Well it turns out my speedometer reads 4MPH fast. But now if I do the actual GPS speed, my RPM is definately higher than what it should be. Unless the RPM is off as well. At 70MPH the 4.7 final should be at 3800RPM I believe? According to my gauge now it's above 4k making me wonder if I do infact have a different final drive? Or if my gauges are all kinds of screwed up. Those with the 4.9 final can you do a simple check for me? Just say what RPM and MPH or KMH you are at preferably below 70MPH because that would require me to take a highway trip and pay tolls to find out the exact RPM my gauge reads. Thanks

 

this link might help you

you enter gear ratios tire size etc, and it tells you speeds etc

http://www.zealautowerks.com/transcalc.php

 

Gearing shouldn't effect your speedometer read as the speed sensor takes the read after all gearing has taken place (on the output of the diff). What does make a difference in your speedo accuracy is tire size. If your tires are 4% bigger than stock, you will indicate 4% low. If your tires are 4% smaller, your speedo will read 4% high.

Keep in mind, OEM speedometers are not 100% accurate in the first place, so custom tire size could offset a variance or compound it...

 

Right, I know tire size changes the speed but what about RPM? Gearing changes RPM

 

Awesome to compare BTW. Tomorrow I will try 5th gear at 3000RPM using GPS and with the 4.785 I will be at 55.15MPH and with the 4.9 53.85MPH. Thats rather hard to tell the difference of 1.3MPH. The only way to really tell the difference is to drive much faster to make it more noticable or stay in a lower gear to not break the speed limit so much. Maybe I will try a 4th gear highway cruise at 5k. Not until I break in my new clutch more though.