I have an ‘88 4Runner 3.0 Auto with 4.10 gears on 33” tires. My speedometer is off by about 15 MPH at 70 gps speed. I don’t know how many teeth are on my speedometer gear. Is there a way to figure that out without getting into the transmission? How many teeth do I need for it to be somewhat accurate?

 

you sure about that? '88s with the 3.0 and auto were standard with 4.30 gears, rather than 4.10s. they also had a slightly deeper transfer case ratio of 2.659:1 vs 2.566:1 for the manual transmission models

 

My axle code is G292.

 

if it is, it should probably truly be a G294, as toyota misidentified a lot of the rear ends in these trucks. a 292 is a 2-pinion with 4.10s, while a 294 is a 4-pinion with 4.10s.

 

Are you actually traveling faster or slower than indicated by the speedometer? I had an issue where the speedometer head had lost some of its magnetization and read low. The fix for that involved buying a replacement from a salvage place.

 

When going 75MPH the speedometer reads ~65. The whole needle moves a bit up and down when driving, but it reads about the same speed.

 

this is pretty common in older vehicles. the odometer is directly attached to the cable, so it reads pretty consistently, while the speedo is "attached" by magnetization, so it can vary by how much it is "off". having a gps unit readout is handy, although having owned one of my '87s for 9 years, i can reasonbly determine my speed from the tachometer.

 

Not really an answer to your question, but one solution for inaccurate speedo is to get a correction box from an outfit like Portland Speedometer:
https://www.speedometerserviceportland.com/
Just a box that goes between the transmission and the speedometer cable, they will provide instructions for getting them the info they need to correct the speedo. Or you can velcro your phone to your steering wheel and use an app.

 

That sounds familiar. The oscillation is wear in the cable. For my 1991, I was able to get a new cable from Toyota. That took care of the oscillation. My speedometer head was shot to the point it squealed. Yours may be ok or it may need to be remagetized.