Drivetrain Can’t identify correct speedometer gear

[townethug]

I recently used LMR’s speedometer gear calculator to identify the correct speedometer gear for my fox. According to that I should be running a 23 tooth but when I put it in the speed still reads significantly higher than I’m actually going.

Here’s my setup.
‘90 T5
3.73s
275/40/17

At this point I’m not sure where things went wrong, hoping y’all could confirm my math or offer suggestions to track this down.

Thanks!

 

[AeroCoupe]

Do you know how many teeth the driver gear on the output shaft of the T5 has?

 

[townethug]

I don’t, I assume it’s stock and I can’t imagine why the previous owner would have
changed it. The gears and wheels are my doing.

 

[General karthief]

Tire height?

 

[AeroCoupe]

You need to verify if it is a six tooth (black), seven tooth (yellow), or 8 tooth (translucent green/yellow) and that the teeth on it are all in good working condition.

 

[AeroCoupe]

Went out in the garage and took a picture of the 7 & 8 tooth gears.

I pulled the 8 tooth out of my T5 and went to a black 6 tooth when it was gone through back in the late 2000’s. This was so I could run a driven gear that was below 20 teeth. Just for longevity as it seems the ones that are 21 tooth and up do not last.

 

[FastDriver]

Agreed. Don't trust my memory, just use the calculator on LMR, but I think you need a 23 tooth, which has really thin teeth that will tear up every couple months if daily driving, and will be off by a few mph at some point in the range. Extend their life by ensuring a lubed up cable with very smooth bends, but they'll still tear up. Or, you could put a 20 tooth in, that will last and at least have a reference that's 20% higher than actual speed.

I'm not saying this will be worth your while at ~$400, but I restored the speedo in my daily from an '03 Cobra T56's digital signal using an Abbott Cable-X to drive a cable. This allows the user to match precisely to any gear/tire combo & doesn't have a gear to wear out. Foxes use a digital signal to the EEC & Cruise Control that you could tap into. It's probably not more pricey than paying someone to pull and tear into your transmission to install a 6-tooth drive gear, and it will guarantee a perfect speedo.