its a '68 and i have had it about 2 weeks. the speedometer seems to be 5mph slow (so when it says 40 im going about 45). i was thinking it has to do with not having stock wheels/tires. they are bigger than the stock ones. the car came with the wheels when i bought it. i was just wondering what to do to solve this?

i am going to buy a new speedo to mach the new tachometer. would this problem go away with a new guage?

 

This problem will not go away with a new gauge. What you need to do is figure out how the wheel diameter affects your rear end gears. (ie. taller tires makes the gears numerically lower) there are calculators for this on the web. after you figure that out you will need to change the gear where the speedometer cable connects into the transmission. BTW if your original speedo is a 120 and you put in a 160 this will also affect the accuracy.