I finally got a chance to determine the accuracy of the speedometer in my '96 Ram 2500 5.9 Magnum 4x4 with 3.55 gears and 285/75/16 tires against my Garmin Navigation unit. At lower speeds, the speedo and the Garmin read neck and neck within about one MPH difference up to 50 - 55 MPH. At 55MPH on the Garmin, my speedo read 52. At 80 MPH on the Garmin, my speedo read about 75 - 76 MPH. Is this the experience of others or can the Dodge dealer flash the PCM to make this more accurate at higher speeds?

 

I've personally found (by a ticket, doh!) that my speedo is off by 3 mph.
And I don't know of any tricks that can be done either.

 

I'll bet 285s where not the stock tire size, so your speedo will be off a bit.

I run 265/70/17s on my 2wd and the speedo is off a bit too.

 

No - stock tires were 265/75/16. Was just curious as to just how accurate these speedos tended to be if nothing has been changed. Don't mind paying the dealer for a PCM flash if it will make the speedo accurate to within about 2 MPH deviation throughout the range (maxing out between 85-90 MPH), however, not sure if these speedos can be made that accurate..

 

I think the SuperChips can adjust tire size..

 

That is one of the options that come
with the Superchips.
Probably cheaper than the dealer
if they can even do it.

 

94 - 97 speedo can be corrected via gear change at the speedo pickup. 98 up is done via tuner or dealer.

http://www.go.jeep-xj.info/HowtoSpeedoGears.htm

C/P from my note's

Chassis Diagnostic Manual. There, in section 29A, it explains the procedure needed to program the CAB for specific tire sizes. [The CAB sends a pulse stream to the PCM, which drives the speedo/odo; to change the calibration, the CAB needs to be told, among other things, the number of teeth on the RWAL sense ring and the rev/mi of the installed tires.]

So, if you change your tire size and your dealer hasn't a clue as to correcting the speed/odo, point him to the Chassis Diagnostic Manual, section 29A. It's all there.



NOTE: the 98 1/2 models do not use speedometer gears and the speed metering is taken from the ABS sensors. To change "gears" for different tire sizes requires reprogramming of the PCM. Dealers have been having problems with this procedure, and many axle ratio/tire size combinations are not available.

 

I think just about all vehicles out of the factory show 3-5 miles faster than actual speed

 

I know you have a 96 and it's done with gears, for the 98 and up, you need to know the revolutions per mile of the tire (594 for my 35" Generals) and the number of teeth on the abs tone gear in the rear end (120 for the dana 70). These are needed by the DRBIII tool the dealer or repair shop will use. Don't know what the Superchips needs.

 

merc225hp, that's exactly what I needed. I can perform the calculation and install the correct speedo pinion gear into my transfer case to try to get my speedo to read more accurately. Sometimes, its good to have the older model truck - something I can fix myself...