Ok so my truck has a stock tachometer and it reads wrong it usually reads 4000 rpms when I know its not that high it occasionally works, on cold days usually and it acts like 4000 rpms is base line like as I press more on the throttle it will go up then when I release it will go back down to 4000 rpms please help me ive been driving by ear ever since I got it and I would like to see what the rpm s are all the time