Speedo out of calibration

[RobbyMartin]

I have tried to get my speedo correct with my gear ratio and tire size that then manual suggests inputting but I am still 5 MPH off according to my hand held GPS. I have 4.56 gears and 315/75/16 tires and I've been toying with different tire size numbers to get it closer but I am stuck at 5 MPH off.

Any suggestions?

[Jackpine]

According to the Tire Size Calculator here: Power Hungry Performance Forum - FAQ: Utilities a good starting point for circumference would be 2652mm.

The calculator may still not open in Internet Explorer, but it works fine in Firefox.

But, you didn't say if your speedo is faster that actual or slower than actual.

If it is showing faster than actual speed, your Tire Size (TS) is too big, Try reducing it.

You also did not say if you are talking about the dashboard speedo or the Gryphon speedo. If you get the speed on the Gryphon to exactly match what a GPS is telling you, which is your true speed, the dashboard speedo will probably be about 2 mph faster at 70 mph due to mechanical errors. However, your odometer will be correct in that condition.

- Jack

[RobbyMartin]

The Gryphon and dashboard are almost the same but GPS speed is 5 MPH slower.

[Jackpine]

Sounds like you need to reduce your tire size to bring the Gryphon's speed down to match the GPS's speed (if you trust your GPS). Once you get the Gryphon speed accurate, the dash speedo will just be whatever it is.

You could also base this on distance. Longshot270 posted this approach: Drive 10 miles on your GPS. Divide that by what your trip odometer says. Multiply that result by the current TS you have been entering. The resulting TS should be very close to what you need. If you drive farther, say 20 or 50 miles, your results should be even better.

- Jack