Ok, so I was thinking the other day as I was cruising down the interstate....

[The Lady In Red]

My speedometer was reading 70 MPH .  Its a factory 305 but someone swapped in a TH350 trans in place of the 700R4 .  Its got 3.08 gears.  Should I be concerned that my speedo is incorrect because of the tranny swap? 

I never really was a fan of counting between mile marker posts to determine speed. I don't think its accurate enough.

[zieg85]

I wouldn't be concerned as long as you know how fast you are going.  Something like that would bug me.  It is a fairly easy fix but you have to know how far you are off and what color gear is in there.  If you have a GPS you can find out how far you are off.  My $.02

[Irish_Alley]

How far off is it? It would be off if the new trans came out of something with a different ratio as you have. if its going to bother you every time you look at your Speedo then yes fix it but if it was me I would just put in a tach until you get around to replace the gear or get the Dakota dash Chris sells were you can calibrate it easy for anything

[Lt.Del]

two things:
GPS
Watch w/ a second hand.

 Go 60 mph.  It should take you exactly 60 seconds between yardsticks.  If it takes 55 seconds, the speedo is reading slow.  If it takes 65 seconds, speedo is reading fast. Make sure tires are inflated correctly.

GPS is best way though.

[frito-bandito]

Once you figure out if your speedo is showing fast or slow, you might be able to make it read rigjht with different tires.

[malibu795]

two things:
GPS
Watch w/ a second hand.

 Go 60 mph.  It should take you exactly 60 seconds between yardsticks.  If it takes 55 seconds, the speedo is reading slow.  If it takes 65 seconds, speedo is reading fast. Make sure tires are inflated correctly.

GPS is best way though.

tire dia can through it off as well