new rear now MPH is wrong

[fordkid68]

hello every one.


well I just got done changing the rear ends in my 66. and now my gauge is saying I am going slower then I really am. could this be cuz maybe I have diferent gears in the new rear end. I thought they were 3.20(the same that is in my old rear end) but maybe I am wrong maybe there 3.25 or something ellse. but maybe I am way off track. here is whats going on. it will say I will be going just under 20 but the engine sounds like it is winding up higher then that. so I shift to second(it is A 3 speed) normally I thought you could go 25 or 30 before your engine really starts to wind up as high as it was. and once I get to about 30 it will just plain stop going up. I even took it on the freeway and was pacing with the other cars and it said I was going 33 to 35. it wasn't staying in the same spot. so what do you guys think is wrong.

other then that I had no problems changing the rear end. witch is why I am conserned about this problem cuz normally no matter what size job you have you always will have problems. so is this going to be hard to fix or what.

well tell me what you guys think is wrong.


thanks
ford kid

[Gearhead999]

Yep, the rear ratio will change your cars speed to what the speedo is reading.

Not a big deal to repair, usually. Add or subtract number of teeth on speedo driven gear, one tooth approx. 6 mph. Add teeth will slow speedo down, subtract teeth will speed speedo up.

Or, just take it to a speedo repair shop and have them calibrate it.

[John Z]

I just fixed this on my car. You need to change the gear that goes into the transmission at the end of the speedometer cable. Mustang parts places sell them. I bought mine at the Ford dealer, but I have a T5 transmission from a newer car, I don't know if Ford still sells these parts for the older transmissions. The easiest way to determine the number of teeth you need is to get on the freeway and drive a CONSTANT indicated speed and measure the time it takes to go one mile. I used the mile markers that are on the side of the road. Use the time for a mile to compute your actual speed. For example if it takes 62 sec to go one mile then your speed is 3600/62 or 58 mph. Now pull the gear from the transmission, one bolt and a clip, and count the number of teeth. Multiply the number of teeth by the indicated speed divided by the measured speed to find the number of teeth you need on the new gear. For example, if the indicated speed was 50 and the measured speed between the mileposts was 58 and your current gear has 20 teeth, then 20*50/58 = 17.2 so you need a new gear with 17 teeth. Nothing to it!

[Bake]

That is one complicated thing to do to get you gears right. what about with new mustangs, like a 2000 is it like that or can i program my computer by plugging something in and saying im using this ratio or something?

[fordkid68]

John Z do you mind explaining that again. cuz I have read your post 6 times and am still confused. but at least some one knows whats wrong. thanks for the reply.


thanks
ford kid

[John Z]

Try again - step by step
1. You need to know the amount of error in your speedometer reading.
a. Indicated speed = speed your speedometer is curently reading.
b. Actual or measured speed = how fast your car is really going.
c. Measure your actual speed. You can estimate your actual speed by measuring the time it takes your car to go one mile. Most interstates have mile markers. These are small green signs on the side of the road that have a number on them. The numbers either increase or decrease depending on the direction you are going. If you measure the time in seconds it takes to go one mile, you can compute your actual speed as 3600/Time. For example, if it takes 62 seconds to go one mile, your average speed is 3600/62=58mph. If you kept your indicated speed, on the speedometer constant, e.g. 50 mph, then you can compute the error in your speedometer.
d. Error in speedometer = (indicated speed)/(measured speed) for the above, the error would be 50/58 = 0.82. This means that the cable that is driving your speedometer is only turning 0.82 revs for every rev that it should be turning. To make the cable spin faster, you need to change the gear at the end of the speedometer cable.
e. Remove the end of the speedometer cable from the transmission. For a 3 speed it should be on the driver's side. One bolt, gently pull on the speedometer cable to remove from transmission. There will be a plastic gear on the end of the cable. It is held to the cable by a "C" clip. Remove the clip and slide the gear off the end of the cable. Count the number of teeth.
f. Multiply the number of teeth by the error to determine the right number of teeth. If you have 20 teeth on the existing gear, then 20*.82 = 17.2, so you would need a gear with 17 teeth.

[mustangman65_79]

Hey, what happend to my posts I put in here?!?

[fordkid68]

John Z thanks for explaining it again I really appreciate it. I understand now.


thanks
for kid

[John Z]

Let us know how it turns out.