JohnnyT/A

Can anyone give me a concrete answer on how to get the speedometer working accurately again? I've been reading some threads about this but I felt that there was many different ideas to the solution.
So I use to have 273 gears now I have 373 gears with a 700r4 with 255x50x16 tires in the rear. Please help

red rock

iTrader: (2)

This web page may help you. http://www.transmissioncenter.net/sp...__va.htm#700r4 You will need to know your driven gear size which if you take the speedometer housing off, you will see the color and determine what size it is with the chart. Then you can get the right speedometer gear for your tires and rear end. I checked your tire size and it looks like they are 25.5 inches in diameter.

John62591

Having a concrete answer with cars is not likely, just probabilities narrowed down to the root of a problem.

If you can get underneath the vehicle look to see if there is anything obviously broken or dirt on the cable and where the speedo gear is all the way to the Firewall and clean it out the best you can. You may need a partner to help diagnose whether the cable is the problem.

You will have to take out the dash if you want to check the gauge for corrosion/rust/just plain crap.

JohnnyT/A

John62591 the speedometer actually works but what I was saying is that is not accurate due to the 373 gears being installed

scoflaw

iTrader: (2)

Verify your speed with a GPS. Do the math, and figure out how many mph your off.
Then figure that 1 tooth on the driven gear = 3mph.

FRMULA88

If you changed the rear end and or rear tire size from the OEM setup,
then you need to change the driven gear in the transmission to restore speedometer accuracy.

assuming the new rear tires are the same height as OEM ( 25.5 is close enough just use 26" on the chart)the only difference is the 3.73 vs the 2.73 stock rear end.

When I did this change back in 1998 I simply called SLP performance for the new driven gear. You just need to check the color of the drive gear.. use the chart and replace the driven gear based on the color of your drive gear, your tire height and the new gear ratio. Normally the drive gear is grey so for 3.73 with 26 tall tire you need 43T driven gear or the "purple" driven gear.

This is pretty straightforward but if you don't want to do it yourself, (if you actually read the link from the first reply to your post you would have this figured out already) a reputable transmission shop can do it for nominal fee.

FRMULA88

before apps on a cell phone you just removed the driven gear assembly from the trans and checked the colors of the drive and driven gear.. then using the chart... you just changed the color of the driven gear accordingly.

having some fancy cell phone app does eliminate the need to actually do the work... just another excuse to play with the phone.

RedLeader289

iTrader: (2)

Taking the tailhousing off to replace the driven gear is a hassle when all you have to do is swap the speedometer gear (it's a ratio between the two, so yes you could change the driven gear but you could also just as effectively change the speedo gear like the first post talked about). They are pretty cheap, and easy to install. I actually need to do the same thing for my car because I've went from a 26" rear tire to a 28".

sofakingdom

iTrader: (1)

No...

There are something around 40 teeth, give or take, on the driven gear in a 700. Therefore 1 tooth on the driven gear = about 2½%. About ½mph @ 20, 1mph @ 40, 2 mph @ 80, 2½ mph @ 100, etc.

Problem may be however, not all driven gears, will work with all drive gears. The split is between #s of teeth on the driven gear, so depending on what gears you have now vs what gears you need, you might have to change the housing the driven one goes in, in addition to the gears themselves. Won't know that for sure until you look and count.

What red linked you to is the page you need: speedo gears.

Now all you have to do is figure out which ones you need.

The way to do this, is to forget about the speedo for a minute, and instead, think about the even simpler instrument that's involved; the odometer.

All that is, is a turns counter. It counts 1000 turns of the speedo cable (driven gear) per mile. It has wheels with cogs on them that each time they spin once, they bump the next one over, 1/10 of a turn. Just WAY too simple. The first one is a pair of gears that are each 10:1, making the result 100:1 So, the tenths of a mile gest bumped 1/10 of a turn, every 100 turns of the speedo cable; then every time that one turns it bumps the miles one 1/10 turn (1 mile), which every time that one turns it bumps the tens-of-miles one 1/10 turn (10 miles), and so on up the line. Then, if the cable spins that many times per mile, the speedo will also see .... 1000 spins per mile, and will show the right speed. Pretty much a total no-brainer.

So, all you need to do now, is figure out what speedo gears you need, to make that cable spin 1000 times each mile. Way too eeeeeeeeeeezy.

MEASURE your tire radius physically with a tape measure, from the ground to the EXACT CENTER of the wheel, with the tire PROPERLY INFLATED. (the wrong tire pressure changes the tire size enough to throw your gear calculations off by potentially several teeth... GET IT RIGHT) DO NOT try to "calculate" the diameter based on the "trade" size (that 255/50-16 thing), it will be WRONG every time. MEASURE.

Once you get that number, multiply by 2 × pi. The number you get by doing that is how far the tire rolls each turn. So let's say you walk out to your car, and your tire measures ... 13-3/8" from ground to the EXACT CENTER of the wheel. (nice random plucked-from-thin-air number; MEASURE yours and see what you get) That's 13.375". 13.375 × 2 × pi = 84.0376". OK, that's how far your tire rolls. Now, divide the # of " in 1 mi (12 × 5280, which is 63360) by that #. That comes out to 753.95. Now, multiply that by the number of teeth on your rear end's ring gear, and divide by the # of teeth on the pinion (that's more precise than using the "rounded off" 3.73 or whatever); for 3.73 gears, those #s are 41 & 11. That works out to 2810.2. So you have now determined, your drive shaft spins 2810.2 times per mile. Divide that by 1000; you get 2.8102. Now all you need is speedo gears THAT YOU CAN ACTUALLY BUY, as close as you can possibly get to the ratio of 1:2.8102.

Now go to the page red linked you to, and look down the list. So for example you might observe that YOU CAN ACTUALLY BUY a 15, 17, etc. tooth drive gear; so you need whichever one of those, coupled with whichever one of the driven gears YOU CAN ACTUALLY BUY, that has 2.8102 times as many teeth. Well, turns out that 17 or any higher # of teeth on the drive gear, causes the # of teeth on the driven gear to be off the list; therefore you CAN'T ACTUALLY BUY any such gears; can't use any of those; looks like it's gonna hafta be a 15-tooth drive gear. So let's see; 42/15 = 2.8000, and 43/15 = 2.86666. Looks to me like 42 and 15 is the combo for that situation. At that ratio, your speedo will be 0.4% off; i.e. 0.4mph off @ 100, 0.2mph off @ 50, etc.; so close, that the wear on your tire tread over 3000 miles, or 1 psi of inflation, or compressing the tires farther by having an extra passenger, will create a greater error than the error in the gears, and therefore, the error in the gears might as well not be there. (we call that phenomenon "measure with micrometer, mark with chalk, cut with axe") You would order a 15-tooth drive gear and a 42-tooth driven gear, and maybe depending on which speedo gears you have now, you might need the housing too. The brown truck comes, and then $20 (or maybe $70) and 10 minutes of work later, your speedo will be as PERFECT as it can possibly be.

See, not so tough, is it?

red rock

iTrader: (2)

I couldn't have explained it better.

JohnnyT/A

Thanks for the replies. Before I remove the speedometer gear housing, should I drain all the transmission fluid first?

RedLeader289

iTrader: (2)

No, just be sure the car is parked on a level surface and you shouldn't lose much IF any by unscrewing the housing.

JohnnyT/A

Would I be correct to say the drive gear needs to be changed if the speedometer is off around 25 mph to high because if that's true I'm gonna have to bring it to a transmission shop because removing the tail housing of the tranny is something I don't wanna get into. If it's just changing the driven gear by removing the speedometer gear housing, I could handle that but after getting off the phone with a guy at the local transmission shop, he was telling me that the drive gear had to probably be changed because the mph is so far off yet the consensus on here is that I should have the 15 tooth (gray) drive gear already in there, and I should just have to install the 43 tooth (purple) driven gear on the inside of the speedometer gear housing. Rrrrrrr I'm so confused!

FRMULA88

You won't know until you or the trans shop take a look

My car had the grey drive gear so we just changed the driven gear. that is not to say EVERY 3rd gen with 2.73s has it. Your trans guy said the same thing... HE NEEDS TO TAKE IT APART AND SEE to be sure. once you remove the driven gear.. you can seen the drive gear.. if it's grey you leave it alone... not rocket science

jrhaus76

Call bowtie overdrive. Order new drive and driven gears. They are $80. Replace them BOTH and be on your way.

You need to give them new rear gear size and rear tire size. I just swapped to 3.70s. Haven't changed the drive and driven gear yet. I'll report back when I do. Too friggen cold out now to work on it.

JohnnyT/A

I understand frmula88 but what the guy also said was it was going to be a hit or miss type of thing where he would have try several different combos which translates to me as bullish?t and lots of $$$$. Also I just went to Bowtie Overdrive web site that jrhaus76 just mentioned and typed in my rear wheel tire size and rear gear size and it's telling me that I need a 19 tooth drive gear and a 34 tooth driven gear???? Now I'm really confused I thought I should need a 15 tooth drive gear and a 43 tooth driven gear? At this point I just wanna buy the correct combo and take it to a tranny shop and be done with it, BUT now that website just through me for a loop!

FRMULA88

there is nothing hit & miss about simple math..

sofa gave you the detailed explanation to figure it out long hand..

OR just use the chart on the link from the first reply.

the important part is you need to See what is in there to select the correct driven gear.

try not to get geeked out about it, it is much easier to just swap out the driven gear than replace both.

if you don't want to deal with this. I assume you have the electronic speedometer.. you could just replace the VSS buffer box (the yellow box under the passenger side dash) with an adjustable one to recalibrate the signal to the speedometer.. then you can play with the GPS on the phone..

but the driven / drive gear is simple..

scoflaw

iTrader: (2)

Larger drive gear will make the speedo turn faster. Larger driven gear will make the speedo turn slower. 15/43 should get you close, and no way would 2.73 to 3.73 make it show 25mph faster. 1 screw, take a look at what you got to start with, more than likely you got the grey 15t drive so changing the driven is simple stuff.

jrhaus76

You MUST call. The lady said the calculator is WRONG. (That's nice).

I have mine at the house. If you want to wait a week or so, I'll do a write up on switching them out and if it fixed my speedo.

jrhaus76

Yes it will, if you're going fast enough It's off by a percentage.

I went 2.73 to 3.70.

At running 65 on the interstate my car reads over 90. Lol

FRMULA88

LOL ! I was going to say that second site you went too must be wrong..

FRMULA88

Correct I recalibrated my speedometer in 1998.. 2.73 to 3.73

and I got to do it again last year when I changed the 700R4 to a 400.
40 tooth reluctor wheel, VSS, and Dakota Digital box.

and I get to fine tune this when I change the 3.70 to a 4.30 and 28"tires.
maybe I will use the GPS app for that LOL !

red rock

iTrader: (2)

I don't know what to say. If you would have clicked on my first link, and scrolled just a little, you would have found the speedometer calculator.http://www.bgsoflex.com/speedo1.html I just checked, and a 15/43 is what i come up with. But that all depends on what your speedo gears are now.

JohnnyT/A

I'm about to order a 15 tooth drive and 43 tooth driven but now I see there's and option for the driven gear between whether it's a cable driven speedometer or electronic. Does anybody know if the 87 trans am switched over to having an electronic speedo by then?

jrhaus76

I'm under the impression the only difference is how the signal is sent. It is still read by the drive/driven gears... I think. I'm pretty sure my 89 L98 is mechanical from being under there 11 billion times.

I got my gears in the box. I'll update everyone with pics and whatnot when I swap them.

JohnnyT/A

Actually if it is electronic, there is a different one.

FRMULA88

Firebirds have electric speedometers beginning in 1987.. Camaro a few years after.

2 ways to tell.

1: you will not have a speedometer cable at the transmission but you will have a VSS (vehicle speed sensor) with 2 wires (purple & yellow..)

2: Passenger side dash.. the 2 wires from the VSS are connected to a VSS Buffer box (yellow plastic box) with some more wiring:
The green wire is the feed to the speedometer. black is the ground, pink is ign, switched power etc.

you can play with the gears or replace the buffer box with an adjustable one... didn't I say this already?!

JohnnyT/A

Yes you did and I decided to play with the gears so upon ordering them there was a choice between cable or electronic.

25thannivZ28

When I had my transmission built (in 2006) I also had the guy install my 4:10's. He swapped out the gear to the proper one as mentioned in these posts (I don't know which one) but then he said it likely still won't be exact. He advised to get ahold of someone at (he recommended) Dakota Digital (D.D.) who could provide me with a speedometer calibration module.

I got one and hooked it up (Electronically) so I can adjust my speedo on the fly in increments as little as 1mph. Either by using a friend in a pacer car next to you or a GPS or whatever. I spliced it somewhere in the harness between the ECU and the Speedo. I can't remember which wire, but D.D. guided me to which wire it was when I was in the process of installing it.

Now this was a while ago so I don't remember who I talked to @ D.D. but I'm sure any one of the techs there can help you.

That's what I did when I went with numerically higher gears & 17" rims. Hope this helps, good luck