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.



[IMG]local://upfiles/87633/65196D0052DF49BCA0F030EE58B3AED9.jpg[/IMG]

 

kind of looks like a GM 10 bolt

 

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.......

 

My guess is ford 8.8 rear. Looks the fill plug is in the wrong place for Ford.

 

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.

 

Maybe this will help.

 

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.

 

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 metaltag? Clean it off and see what it has on it. Gear ratio maybe?

 

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?

 

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.