Speedo gear help

[keith88]

Hi guys last night i got my 1st ticket in the charger coming home from a cruise said i was doing 64 in a 45 ..didn't feel like it and the last time i looked at my speed shortly before he stopped me it said about 50 . I know my speedo is off but have no idea by how much ,kind of hoping some one could tell  what gear it should have to stop this from happening again. I have a 904 with a 15" rim ,tire size is 27" it came with 14 so i know right there is an issue . I did google this size to gear thing but it skips my tire size goes from 26 than list 28 so not sure what i should have . My rear end is a 2:76  any help would be much appreciated.... thanks.

[cdr]

this

[lukedukem]

I have the same issue. My car is off by about 10 mph. I have an app on my phone that does speed. I just use it to see what rpms I'm at for the speed then I can turn it off or just keep it on. I too need to change my gear I my tranny but don't feel like it.
Luke

[myk]

It's real easy guys.  The hardest part is figuring out which pinion gear you need, which that chart above should help...

[keith88]

OK i pulled the gear to look and i have a yellow 33 tooth in it now I have 2:76 gear in the rear , according to the chart above with my 27" tires i should have a 26 tooth gear in it .. I also believe its like 20 mph off so dose that make since.

[cdr]

yes it makes since, also i am not sure but i thing the 26 takes a different holder ?  my car came with 2.76 & a 28 tooth, but had shorter tires than yours.

[John_Kunkel]

27" tire and 2.76 gear requires a 27-tooth pinion. Keep in mind that the correct pinion only ensures a correct odometer not a correct speedometer. Even with an odometer that's right on, the speedo can be way off due to its design. (and age)

[Just 6T9 CHGR]

There is a speed app for the iPhone that uses GPS that is fairly accurate  +/- 1-2 mph

[keith88]

Thanks for that info i just ordered a 27 tooth gear and downloaded that app ill try it on the way in to work tomorrow.

[keith88]

OK i got my 27 tooth gear in today it didn't go as planed 1st i am still off 5 mph at 45 and 10 at 65 but it dose not increase from there so that's good. looks like i need a 25 tooth gear but i need a new housing to mate with it anyone know where i can get one couldn't find one searching the internet. I also stripped out the bolt that holds the housing clamp as well as broke the  o-ring couldn't find on of those locally so i ended up supper gluing it and helicoiling the hole ...what a day!

[XH29N0G]

I hope I have not missed something in the thread, but I think the next step is to check the speedo gear against the odometer.  You may have the right gear, but the magnets on the speedo head have weakened, or the needle is binding slightly.  The check against the odometer will tell you whether the issue is with the gear or the speedo itself. In principle you could choose a gear that works with a bad speedo and have it work, but I would try to get it right if possible because it may be more likely to stay right.

[keith88]

Can you explain a little more please ,the odometer should not be right any way if the speedo is off right? Not sure how to check that?

[XH29N0G]

I think the odometer is simply geared to the speedo cable.  It counts the number of times the speedo cable turns.  This means if you change the pinion gear in the transmission or the gear ratios, tire sizes etc.  the odometer will change.

The speedo mechanism is not directly counting revolutions.  In my car, the speedo mechanism is a needle that is on an assembly with a return spring (called a hair spring).  Inside this assembly is another assembly that is attached to the speedo cable.  The two assemblies are not connected.  When the speedo cable rotates, it rotates the inner assembly.  The inner assembly has a magnet (I think) and the spinning of this magnet causes a force to be exerted on the outer assembly.  The more it spins, the more force.  The force causes the outer assembly to rotate against the return spring and this causes the needle to move.

Where it can go wrong is with the return spring (it might weaken over time), with dust getting into the outer assembly (it might bind up), or with the magnetic coupling (I gather it can weaken over time).    There must be something on the web describing this.  If I find a description/picture, I will post the link.

[keith88]

Thanks but still not sure what one has to do with the other as far as how it reads the speed. I started out being 19 mph off got that down to between 5- 10 mph off  going from a 33 to a 27 tooth so i was just figuring on a 25 tooth should put me  with in  range of the cop not giving me a ticket issue. But if you find info on the odometer thing by all means i still would like to see it ..thanks.

[cdr]

get on the freeway & check your odometer reading against the mile markers on the road, that will tell you if the gear is correct, if so you need your speedo recalibrated

[XH29N0G]

I think you will probably be able to solve the speedometer reading by trial and error (It sounds to me like you have found the right pinion gear right now, but the only way to check is to check the odometer reading).  

What I found on how stuff works is:  http://auto.howstuffworks.com/car-driving-safety/safety-regulatory-devices/odometer.htm and http://auto.howstuffworks.com/car-driving-safety/safety-regulatory-devices/speedometer3.htm.

I think the key point is that the odometer and the speedometer are two different devices that are both driven by the speedometer cable and the pinion gear in the transmission.   This means either one could be right (or wrong) in its reading.  You are interested in the speedo.  All that I was suggesting was that the speedo is usually calibrated to the odometer, and the odometer is simply set by gearing.  

You had an incorrect pinion gear (I actually think I do too) but corrected it and the speedo still reads off.  Checking the odometer will tell you whether it is still the wrong pinion gear, or whether it is a miscalibrated speedometer.  

I am not an expert in this area so I could be wrong.  If someone else corrects what I write, follow their advice.  

[keith88]

Yea but if your speedo is off wouldn't your odometer be off with the mile markers?

[XH29N0G]

Yea but if your speedo is off wouldn't your odometer be off?

Not necessarily.  If the speedo and odometer are off because of the pinion gear, the answer is yes.  If the speedo is miscalibrated because of a worn spring or dust in the mechanism or something with the magnet, then no.  This is what is so crazy about the whole set up and what I meant when I said they are two separate devices that are both run by the same cable. 

[keith88]

OK that gives me an excuse to go for a ride in the morning and see what i come up with. Dame this stuff is confusing!!

[cdr]

example if your speedo reads fast but the odom only clicks 9 tenths in a mile then both are off, if you click 10 tenths in a mile but the speedo reads wrong then the speedo is out of calibration .

[ws23rt]

Thanks but still not sure what one has to do with the other as far as how it reads the speed. I started out being 19 mph off got that down to between 5- 10 mph off  going from a 33 to a 27 tooth so i was just figuring on a 25 tooth should put me  with in  range of the cop not giving me a ticket issue. But if you find info on the odometer thing by all means i still would like to see it ..thanks.

One thing I learned many years ago is that knowing the speed one is traveling is the responsibility of the driver.  I have tried all of the issues that I thought would explain my speed when in front of a judge.---I do mean all of them--

The overwhelming question from the judges is how long have you been driving?---Their is no answer to this question that will get you out of the conviction (that I have experienced). --If you have a license you should know how fast you are driving.

Getting a 45 year old car to be accurate in how any of the gauges perform is a task that is as big as how accurate is good enough?

If you have the right speedo gear for your rear gear and tires and the mph on the dash is not right (+-5mph) it has to be a problem with the mag. pic up in the speedometer.

I would say understand what you see on the dash and if it is +- 5 mph be happy.

[keith88]

Thanks but still not sure what one has to do with the other as far as how it reads the speed. I started out being 19 mph off got that down to between 5- 10 mph off  going from a 33 to a 27 tooth so i was just figuring on a 25 tooth should put me  with in  range of the cop not giving me a ticket issue. But if you find info on the odometer thing by all means i still would like to see it ..thanks.

One thing I learned many years ago is that knowing the speed one is traveling is the responsibility of the driver.  I have tried all of the issues that I thought would explain my speed when in front of a judge.---I do mean all of them--

The overwhelming question from the judges is how long have you been driving?---Their is no answer to this question that will get you out of the conviction (that I have experienced). --If you have a license you should know how fast you are driving.

Getting a 45 year old car to be accurate in how any of the gauges perform is a task that is as big as how accurate is good enough?

If you have the right speedo gear for your rear gear and tires and the mph on the dash is not right (+-5mph) it has to be a problem with the mag. pic up in the speedometer.

I would say understand what you see on the dash and if it is +- 5 mph be happy.

well my issue will be that for 30 years i was used to driving a 69 camaro not a 69 charger that is a lot bigger and dose not seem that it is going as fast as it is because of its size . I had no idea that my speed was off until i got pulled over. i was never going that much faster then traffic around me to think it was more than a little and not a lot like it was.

[XH29N0G]

Well, at least you get to do lots of test driving. 

Have fun.

[keith88]

And loving it!!

[ws23rt]

Thanks but still not sure what one has to do with the other as far as how it reads the speed. I started out being 19 mph off got that down to between 5- 10 mph off  going from a 33 to a 27 tooth so i was just figuring on a 25 tooth should put me  with in  range of the cop not giving me a ticket issue. But if you find info on the odometer thing by all means i still would like to see it ..thanks.

One thing I learned many years ago is that knowing the speed one is traveling is the responsibility of the driver.  I have tried all of the issues that I thought would explain my speed when in front of a judge.---I do mean all of them--

The overwhelming question from the judges is how long have you been driving?---Their is no answer to this question that will get you out of the conviction (that I have experienced). --If you have a license you should know how fast you are driving.

Getting a 45 year old car to be accurate in how any of the gauges perform is a task that is as big as how accurate is good enough?

If you have the right speedo gear for your rear gear and tires and the mph on the dash is not right (+-5mph) it has to be a problem with the mag. pic up in the speedometer.

I would say understand what you see on the dash and if it is +- 5 mph be happy.

well my issue will be that for 30 years i was used to driving a 69 camaro not a 69 charger that is a lot bigger and dose not seem that it is going as fast as it is because of its size . I had no idea that my speed was off until i got pulled over. i was never going that much faster then traffic around me to think it was more than a little and not a lot like it was.

I do get what you are saying.  Some cars feel like they are going 60 when they are only doing 40. Also some cars look like they are going fast to a cop just cause it's a fast looking car.  I have a new car that feels like 80mph at 120mph.   The judge will not feel or accept your problem.