I just bought A 2000 Dodge Dakota Sport with 4.7 L V8 Magnum with 167.000km on it and i've notice that being in P wth engine on the idle goes up and down ( from 650RPM TO 750 OR 800 RPM ) so the idling is not stable,doesn't matter if engine is warm or cool.
i also notice small muffler leak ( the leak is located before the 1-st oxygen sensor, wondering what could cause the idle fluctuation ?
the previous owner changed the rims from 15' to 16' right now truck has 265x70rx16 on it,does affect the speedo or something else that i could worry about ?
thanks in advance, IRENE

 

Sounds like it may be your Throttle Position Sensor (TPS), but a dirty or failing Idle Air Control (AIC (just caught this I meant IAC, what a goober I am ) will affect idle also, if the exhaust leak damaged your O2 sensor it would mess with mileage not sure about idle and yes your speedo might be wrong. Do a search using the search link and you should find more precise answers to your questions. There is a site you can use to calculate the difference your speedo will, but I can't remember it so maybe somebody will chime in.
Hope this helped a little.

 

265/70R16 is a stock tire size.. well it is on my 02 sport. I just put some 265/75R16 on it so My speedo is out. But, the tire size wouldnt effect the idle.

 

iac**