Guys what do you think is more accurate the GPS/Garmin or the speedometer is these cars? I see my 89 Firebird appears to be 1.5 miles an hour off slow at highway speeds. I think if changed the gear with 1 tooth less it should be closer.
if 1 tooth is equal to 1.5 MPH.

Thoughts?

 

Is 1.5 mph enough to worry about?

 

Last time I used a garmin i found it to be dead nuts accurate to the exact mph. There's a faq top of page that provides the speedo gears necessary for every rear gear ratio. Not uncommon for a speedo to be a mph off. Your not going to correct that by changing speedo gears.

 

I think you could see 1.5 mph difference just by moving your head slightly looking the gauge, unless you have digital dash. Well within accuracy tolerances for the whole system. You won't be able to get it anymore accurate than that.
A better way to check the actual accuracy of the speedo calibration is to use the odometer on a known distance. You can use gps or mile markers or a track. The longer distance you measure , the more accurate the deviation measurement. There may still be an additional discrepancy between the actual "log" (counting of revolutions) and the actual gauge display (the needle), but no way to change that besides installing a more accurate gauge.

 

I think one tooth will give you more change than you want. I would be happy with the 1.5 mph off and call it close enough.

 

The odometer may be totally accurate and the speedo off. So don't assume the gears are the issue.

The 1.5 is only "off" at one speed. So the GPS may be accurate but you may be as close as possible given the speedo gear combinations (you didn't post yours, as no one seems to do.)

There are 4 speedo gear combinations for your trans. / rear gear combo assuming stock height tires.

So there's a bit more math to do in order to determine whether you need to make that change.

 

A tooth change is usually closer to 5 MPH change than 1.5. I would think 1.5 MPH would be more from tire wear than anything else.