speedometer too slow

[Ribbedroof]

To provide a little clarity on build sheets...

Per GM, the broadcast (build) sheet was never intended to be left in the car. They were attached to the components used to construct the cars, and were to be removed and discarded upon assembly. This info came from GM via a state patrol Vehicle Theft Officer during the investigation of a late-model GP in the mid-80s. However, obviously, many line workers did not remove them if they were not going to be seen after assembly.

This is why many people are not able to find a build sheet for their car....they aren't supposed to be there in the first place.

As a footnote, the GP turned out to be a tag job....seller (a bulk oil dealer) had been peddling stolen cars for a few years to good customers. They got tripped up by retagging the GP to a year that some of the content on the stolen car was not offered.

 

[mickeypowa]

hi guys!
I have some news :
so I did the tape test and I get 1.12:1
I mean when I make the wheel do 1 complete rotation, the driveshaft do 1 and 1/8 rotation so 1.12
Is it a normal rear end ratio ? it seems low ? maybe that's why my speedometer is too slow ?
note that when I spin a rear wheel, the other rear wheel doesn't move at all.

and my rear tire are this type : https://www.summitracing.com/parts/mtt-6653/overview/
it's written 26x10.00r15 sportsman on it.

so what can I do now ?

 

[MrSony]

Does your car have a posi rear end or an open diff? How many marks does it leave when you do a burnout?

 

[MrSony]

If it's a posi rear end, or limited slip, you only rotate the tire one full 360 degree revolution and count the number of times the driveshaft turns a ful 360 degrees as well. If it is an open diff, you need to spin the drive wheel (rear passenger side) TWO revolutions to get an accurate statement.

 

[UNGN]

You have to spin the driveshaft and watch the wheels..both wheels.

Put the car on jackstands and put it in neutral.

Put a piece of tape on the bottom of the driveshaft and the bottom of BOTH tires. Spin the driveshaft and watch the tires to make sure they are spinning equally. Count the drive shaft revolutions for both tires tape to go 360 degrees.

If you spin a tire and watch the driveshaft, the differential will mess up the reading.

 

[MrSony]

If the driveshaft turned 1 1/8 times and that as a decimal is 1.12, You either have 2.29 rear end gears or 2.56 gears. Since you got 1.12, that leads to my previous thought that you have an open differential. For open differentials you take the number you got in decimal form times 2, which is 2.24 which is damn close to 2.29. These cars came with 2.29 gears as standard equipment with three speed automatic transmission, but I think you could've gotten 2.56 too, but 2.29 seems more likely.

 

[Ribbedroof]

Just pop off the rear cover and count teeth....gear lube could stand to be changed by this time anyway.

 

[Ribbedroof]

If the driveshaft turned 1 1/8 times and that as a decimal is 1.12, You either have 2.29 rear end gears or 2.56 gears. Since you got 1.12, that leads to my previous thought that you have an open differential. For open differentials you take the number you got in decimal form times 2, which is 2.24 which is damn close to 2.29. These cars came with 2.29 gears as standard equipment with three speed automatic transmission, but I think you could've gotten 2.56 too, but 2.29 seems more likely.

Rear axle ratios varied from year to year, by engine/trans selection, and by emissions standards as well as body style.

 

[UNGN]

If you spin a tire and watch the driveshaft, the differential will mess up the reading.

hi guys!
I have some news :
so I did the tape test and I get 1.12:1
I mean when I make the wheel do 1 complete rotation, the driveshaft do 1 and 1/8 rotation so 1.12
Is it a normal rear end ratio ? it seems low ? maybe that's why my speedometer is too slow ?
note that when I spin a rear wheel, the other rear wheel doesn't move at all.

and my rear tire are this type : https://www.summitracing.com/parts/mtt-6653/overview/
it's written 26x10.00r15 sportsman on it.

so what can I do now ?

The other tire not moving at all is a little disturbing if it was completely in the air (maybe your brakes drag), but if if you have an open differential (likely) and and the other tire didn't move at all and you spun the tire instead of the driveshaft like you are supposed to do, you likely have a 2.29 or 2.41:1 rear axle ratio. If the other tire moved even a little, the test is meaningless (which is why you are supposed to spin the driveshaft and not the tire).

 

[Kwik_Cutty88]

Left hand drive Cutlass, cool man. Speedometer cable gear inside the transmission is probably broken, it's plastic so it breaks over time. What size tires and what rear end ratio do you have? That will determine what gear you need.

All Cutlasses were left hand drive?

Also OP, consider using this calculator. You'll need to know your speedometer gear drive size, tire diameter and rear end ratio. This will tell you what driven speedometer gear size you'll need.

http://www.tciauto.com/tc/speedometer-gear-calculator/