I just went from R16 tires to R16.5 and have realized that the speedo is off by appox. 10 mph. My parts guy says they no longer run off of the trans were you could change gears and adjust the speedo readout that way. He says that with the electronic speedos that the sensor is in the rear axle and you do something to the speeometer itself. Anyone know what to do to adjust it?
thank you dave B.
They have something called a reluctor (a worm gear still engages with the tranny-diveshaft connection, but is connected to a magnet with like 16 poles instead of a mechanical line) that senses RPMs of the driveshaft and sends it to the computer. From there it's multiplied with the size tires you have on the vehicle plus some other functions and then sends it to your output being the speedometer. To change the computers default tire size, you need a reprogrammer or just go into your local ford dealership and get it updated. All you enter is the hight of the tires and presto, working like new : )
On many newer vehicles, this would require that the main computer be programmed with the change in the final drive ratio. You will probably have to take it to your local dealer and see if they can program a custom ratio.
The recalibration of the speedometer depends on your make, model & year vehicle. Some can be recal'd with a switch sequence while some require reprogramming the ECM or other devices. If you have a '96 or newer OBD2 vehicle, it is likely that you will need to get it done by a dealer or someone else capable of re-programming the ECM.. Note that it is important that this be accurate if you have antilock brakes or traction control for them to work correctly.
Note that
Not really...the ABS/TC senses the difference in speed between wheels, and doesn't care how big the tire/wheel combination is as long as all 4 wheels are the same.
But, the OP's tranny won't shift right until the speedometer is corrected, and his odometer will be off as well.
Either it has to be reprogrammed for your tire/wheel combination, or you can get a Superlift Truspeed unit, which corrects the signal.
I put one of those units on my Superduty when I put the 35's on it. Easy to install, PITA to get dialed in by yourself.. Lots of little adjustments and test drives with my GPS receiver finally got it correct.
The "ballpark" settings the instructions show are pretty close tho.
Are you sure that, by increasing your rim size by a half inch your tire circumference increased enough to raise your speed by 10 mph? That sounds like a lot. There must be something else must be involved. Also, at what indicated speed does this increase seem to have occurred....65 mph...100 mph...etc. Do you have a tach from which to check this increase? There is a lot involved. I went from 9.50-16.5 to 12.00-16.5 on the rear of my truck and the speedo finally registered correctly! Remember, the factory usually sets speedos to record higher speeds than you are actually experiencing. Some AAA locations have the ability to accurately check your speedo. Good luck!
On Tue, 08 Mar 2005 02:49:16 +0000, Ernie Sparks rearranged some electrons to form:
You can also use a stopwatch and highway mile markers.
MotorsForum website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.