speedometer calibration

[jimr4115]

still can't get help or an answer to my problem. took off the oem P225/60/R16 and installed a set of custom 225/55/R17" tires and rims. Checked a supplied chart from Discount Tire and made the comparisons of the 2 sizes so I wouldn't affect my true road speed. Well this change has affected the indicated in dash speedometer speed, as compared to my readout on my GPS by about 30-35%. I>E from a dead stop, the speedo doesn't even start moving till after 15 mph. At 60, with a GPS readout of 60 mph, speedo reads 40mph. I have disconnected the negative battery terminal and thought this would allow the wave length, read by the tranny speed sensor, to adjust and re-calibrate everything. Didn't happen. Ordered a new speed sensor and installed. Still no changes after another battery disconnect. Just can't believe that, with above changes made, this is coincidental to anything. Believed the ecu would recognize the tire change and new rotation and make any and all new adjustments.  What am I missing and where have I failed. Any suggestions, comments solutions would be greatly appreciated. 

Anyone know of a obd scanner capable of reading and having the capability to make the changes directly to the ecu?

[sha4000]

Are you sure it's the rims and not the cluster itself? Did you remount the OEM rims to see if the the speedometer goes back to normal?

[M.Yasir]

but how is that possible that by changing the rim size it will affect the behavior of the speedometer in the dash? i think you should go to an electrician or find a new console from the junkyard to rule out any possibility,

OBD scanners which are capable of programming the ECU are expensive and it will be not worth purchasing it in my opinion just for this problem,

I have a similar case in my broher in law Ford Mustang GT (2006), but in that car it is the other way around speedometer reads more speed than actual as per GPS of about ~10 km/hr

[CELSI0R]

The ECU will not recognize a change in wheel size, and relies on the car having the same size wheels (in total diameter, including rims and tires) as it did stock when determining speed.

If the speed was reading fine when the stock rims and tires were on the car, then changing the cluster may not necessarily fix the problem. As sha4000 suggested, I would try putting the stock rims back on the car, to see if the speed is reading properly again. However, your new tires should not have resulted in such a dramatic change in speed. I calculated what you should expect using a calculator on TireSize.com:


 

On the other hand, you mentioned that the speedometer needle doesn't start moving until about 15 mph. Try hooking up a scan tool, and have a passenger check if the speed is correct coming from the ECU's diagnostic port as you drive. If it is, then the problem could be the cluster.