Speedo gear questions

[annoyingrob]

Here's an odd question. Currently I have an R154 with a 4.3 rear-end, AND appropriate speedo gearing in the transmission for it. My only conclusion is that the revious owner blew up the 4.10 torsen, replaced it with a 4.30, and somehow found a speedo gear out of an old toyota truck or something.

Anyways, I'm replacing the 4.30 with a 3.54 gear set, and I want the speedometer and odometer to be accurate. Also, due to tire sizing, my odometer is exactly 5% slow.

So, by my calculations, if I swap in a 3.73 driven gear into the transmission, it should account for the 3.54 rear-end with a 5% larger diameter tire right?

 

[annoyingrob]

wait, I think I'm calculating the wrong way to offset for tires. Now I'm confused. I would need ideally some sort of 3.37 gear to make it work right?

 

[annoyingrob]

Ok, let's add a little info tho the thread. This would be helpful if people would chime in if I'm right or wrong here.

All pre-89 R154 transmissions came with a 10 tooth speedo drive gear, as did all JZA70s. This is what I would assume toyota refers to as their "short ratio" drive gear

most 89, and all 90+ R154s have 11 tooth drive gears (the "tall ratio" gear)

I believe a 3.73 rear-end used a 30 tooth driven gear correct? (IE: a 30/10 gear)

That would mean the JZA70s with a 4.10 used a 33/10 gear, and my car likely has a 35/10 gear in it for the 4.30.


If I want to match a 3.37 rear-end gearing (3.54 with 26" tires), I would need a 27/10 gear.