speedo/gear relation

[lukedukem]

i was taking a criuse sunday and my wife was behind me in my truck, she called and said why are you going 50. my speedo said 60.
what would cause this, lower gears. it came factory with 3.23, i haven't crawled under it to check if there is any tags or marking to see what they are now. but would this be the issue. also rpms were somewhere around 2500. idk
                                                                                     

[440]

Is it an auto or manual ?

You probably just need to change the the speedo cable drive gear.

[lukedukem]

sorry, yeah its auto... if the previous owner put lower gears, should there be a tag. or how can i find out what they are now. is there a conversion table .

[440]

If there isn't a tag (or it's wrong) jack up the rear of the car. Mark the top of the tire with chalk and also the driveshaft yoke. Have a friend turn the tire one revolution and count the rotations of the yoke. This should tell you what gears are in the rear.

If it's an open diff you'll have to spin the yoke instead.

[Charger RT]

if the rear is an open end it is easier to leave one tire on the ground and spin the tire off the ground 2 revolution while counting how many turns the drive shaft turns. It will give you the correct number.
Tim

[lukedukem]

ok. i'll try it, but regaurdless, would this cause the speedo to read higher than i'm actually going?
a dumb ? but what does the open diff look like, cover on back or third memeber you can change out
luke

[440]

Your car should have either a limited slip diff or open diff. If you jack one rear tire off the ground can you turn it fairly easy?

By figuring out what rear gears (and tire size) you can get the proper speedo drive gear in the transmission.

[Musicman]

By figuring out what rear gears (and tire size) you can get the proper speedo drive gear in the transmission.

What he said 

[440]

This should give you an idea. http://www.mymopar.com/speedometer.htm

[lukedukem]

perfect. thanks 440
now i just need to wait till 5am to get off work and go home and wakeup wife to help me. thats probably not gonna fly. 

luke

[Cooter]

Simple....Use a GPS...

[John_Kunkel]

Speedometers can be in error even if the drive gear is correct, the odometer is the best way to check for the correct drive gear.

[lukedukem]

thanks for the help guys, i'm just trying to find out why my car says i'm going 60 but its only going 50 in reality. my fault for rambling on about gears and stuff. i'm not sure what is causing this...
yes, first thing is i'm going to find out the gear ratio. the gps thing just confused me more. thanks for the link to that table 440. i'm working nights this week but this is the last one. i'll be off till monday so i have the wekkend to get it ironed out
luke

[Musicman]

Speedometers can be in error even if the drive gear is correct, the odometer is the best way to check for the correct drive gear.

That's a good point John 

[440]

There is a little gear inside the tranny (reasonably easy to replace) that drives the speedo cable. They come in different teeth count to compensate for different factors such as different gear ratios, different tire sizes etc.

If you changed nothing on a unmolested car except went to a larger diameter tire the speedo for example could read 50 while your doing 60. Same on the other hand smaller tires would make the speedo read high. ie read 70 when your doing 60.

Same with gear ratios, so it's important to know what gear ratio you have and tire size. Someone at some point either changed transmissions, rear end, or tire size from stock.

Cooters suggestion of a gps is a good idea to double check your speedo.

[lukedukem]

There is a little gear inside the tranny (reasonably easy to replace) that drives the speedo cable. They come in different teeth count to compensate for different factors such as different gear ratios, different tire sizes etc.

If you changed nothing on a unmolested car except went to a larger diameter tire the speedo for example could read 50 while your doing 60. Same on the other hand smaller tires would make the speedo read high. ie read 70 when your doing 60.

Same with gear ratios, so it's important to know what gear ratio you have and tire size. Someone at some point either changed transmissions, rear end, or tire size from stock.

i get it now. i'm not sure what stock tire size was but its got 275/60 r15 now. that wouldn't be enough to make it read wrong(plus its reading 60 and going only 50, so it would have to be a smaller tire), but the car is a bb and factory it was a sb engine and tranny. but i thnik its the rear end. i mean the RPMs were getting up there too.
thats were i'm gonna start today after i wake up. find the rear end gear ratio and go from there.
if i can crawl under the ar and get some numbers off it i'll need yalls help coding it, so i'll post the results when i get done.
thanks
luke