Speedometer is off after ring and pinion gear was changed

[mexiwaiian]

To whom this concerns,

I looked through some of the posts, however non of them seem to really answer what I am looking for. I have a 1991 240sx fastback in which I have the R200 rear end with ring and pinion gears changed to 4.636. Now the speedometer shows my speed to be a good 20MPH or so faster than actual.

I was under the impression that I can change my speedometer pinion gear which can help get my speedometer to be more accurate. Can someone comment on how to determine what is needed or how I can calculate the right one? There are 17t, 18t, 19t, 20t, 21t, and 22t sizes through http://www.courtesyparts.com and don't really know how to determine what is the right size.

-Moses

[churro]

Zombie thread

Seriously though, I'm changing the final ratio to 3.69 and I'd like to know if there's a way to calculate the number of teeth on the speedometer pinion to read accurately.

[got chub]

my 95 manual is all stock gearing and i have 20t on my speed sensor that reads 3x my real speed dont know if that help but there ya go

[MrPresidentQ45]

I'm looking to swap in steeper gears and change to a different tire size, so I'm making this as difficult as possible for myself. If I understand correctly, my 240 has no speedometer cable but instead a sensor. It sounds like there is still a gear on the sensor (?). If that is the case, you should be able to change the number of teeth to correct for changes to overall wheel diameter and gears. I looked into this for another vehicle, and if I remember correctly, 1 tooth = appx. 5mph, and adding teeth should slow the gear. I would suggest checking with a local transmission shop... when in doubt, ask the experts.

[codyace]

Wow, twice tonight I don't have specific help but can point you in the right direction...

I forget if it was on FreshAlloy or Zilvia (ya know, the other forums) that I asked this same question, and got a reply about monkeying with the speedometer itself....

I didn't want to break anythign, so I just went to a gear calculator site, knew what rpm's were what speeds, and went from there lol.

[Hijacker]

zeroyon.com has a writeup on using the calibration dial on the speedo to add a 180 mph gauge. The guys on zilvia or FA prolly used that as a jumping board to figure out how to change the speedo to correct for a different final drive.

As for changing the mechanical pinion on the sensor, I wouldn't know where to start with calculating the necessary gear ratio.