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06overland 03-06-2012 11:04 PM

Speedometer vs odometer
Hey everyone, I have a situation that doesn't make sense to me. I have a 2006 wk that came with 245/65-17 tires. I installed an RC 2" lift and BFG AT/KO tires in size 265/70-17. I just got the diablo sport predator tuner to calibrate the new tire size, but something is still wrong. Here is what I did:

Calibrated for 265/70-17
Test drove on the parkway and compared tripometer to mile markers (supposedly accurate to +\- 10 ft) and compared my speedometer to mph on my phone gps (at constant speed)

Below are my results:

Calibrated for 265/70-17
Speedo = 50mph
Gps = 47mph
Mile marker distance = 10mi
Odometer reading = 10.25mi

I interpreted this as meaning that the car thinks I am going faster/farther than I really am. The speedo shows +6% over and the odometer shows +3% over. I then calibrated again for a 6% smaller tire. This is what I found:

calibrated for 265/62-17
Speedo = 50mph
Gps = 50mph
Mile marker distance = 10 mi
Odometer reading = 9.7 mi

My new results show the speedo dead on and the odometer off by -3% under. I know that measuring using mile markers isn't perfect but it is better than 3% for sure. I also know that phone gps isn't perfect, but it is an average and on cruise control it stays perfectly still as does the speedometer.

My question is what is going on. Why are the speedo and odometer not showing relative results?

Sorry for the novel, and thanks in advance!

Knightrider03m 03-06-2012 11:38 PM

Re: Speedometer vs odometer
It maybe your phone that is causing the problem. I know that the speed on my gps would go from 70mph to 0 mph then back up to 70pmh from time to time. If your phone did that, often enough, it could show for the .3 mile differences. I have however, found more reliable speed on a real GPS system like a Garmin or Tomtom but still its not stable.

Take another car, drive it from your point A to B and see how much the mileage is, then take your car and do the same, if both cars matches up then you are good.

06overland 03-07-2012 08:35 AM

Re: Speedometer vs odometer
Thanks for the input Marcus. What is confusing me is why when I program my actual side wall numbers it is so far off.

Knightrider03m 03-07-2012 12:25 PM

Re: Speedometer vs odometer
What was the numbers when you didn't have the tuner but had the bigger tires?

06overland 03-07-2012 02:10 PM

Re: Speedometer vs odometer
Speedo was about 7% low.

hemimds 05-09-2013 07:40 AM

My 2005 hemi with stock tires shows 5 km/h faster than both my aftermarket radio GPS and my portable GPS .

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