Tachometer is off?

When I first got my truck for a couple of months I noticed that the RPM’s seemed a little high. I have recently installed a scan gauge and stereo that connects to the OBII port. Both of them show my rpm’s around 1500 to 1600 less than the gauge. Anyone have an idea why?

 

I have a scangauge - no difference in rpm between it and the dashboard. What are you showing on dash and scangauge at idle with a/c off (and fan speed off)?

 

2,000 on the tach and 1350 on the scanguage.

 

dawgspit
My Tacoma is a V6-6 speed manual. Have had ScanGauge about 7 years. Today on a 45 degree day I observed the following with truck fully warmed up (10 mile drive) : HVAC fan off (insures a/c not running) ScanGauge shows 720rpm with transmission in neutral and dashboard very close. Try your truck in Park or Neutral if automatic. Also my experience on cruise control at highway speed if dashboard says 3000 rpm, ScanGauge shows 2950-3050 - close enough for me!

Both of your readings at idle seem way high, at least on an engine that is fully warmed up. One thing you might try is record dash rpm as above, then unplug your accessories from the OBII port and see if your rpm (dash display) changes.

 

Do you have the options wrong on your scangauge? IE your scangauge is setup for a 4cyl when you actually have a 6cyl?
2000RPM / 6 cyl = ~333
333 x 4 cyl = ~1350RPM

 

Not completely sure - but be sure the H controls are not on defrost as well as it may kick in the AC .

 

HVAC fan off prevents the a/c compressor from running at all.

 

OK I may be way off on this (Or it was so long ago it has since been changed ) - but I remember reading something about the vehicle MFG's wanting to be sure that there is no buildup of fumes in the cab/interior so the fan would go to low low but not shut off when the engine is running .... ????