speedo is all whacked out

[ParatrooperAlex]

i have a '78 Z28 with 235/65/15 tires, on the '81 Z28 wheels. when im going 25MPH the speedo reads 40MPH, 40MPH reads 60 MPH, 60MPH reads 80MPH, ect. i was searching the threads and found a tire conversion calculator, but it says i should only be off by 5mph at the most. what can i do to fix this without having to buy a whole new speedo?

[sks52587]

there is a plastic gear in the tranny, that could have gone bad, dont know wy. Also, did you change your rear end ratio? that will throw it way off!

[BBCAM]

You need to do three things , know your rear end ratio, your tire circumference , and what transmission you have .. then you need to go to a transmission shop and get the correct ratio gear set for the trans to match the tire size and rear end ratio the spedometer has nothing to do with it it only reads what it recieves from the trans gears , and im gussing that somewhere along the life of your car someone has changed the trans to one out of a car with a different rear end ratio causing the error in your speedo thats the same thing that happened to my 78 Z and it was as simple as going to the trans shop and measuring the tire and knowing the rear gear, the tech at the shop set me up with the proper output shaft and driven gears to match the parts I have all I had to do is remove the tailshaft housing and the gears are right there , one connects to the spedometer cable end and the other rides on the output shaft and is held on by a simple clip ...

[d-z28]

i have that same problem but mine is 30 mph off due to tire size 410s and new trans i just havent got around to gettin it fixed no time

[btchncamaro]

you could just have a stretched speedo cable too. let me know the color of the nylon gear in the trans,(they are color coded) at the end of the speedo cable. i dont even have one and just need something close. my set up is: tire circ. 28" rear end ratio 4.10's, and tranny is a th350. even the stock color would probably help me out. i bought the car rolling.

[BBCAM]

They are color coded, but the drive gear is different depending on application and if you dont know which you have you will never get close and risk chewing up the gears .. if you know how the car was originally equipped (rear and tire wise ) you could get close if you have the OE trans and adjust for any ratio change ..you would still need to use the above formula to get it right , the gears are cheep and they are not hard to put in .

[ParatrooperAlex]

the gears should be stock, but is there a way to figure it out with out taking the gears out?

[d-z28]

mark a line on the drive shaft then turn the wheel one complete turn while counting the turns of the drive shaft if it turns 3and 3/4 turns then it is a 3.73 rear ratio and so on

[ParatrooperAlex]

yeah, i thought it was somthin like that, i'll check it out this weekend.

PS- check out my thread about euro lights in the electrical section

[Louvit]

I have 373's and mine if off 10 mph