Ok, I've done TONS of research on this, and thought I had the right speedo gear twice! And according to me, it should be right, but my speedo still reads inaccurately.

Ok, went from 2.73's (confirmed by RPO code) to 3.23's (also confirmed, counted teeth). I've used a couple speedo gear calculators out there and both give me the same combo. I have originally a 17 tooth drive, and a 38 tooth driven, tire diameter of 24.5-25.0". This combo clearly worked fine before. Now, using my original 17 tooth drive gear, I should need a 43 tooth driven.
I did swap gauge clusters, but both use 1001 revs./mile, and I didnt swap the circuitry or anything like that. I swapped out the VSS to the other P/N that is needed to compensate for the offset, reads the same as before so no problem there. Speedo reads about 10mph off at around 55mph (reads 40).

As far as I'm concerned it should be right, but I just cant figure it out for nothing! Please help!

 

That's your problem. Every single "calculator" out there I have seen gives different gear teeth #'s than what GM put in cars.

Just get the right gears for a 3.23 f-body from the dealer and you'll be set. I'm pretty sure they were 17 & 41 but check with a dealer. Actually, I might have a 41 tooth VSS driven gear, hit me up if you want it cheap after you verify.

 

Without doing the math, I think 41 tooth will do it, by how much its off. I just cant afford to keep going through gears at 5.50 a pop. I'll figure it out and let ya know.

 

Well, I've run the numbers in a few equations and I'm still getting a 43 tooth driven. Tire size is 245/50R16, differential ratio is confirmed to be 3.23. I have no idea what is going on here. Anyone else have any ideas?

 

You're not hearing me, man.

Go put in a 215/65R15 tyre, 3.73 rear and get the gears for a 700, a 200-4R, or a T5. Show me one single online "calculator" that gives the right gears for that combo. There aren't any. I've already done up gears in each one of those trannies for 3.73's and stock height tyres. Do you believe the guys that tell you 43 tooth driven, or do you believe me?

Better yet, go put in your 2.73 combo. See what goobered up numbers the "calculator" gives you.

GM > online calculators.

 

Do a search on my userID for the phrase "SMUC". Look at older posts first. I tell exactly how to calculate ratios for any combincation that can exist. Once you get all the possible tooth counts that are close, it boils down to which ones actually exist.

The number is actually 1000 revs per mile, not 1001. But, by the time you get out to the 4th significant figure, you'll find that .5 psi in your tires, or 1/32" of tread wear, will cause far more error than that. And furthermore, all of that other stuff changes on a continuous basis; so even if you do happen to get it perfect this time, who knows whether the next set of tires, or in teh winter instead of summer, etc., it will still be "perfect".

Also, measure your actual own tires, with them on the car, at normal pressure, sitting on the ground. Measure from the ground to the center of the wheel. Don't waste your time and effort "calculating" the tire diameter from the size specs; there's more than one whole tooth of error available right there, just from different mfrs' variations in their tire sizes.

Don't forget, you'll never get it perfect, the best you'll be able to do is to get it within 2 or 3%. Don't get all hung up on "exact", it's not something that's that precise.

"Measure with a micrometer, mark with chalk, cut with an axe"

 

HAHA, ok. I get the math plenty well, I agree, it's not hard stuff, but for some reason...it's still off. I've used many online calcs., always get 43 tooth driven. I used RB83L69's SMUC equations, and I work out to get a needed ratio of 2.52, which would call for a 43/17.

Here's some facts:
Tire Diameter 25.8"
Rear Ratio 3.23:1
Driven Gear 17 tooth

 

jmd,
I hear what your saying. What I had done was find the correct tire size that gave me a 17/38 drive/driven ratio with 2.73's (all originally on my car) and the online equations gave me a 24.5" diameter tire. Then I had used that 24.5" to calculate the driven needed with 3.23's. But...you are right, they are still off. Using my tire dimensions of 245/50R16, you get a diameter of 25.8", and when plugged into the equations (online), it gives a driven a couple teeth off, as you say. So yes, you are right.

But, like I said, using RB83L69's equations by hand, I'm still getting 43 tooth with the new tire diameter. This should not be this hard to figure out!

 

All I can suggest is to go with the closest factory combo you can find. Mine are easy because they installed 3.42 and 3.73's in plenty of cars and I stick with stock height tyres. When you change something in the equation, you can alter the gears' ratio to each other accordingly. If you started with a 3.73 & factory gear combo and you were to shorten the tyres by 10% in rolling circumference and change the speedo gear ratio by 10% to slow down the speedo cable, you'd be fine.

I don't know what is up with RB's math; dude is spot-on about everything else. I ignore everything but what GM put in cars. Years of seeing "the math" give the wrong speedo readings makes me that stubborn about it.

-Matthew (15/45, 7/21, 10/30 and other combos over the years )

 

The formula always works. It has to. It has no choice. It's all numbers. That's how you can take your speedo out and send it to a speedo shop and get it certified, without the rest of the car.

If it's not coming up with the right anwer, then you're not doing something right; either you're "calculating" (guessing) your tire size based on the mfr's dimensions instead of measuring your actual tires like I described you should do, or your gears aren't what you think they are, or something.

However, your speedo itself could also be off; and if that's the case, then no amount of calculations based on the correct numbers will wver make it read right. You'll need to swap it or repair it to cure it.

You can easily check the gearing itself, independent of the speedo, by using the odometer. Drive aong a measured stretch of road with mile markers, like an interstate or something, for a good distance, I'd suggest 10 miles. Drive at a constant speed showing on your speedo, I'd suggest 60 mph. If the gears are right, then the odometer will be right, since all it is, is a turns counter (speed coesn't affect it). Then if your speedo is off, you'll be able to tell exactly how much by the time it took to cover that 10 miles with the speedo showing 60; since at 60 mph you ahould be travelling exactly a mile a minute, it should have taken exactly 10 minutes. If it takes more, your speedo reads lower than actual road speed, and if it takes less, it reads higher.