Like the title says, can a leaky intake manifold gasket cause an innacuarte reading on the speedometer? Its usually five miles behind when I compare to my gps? Theory being that maybe air is leaking from the engine and thus affecting engine performance and speedometer reading?

 

Not a chance.

 

Swartlkk, you would be proud of me. Before I started this thread, I searched the Blazer forum for- inaccurate speedometer, intake manifold gasket and speedometer and nothing turned up.

If its not the intake manifold gasket, then what could it be , besides I need to get a new car ?

 

Every speedometer has inherent inaccuracy. They can only be as good as they were originally calibrated to be and given production tolerances, they were calibrated to the conservative side.

Something as simple as changing the brand of tire on the truck can show a difference in the reported speed. Changing the tire size or the gears in the drive axle(s) will definitely cause a difference in the reported speed.

Engine modifications or malfunctions will not cause any issues with the reported speed. The signal that ultimately determines the position of the speedometer starts at the VSS (vehicle speed sensor) located in the tail housing on the transmission (2wd) or transfer case (4wd). That signal goes to the PCM (power train control module) where it is manipulated by the corrections for tire circumference and the gear ratios of the drive axle(s).

What you have to determine is how far off it actually is. Make note of the discrepancy between GPS and speedometer at a variety of speeds and calculate the percent difference.

So if the 5MPH difference was at 60MPH, that is ~8% off which is a lot. You basically have to determine if things have changed. Going from 235/70R15 tires to 235/75R15 is good for 3.3% decrease in reported speed vs. actual speed.