Speedometer Calibration

[TurboBeagleBuggy]

I was on a four lane yesterday getting passed, odd. Anyhow I changed my tire size since I have bigger tires on my truck and it was 4 mph slow at 65 mph compared to the GPS. At 45mph the speedo was 2 mph slow when compared to the GPS. Any ideas?

[never satisfied]

Mine did the same thing. Not sure why.

[emarkay]

GPS, as in Global Positioning System?
It's only going to be accurate on a 100% level road - GPS does measure linear distance as well as elevations, but I don't think it does both in real time, correct?

Compare your dash speedo and your EFILIve dashboard indicated speedo to see if there are any mechanical issues in the cluster first...

[Kennedy Diesel]

Tire diameter will vary with wheel width and manufacturer.

Figure your percentage of error and alter the "counts" by same.

I've found the OE speedo to be approx 2mph high at 60 mph when perfect at 50 and 70...

[TurboBeagleBuggy]

GPS, as in Global Positioning System?
It's only going to be accurate on a 100% level road


Yes global positioning system, I was on level road when I checked it. I also was able to confirm the GPS speed with the Attitude monitor since I have it setup for my tires to display corrected speed. I also noticed the same results John on the OE tires.

[Blacky]

Speedo calibration is a tricky business. Some GM divisions use a GPS system and just change the pulses per mile value until the speedo reads what they want it to read.

EFILive's calculated pulses per mile based on tire size is, at best an approximation and is intended only to get you "in the ball park" - usually within 2-3%. As John said, adjust the pulses per mile by the same percentage that your speedo is wrong.

Regards
Paul

[Kennedy Diesel]

Or fudge the tire size by a c-hair and let the program calculate ppm.

IE: 282-75-16 etc.