I bought a complete rear end from a junkyard and put it in my car. At the time I was positive it was either a 3.23 or a 3.42 gear. I know when my gauge sais 50 mph Im actually going 45ish, so this is what I came up with 2.73/3.08=.886 or 88.6% then 50mph/100=.5 and .5x88.6=44.3mph. Somone please chime in and show me I'm wrong.

 

When you put fresh fluid (or, any fluid at all, maybe??? lots of times junkyards drain those before selling them) in your "new" junk rear, how many teeth did you count on the gears?

 

Actually I did'nt count, but it was already drained. If its that important let me know and I will take the cover off and count.

 

It's not at all important to me.

But if it is to you, and you want the real truth and not just a bunch of random guesswork, the only sure thing is to look at it.

 

It does matter to me. It'l probably be saterday before I have time to look but I'm going to get back to you on that for sure if that's . Thanks sofakingdom.

 

The oil in your diff needs to be changed just like in your crank case, or your tranny (not quite as often). It's just rarely done by the average guy, so the rear end is usually woefully past it's regular service. Also sofakingdom has a special place in his heart for our dear little 7.5" 10 bolts, and he wants to save as many of those factory gear sets as he can.

When you're eyeballing the ring gear, it'll have a stamp on it somewhere along the outer diameter with it's teeth, and the amount of pinion teeth it matches to. You'll see something like this:

42:13 - 3.23
41:12 - 3.42

And that's how you know.

 

i.e. 42 divided by 13 = 3.23, you could jack the car up and do the old count the rotation of the drive shaft and rear wheel if you don't want to pull the cover.

 

I dont understand where the 42 and 13 come from. How does the old wheel and driveshaft thing work? Do tell.
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I'm actually a little suprised one of the seniors on this board didnt know right off the bat what rear gear would offset your mph by 5mph.

 

The number of teeth on the pinion and the ring.Poorly.... if the wheels don't rotate EXACTLY the same (i.e. if it's an open rear and one wheel rotates more than the other) it introduces error.At what mph? "offset by 5 mph" at 0? at 50? at 100? In order for that sort of guesswork to have any hope of accuracy, where you look at how much something changes and try to use that to figure out what the new thing is, you have to know the old thing.None of this makes any sense. If you "know" you have one of a certain choice of gears, then why are you using some other ratio in your "calculations"? Where does 100/50 =.5 have anything to do with any of that? All of that is just gibberish. That sort of thing is why peoplep who know what they're doing, AVOID all of that; and jump DIRECTLY to the FACTS, which is, look at the gears.

The closest you can come from the numbers you give, is that if the speedo reads 50 when you're really going 45, then your speedo error is (50-45)/45, or 5/45, or 1/9, or 11%. Meaning your new gear is 11% higher than your old one, or, 1.11 times the old one. If your old one was 2.73, then 2.73 * 1.11 = 3.03. There's no such ratio. Could be 3.08, maybe, though. Who knows.

Guessing at what gears you have is non-productive. It almost never works. Since you don't even know if there's fluid in there, let alone what condition it's in, it's time to put all that aside, and go get busy out there in meatspace.

Get the numbers off the gears or count the teeth while you're putting in the fluid, and from there, it'll be so obvious, it'll slap you up side of the face. No errors, no guesswork, no uncertainty, no fuzzy "math", no confusion, just TRUTH and REALITY.

 

I know everythings in good condition. I put oil in it when I bolted it up, and the formula I did came from here. I know i had 2.73s and I divided by each stock rear ratio till I came up with the 5mph difference at 50. I was thinking the higher the mph the more off it would be.

 

Generally so, yes.

In order for "calculating" to be of value, you'd have to know, first, how accurate your speedo was BEFORE the swap BY MEASURING it (IOW not just "assume" that since it's "stock", it's also automatically "right"); then, know EXACTLY how fast you're going AFTER the swap, by timing a measured distance (ideally, THE SAME measured distance, to eliminate yet another potential source of uncertainty) with the speedo at a constant speed; and then and only then, you'd have data with enough precision and accuracy to "calculate" something meaningful. Otherwise, you end up in the "measure with micrometer, mark with chalk, cut with dynamite" scenario, where you can maybe get some of the numbers out to 3 decimal places or whatever, but others, with no measured precision at all. That's where the "fuzzy" part comes from. All it takes is ONE of the critical "factors" in a calculation being uncertain like that, and the WHOLE calculation takes on AT LEAST that degree of uncertainty.

In your case, you don't know by measurement how accurate the speedo was before, and you don't know by measurement how fast you're going now at a given speedo indication, so it doesn't matter how you go about "calculating". BOTH of the numbers you're starting out from, are still basically just guesses.

Well, go take a look, and let us know what you find. We can talk about it all day long, and it won't get you any closer to knowing what you've REALLY got. Talk is cheap, but the parts aren't listening.

 

Reminds me of an incident a few weeks back when a young Captain thought a one mile running track I had constructed was off so he decided to measure it with the odometer on his motorcycle. I had a brief talk with him concerning the level of accuracy between a bike odo and a calibrated Sokkia total station.

Bottom line is you've got access to the EXACT data you need, ie the ring/pinion count. From there you'll install a speedo gear that matches AS CLOSE AS POSSIBLE the EXACT rear end ratio and tire size so that the speedo measures within an acceptable margin of error.

Speedo's are never EXACT. Tire size (really circumference) will vary slightly with heat and a little with inflation. Speedo gears are a 'best fit' between these other variables and the known, unchanging rear end ratio.