How far would my speedo be off if went from 2:73's to 3:42's and how would I correct the speedo to read the right mph

 

I'd like to hear the answer to this myself.

 

ANyone?

 

It can be found in the https://www.thirdgen.org/newdesign/faq/thirdgen.shtml

2.73 to a 3.73 is the example but you could convert it to a 3.42

"Q: How to I calibrate my speedometer for new rear-end gears?

A: Determining the precise speedometer drive gear/driven gear combination required for a rear axle ratio change is surprisingly easy-if you know the existing combination and axle ratio. Whip out your calculator and follow along. Let's assume that your F-Body is currently equipped with 2.73 gears and that you're planning to install a 3.73 ring and pinion. If you didn't change speedometer gears, actual vehicle speed would 73.1 percent of indicated speed (2.73/3.73 = .731). That being the case, what's required is speedometer gearing that will result in the driven gear spinning at 73.1 percent of its current speed. The speedo gear combination for a 2.73 rear (assuming 26-inch diameter tires) is a 17-tooth drive gear and 37-tooth driven gear. Switching to a 45-tooth driven gear (the most teeth available) would translate to the vehicle traveling at 82 percent of indicated speed - still a 10 percent error.

Obviously, the drive gear must be changed to one with 15 teeth if the 73.1 percent ratio is to be achieved. Going back to the original 37-tooth driven gear for illustration purposes, the 15-tooth drive gear would result in the vehicle traveling at 88 percent of indicated speed - that seems like a step in the wrong direction, until the driven gear is changed. (This is where it gets a little tricky because you have to work with a percentage of a percentage). Matching the 15-tooth drive speedometer gear up with a 44-tooth driven gear brings the drive ratio to 74 percent (15/17=.88; 37/44=.84; .88 x .84 = .739, which rounds to .74, or 74 percent). Still not quite enough. Will a 45-tooth driven gear do the job? Run the numbers through your calculator and you should come up with 72 percent. That's about as close as you're going to get.

In case you haven't put all the pieces together, there is a distinct limitation regarding speedometer accuracy and steep rear gearing. Unless driven gears with more than 45 teeth, and/or drive gears with fewer than 15 teeth become available, it will be impossible to have an accurate speedometer with rearend gear ratios lower (higher numerically) than 3.75:1 - unless 27-inch or larger tires are installed."

 

In this case 2.73/3.42 = 0.798

So there would be a 20.2% error. So you're Speedo would read 20% higher than what you were actually going if I understand correctly.

 

From what I get if you go from a 17 to a 15 that gives you .88

Then 37 to a 41 that gives you .90

then multiple those and you get .792

I seriously suggest you redo the math and read the above and what is on the link. At the link it shows the PN for the teeth.

 

The 700r4 web site gives 15/39. that's with 235/60/15 tires and they are only .1 " taller than stock. that's a diffrence of 39 to 41 teeth on the driven gear. Could this be right? I need to know I am about to make the change from 2.73 open to 3.42 posi. At this point I'm not sure what speedometer gears to get.

 

I've tried multiple sites before too. Most of them come off with a different number. I think it's probably do to the fact that they truncate (cut off numbers) at a certain decimal place in the coding. It might also be a rounding feature too. I would feel more comfortable sitting down and doing the numbers out by hand. That's why I said to use the formula above and look up the rest. It's always better to learn how to do something than rely on another person's coding...

One time NASA relied on a coder and the rocket blew up.... guess what the techs didn't make sure the software was working properly on the new hardware so it cut a few decimals off a number and boom lots of money down the drain. NASA even sent up another rocket cause they thought it was the computers fault (usually always human error) and guess what the new one blew up too.

 

http://www.bgsoflex.com/cgi-bin/spee...n&givengear=15

I punched in 26" tires. I was guessing on size it said 15/40.

looks like this place carries a bunch of speedo gears I haven't ever bought anything from them so you might want to ask around before buying.
http://www.transmissioncenter.net/sp...n_______va.htm

 

Using the hand calculator way I get:

37 to 41 = 0.902

37 to 40 = 0.925

37 to 39 = 0.948


15 to 17 = 0.882

2.73 gear / 3.42 = 0.798

so 15 x 41 = 0.795

15 x 40 = 0.815

15 x 39 = 0.836


From the way I understand it you want to get a ratio closest to old rearend / new rearend

I would also assume it's better to come up short (meaning speedo reads a little higher than actual speed). If you went long that means you could be going 55 mph down the highway and say you got a 4% off. You would be really doing 57 mph... ok now that's not bad but count in you're speeding 4 mph over on the speedo with traffic you're now going 61 mph and you get pulled over for going 6 mph over.

Maybe another member should jump on this, since I really haven't ever done the swap before. I'm just going off how I understand the Tech Article.

 

If you hated math as much as I did in high school, the tech article is basically useless and confusing. I changed from 2.73's to 3.42's in my car and I just went to the local dealership and the guy at the parts counter was able to look it up on the computer and give me the right gears for the change. There's only two different gears for the the output shaft of the transmission (grey=15 teeth, and red=17 teeth). You need the grey one for 3.42's. The gear they gave me for the drive cable was yellow. I can't remember how many teeth it had exactly, but that was the color of it. I think that tech article says how many teeth are one each color of gear. The grey trans gear and the yellow cable gear are the ones I used for the 3.42 gears and they worked right. I'm now running 3.23's and they use the red tranny gear and a grey cable gear. My 2.73's used a red gear for the trans and the cable.

 

Part No. Teeth Color Sleeve*
25513042 35 Orange 25007338
25513043 36 White 25007338
25513044 37 Red 25007338
25513045 38 Blue 25007338
25513046 39 Brown 25007338
25513047 40 Black 25007339
25513048 41 Yello 25007339
25513049 42 Green 25007339
25513050 43 Purple 25007339
25513051 44 Dk Gray 25007339
25513052 45 Lt Blue 25007339

*Some applications may use Sleeve No. 25007224 or 10456092 in place of 25007338, or 25007308 or 10456089 in place of 25007339.
Red Drive Gear Combinations

So you had a 15 (grey) and a 41 (yellow). I must have been right since that would give you 15 x 41 = 0.795 and you need'd 2.73 gear / 3.42 = 0.798 An error of 0.3% not to bad.

Dealer method works but if you have a Dealer like mine who can't even order parts for you without loosing them for 2 weeks then finding them again. I wouldn't trust my dealer with an oil change.

*Corrected the wrong PN thanks for the update guys! See that goes to show you should learn to look stuff up on your own and research...

 

That is correct for a 3.42 swap from 2.73. I have the Gray drive gear and Yellow driven gear and my speedo is calibrated perfectly. Very easy to change them too. I got the part numbers from a TPIS catalog but I am sure that they are the same as the ones below.

 

If you have an electric speedo with a vss, does the actual vss have to be changed when going from a 37 to a 41 tooth gear?

 

Yes, the vss shaft is positioned differently to make up for the difference in the diameter of the gear. I speak from 1st hand experience.

 

I'll definitely agree with fireturd on having a dealership with a competent parts guy. I happen to be fortunate to have at least one in my area that actually has a few parts guys that care and know what they're doing. The sad part is that's only one dealership out of probably over a dozen around here! In addition to that, I only go there for parts that I can't get at the regular parts stores, and they've never laid a wrench on any of my vehicles. I value my hard earned money too much for that! And what NovaSS said about the sleeve is correct too. When they sold me the gears for the 3.42 setup, they also sold me a sleeve with a different part # to go with the cable gear. So I'm thinking the sleeve issue probably applies to both electrical and cable driven speedo setups.

 

The sleeve for a 90 up electric VSS for 40 teeth and above is 10456089 not the second last one that fireturd350 posted. The tech data where he got it from is wrong they left out the 6 in the #. This is the one I got from my gm dealer. (that is when I had a 700r4)

edit:

I see you have a cable sender so you will have to find the part #.

 

Here's a pretty brainless way to figure out the parts needed. Bowtie Overdrives' speedo gear calculator.
Math is hard, let's go shopping!

 

That calculator, and all calculators for that matter, I have seen are not correct. The stock gears for 2.73's is 17/37, 3.42's is 15/41, 3.73's is 15/45. What that says is wrong.

 

What about for 3.08's? I switched to 3.42's but dont know if I need to change the drive gear too.

 

The combos above are for autos. I'm not sure on 3.08 manual cars. The 3.42 combo is 7/19 though for manuals. Call your local gm dealer and find out what you have in there now.

 

The 3.45 ratio for the 9 bolt comes through at 15/40, which works out in my math from the 2.77 stock ratio. EDIT: With a 255/50/16 that I'll be going with...

 

Are you sure gm put 15/40 in the 3.45 cars? I know for a fact that they put 15/41 in the 3.42 cars and going with a slightly steeper rear wouldn't make since to go to 15/40. That would be the opposite of what you want. I might be wrong though so who knows. 700r4 says I should put a 15/39 in my car which is stupid to me when gm put in 15/41. Why all the calculators are wrong, I have no idea.

 

I don't know what GM put in the 9 bolt cars, period. Is there anywhere to find out? I thought I had the math worked out, but my car is a 2.77 ratio 9 bolt instead of a 2.73 ratio 10 bolt... I have to either dig into the trans to find out what I have now, or trust a company that claims they know what they're doing with their calculators. Guess I'll have to get dirty before swapping my gears out.

 

I think I might just sit down sometime and write me out a Java program to figure it out on my own. I could do it just like the Math Version but have the computer do it all. I bet I could even have it print out the PN's once it selected the closest gears.

I might try that sometime when I'm free.