Reason I ask is, I was trying to figure out how far off my speedo is, and also verifying gear ratio out back.

I run a 235/60-15 tire, 26" tall.
My axle is stamped for 3.55 gears, and rotating one wheel up in the air counting the driveshaft turns seems to match. (had the cover off once, but forgot to count teeth- d'oh!)
The question is, --or what I've run into is, using a GPS unit, driving along, taking readings of speed and RPMs, and matching them up to an RPM chart of either 3.55 or 3.73, My tachometer and GPS readings give me more of 3.73.

And yes, my speedo reads 'bout 4 mph slow, I can handle that...no biggie.

Is there that much converter slippage to make it look like 3.73- ish on the tach? I use an Autometer tach if it makes a diff.

Why would someone change from 3.55 to 3.73 on a plain jane? Thats not all that different I would think to make any real difference.....
What do you think?

Hold it in 1st and test it above 3000 rpm to insure you are beyond any stall built into your converter.

My stock set-up's with a 3.73 gear and stock tire diameter, I have three vehicles OE set-up the same way, had four, a '91 Nissan 300ZX TT, all read 3K rpm at 60 mph.

Most stock 'Glide torque convertors flash to 1600-1800 rpm. My GPS is within 0.2 mph of the speedo on the Wife's 2006 Impala.