I just swapped a 3.73 posi into my '86 Camaro (it had a 2.73 open carrier from factory). My dad and I did a bunch of research on what my driven and drive gears should be for 26" tires and got them installed but my speedo is still reading 10 mph fast like it was with the 2.73 gears. Is there something in the dash cluster that I need to look at to get it to read right? It bugs me that its not even close to accurate

 

https://www.tciauto.com/speedometer-gear-calculatorc try this

 

I already have a 15t drive gear and 43t driven gear like is says i need. I cant figure out why its still not reading right

 

+/-5mph is about as accurate as the speedometer can really be expected to get. If you're only off by 10mph, you're pretty close.

A factory 3.73 26" tire thirdgen would have come from the factory with a 44 or 45T driven.

 

I recently went through this with my wife's 1987 IROC. None of the calculators I tried would accurately predict my necessary gear combo, so I ended up buying a pack of various sized Speedo gears on eBay and trial and error got me within 1mph of accurate. I had the 15T inner gear. Don't remember what outer gear size I ended up with, but I discovered every gear changed the Speedo by about 2mph at 60.

 

It's the right drive gear, and wrong driven gear.This. For a mechanical speedo Camaro, I'd go straight to the the 45, unless your tires are larger than stock.

 

I meant to say 15 & 45. I looked back at when i bought them and it was the 15 & 45t gears

 

Speedo was off 10MPH before when speedo gears were 'correct' ?

Swap rear-gear, swap speedo gears &,..........

Speedo is still off about 10MPH after while speedo gears are 'correct' ?


Could be the Speedo head itself. Maybe the needle is just set on the gauge wrong and you can spin the needle a bit to fix it. Or maybe the Speedo head is just bad. It's also possible ( not probable! ) that there's some 'drag' in the Speedo cable ( creased cable /binding ).