lakes90GT

I had a shop switch my stock gear ratio to a 3.73 ratio in my '90Gt manual and the guy called and said My speedometer is way off and he hadn't thought of that before now. What do you guys do with the speedometers after changing gear ratios? First mod! Thanks.

PRO50SC

Change the speedo gear to compensate for the gear change. There's a chart floating around here for the info or some one will chime in for what they used. I have 3.73's also but never worried about the difference in the speedo.

Oh, here's the chart although for some reason it's missing the gear for 3.73's for certain years. No idea why.
https://mustangforums.com/m_1108239/tm.htm

lakes90GT

Thanks I hope someone chimes in with the info. He says my speedo says 80 or 90 when I am going 60. Would slow me down a bit though.

gspfunk

You'll need to remove your speedo gear housing from the tailshaft of the tranny and see what color the gear inside is first. Different colors have different #'s of teeth.

PRO50SC

That seems a lil bit too far off. I think at best, mine was only off 10mph at the most.

OnyxCobra

it'll be off more the faster you go i would think. Anyway I think your easiest/best bet is going to be the White 23 tooth speedo gear, I think its an 8 tooth drive? It wont be 100% perfect but it will be very close.