with the lowest ratio possible (for gas mileage, I'm thinking 2.76) and install it pretty soon but I'd like my speedo to read correctly for now. What is this rear end? Ford? GM? is it a Dana? Is my math off or is 3.46:1 a common ratio? My rear end is a 10 bolt, between 9" and 10" and I have attached a picture. Thumbnail Image Attachment (1) < Message edited by Comma -- 5/12/2008 1:18:04 PM > _____________________________ 1981 D150 5.2L auto Next up: Body work Check my gallery and my youtube channel to watch my progress. Post #: 1

RE: Rear end confusion - 5/12/2008 2:08:50 PM    2young2drive   Posts: 27 Joined: 2/19/2008 Status: offline kind of looks like a GM 10 bolt (in reply to Comma) Post #: 2

RE: Rear end confusion - 5/12/2008 2:13:30 PM    javatman Posts: 35 Joined: 3/30/2008 Status: offline is there any way you can get a pic of the front side with the numbers on it or write them down and post them? hard to tell from looking at the rear.my guess is a ford rear end but i would need more info before giving a positive ansewer....... (in reply to Comma) Post #: 3

RE: Rear end confusion - 5/12/2008 9:40:32 PM    charlie1935   Posts: 383 Joined: 3/6/2004 From: DuQuoin,IL. Status: online My guess is ford 8.8 rear. Looks the fill plug is in the wrong place for Ford. < Message edited by charlie1935 -- 5/13/2008 8:44:25 AM > _____________________________ Grandpas like toys too (in reply to javatman) Post #: 4

RE: Rear end confusion - 5/13/2008 7:23:31 AM    turbododgepirate Posts: 124 Joined: 2/4/2008 Status: offline I dunno, looks a lot like the 8-1/4 rear end in my 1985 D100, but 3.55 is the closest thing to that. < Message edited by turbododgepirate -- 5/13/2008 7:32:46 AM > _____________________________ ad astra per alia porci 1986 Dodge Shelby Charger Panther pink, 2.4 turbo motor. 1985 Dodge D150 (in reply to charlie1935) Post #: 5

RE: Rear end confusion - 5/13/2008 8:25:32 AM    charlie1935   Posts: 383 Joined: 3/6/2004 From: DuQuoin,IL. Status: online Maybe this will help. Attachment (2) < Message edited by charlie1935 -- 5/13/2008 8:36:17 AM > _____________________________ Grandpas like toys too (in reply to turbododgepirate) Post #: 6

RE: Rear end confusion - 5/13/2008 9:11:58 AM    Comma Posts: 22 Joined: 4/6/2008 Status: offline So it looks like a Mopar 8 1/4"... I'll have to re-measure, look harder for numbers and re-do the tire-turn method, with internet research I found that 3.46 is a ratio BMW uses, and this sure isn't a BMW rear. < Message edited by Comma -- 5/13/2008 9:19:38 AM > _____________________________ 1981 D150 5.2L auto Next up: Body work Check my gallery and my youtube channel to watch my progress. (in reply to charlie1935) Post #: 7

RE: Rear end confusion - 5/13/2008 9:18:15 AM    charlie1935   Posts: 383 Joined: 3/6/2004 From: DuQuoin,IL. Status: online I always turned the wheel one revolution and doubled the driveshaft revolutions to get the ratio. What is that at the top center bolt, a metal tag?  Clean it off and see what it has on it. Gear ratio maybe? < Message edited by charlie1935 -- 5/13/2008 9:33:14 AM > _____________________________ Grandpas like toys too (in reply to Comma) Post #: 8

RE: Rear end confusion - 5/13/2008 7:44:48 PM    moparborn Posts: 413 Joined: 9/9/2007 From: Pittsburgh,PA. Status: offline Looks like an 8.25. the gear ratio should be on the tag. Count the revolutions of the driveshaft to one revolution of the tire. A 2.76 ratio is not always better for fuel economy,what will it be used for(local or highway),engine size,auto or stick? _____________________________ (in reply to charlie1935) Post #: 9

RE: Rear end confusion - 5/14/2008 7:48:00 AM    Comma Posts: 22 Joined: 4/6/2008 Status: offline I'll check the tag when I get a chance today. It's a slushbox, boy do I miss my old stick, I use it for mostly local driving. Why wouldn't a 2.76 be better for fuel economy? In my experience with getting the best fuel economy possible, driving style and low RPMs are the way to go, which reminds me that I need a tachometer.  _____________________________ 1981 D150 5.2L auto Next up: Body work Check my gallery and my youtube channel to watch my progress. (in reply to moparborn) Post #: 10

RE: Rear end confusion - 5/14/2008 10:25:45 AM    javatman Posts: 35 Joined: 3/30/2008 Status: offline heres the low down on it according to the casting number on the housing... Castings: 2852905, 3723598, 3723599 8.25 Chrysler (mopar) Rear End Info: 1 5/8 pinion, 57.6" wide, 10 bolt cover, 55.6" width (flange to flange) / 51.4 housing flange to flange / 43.02 spring seat center to center. 4/20/83 KD46 represents your unit came out of a 80's model dodge diplomat (possible police car) check it for (positraction/suregrip) it is a good rear end but not heavy duty. I hate to post this here but my earlier post in this spot was inaccurate due to personal error.in other words i was wrong.hopefully this info will help you...... < Message edited by javatman -- 5/14/2008 2:26:23 PM > (in reply to Comma) Post #: 11

RE: Rear end confusion - 5/14/2008 12:01:30 PM    Comma Posts: 22 Joined: 4/6/2008 Status: offline OK - I just got out from under the truck. There is no tag on the rear, just some numbers Bottom of housing (chrysler symbol)  48674                         3723599 Bottom of housing, parallel to cover KD46 4  (20) 83 (pictures attached) Thumbnail Image Attachment (1) < Message edited by Comma -- 5/14/2008 12:25:43 PM > _____________________________ 1981 D150 5.2L auto Next up: Body work Check my gallery and my youtube channel to watch my progress. (in reply to javatman) Post #: 12

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