Hey guys, I need some help. I have a 2000 F-250, 7.3l, CC, 3:73 gear, 4x4. I recently changed tires from a LT 265/75/16 to a LT285/75/16 and now my speedo is reading 5mph fast, if it shows 70mph I am actually only doing 65mph. I checked this with a gps unit and run past 3 or 4 radar units that they setup along the road and all of them show the same results.

I talked with the local dealer and a local off road shop and got conflicting answers. One told me I could have the computer reflashed, and the other said I would need to change the speedo gear in the tranny. Which is correct?

This doesn't make sense to me as I thought the speedo would read "slow" after increasing tire size. Has anyone else seen this issue?

thanks

 

Since there isn't a speedo gear in the tranny, I think that's the wrong answer. The speedo is calibrated with software.

The speedo should read slower with larger tires. Did you check it before changing tires? Maybe it was wrong in the other direction before.

 

No, I didn't check it before switching tires. I have only had the truck for 2 months. I too was thinking that is should be reading slower with the larger tires, but it isn't.

Thanks for the info, I will check with our local diesel shop to see if he can recalibrate it.

 

There is an electronic device that can be wired in and goes in the cover under the steering wheel. It can be adjusted for different tires. Can't think of name of it now I'll try to get back with the name of it.

 

Check out a TruSpeed speed sensor recalibrator. Also I think some of the programmers have options to change tire sizes.

 

The name of the product is Truspeed.

I am confused though, in relation to what you're describing as the speedometer reading faster than actual with larger tires. Almost the only way that could happen was if the previous owner had already had the speedo recal'd with even larger tires on it, but then switched to some 265's when he sold/traded it.

reason I say the above is because I have two Fords (see sig), both with 3.73 gears in the rear end. By clocking both of them for several straight 30 mile runs at 70mph on cruise control (absolutely not even the first interruption in speed throughout the testing time), I determined that with the stock 265 size (same as what you changed from), the speedo reads about 1.5-2.0 mph faster than actual. This actually works in Ford's favor because you'll run out of your mileage warranty before you actually travel those miles because they use the speedomoeter readings for warranty determination.

Anyway, I have also replaced the 265's on my Excursion with 285-75R16s and repeated the timing trip tests to find out that with the 285's, my speedo now reads about 1.5 mph slower than actual.

If you know of anyone with AutoEnginuity, you can reportedly recal your own speedo with that tool if they are running the msot recent release of version 5.4. Otherwise, the dealer can reset the program, or you can purchase the Truspeed device and do it yourself.

Here's a link for Truspeed.
http://www.superlift.com/accessories/truspeed.asp

Here's a link to one source (perhaps the best price)
http://www.4wheelerssupply.com/catal...FQINPAod1Ef17w

Here's a link for an eBay auction on this device
http://cgi.ebay.com/ebaymotors/Super...QQcmdZViewItem

 

Assuming your truck originally came with 235/85/16 and was correctly calibrated, you should read 3.5% slower for any give speed with 285s.

 

1) If your Speedo is currently calibrated for LT 265/75/16 tires which are 31.6" diameter, and you change to LT 285/75/16 tires which are 32.7" diameter, then when your Speedo reads 70 MPH, your actual road speed is (32.7/31.6)(70)=72.4 MPH.

2) Neither answer is correct for calibrating your Speedo. On my truck, the Speedo is controlled by the GEM, not by the PCM. The GEM gets a sequence of pulses from the VSS in the diff. There's a speed ring mounted on the outer perimeter of the ring gear housing, and every pip on the speed ring generates a pulse when it goes past the VSS. So in effect, the Speedo is measuring the rear axel RPM.

3) When you change tire diameters, you need to recalibrate the GEM, and this is done with a special programmer via the OBII port. I had Jody plug his special box into my port just to see what options were available, and it wasn't clear how many different tire diameters can be calibrated for, so you'd have to find some one with this unit to check your GEM options.

 

There are instruction somewhere on here you can print out and take to fords with you, and they can follow the instruction on the sheet to reprogram your speedo. Ill try and locate them...

 

You can have the dealer reset the Speedo up to a 35 inch tire. (# of revs per mile up to a centain number) For some reason there is a limit on how many time the reflash can be done.

I have the TruSpeed and have set it for two tire sizes (35" winter, 37" all other times). Only thing I have found with the TruSpeed is running on Cruse. It seems to surge a few miles per hour up and down, and I don't get as good mileage as I use to.

Dave.

 

Even though the speedo should be reading slower, it isn't. As mentioned earlier, I checked it with a gps unit and ran it by several of the roadside radar units. This was done each time with the cruise set at 70mph and each time it showed the actual speed to be 65mph.

Someone mentioned it may have been recalibrated for larger tires before I got the truck. The previous owner is a good friend and it has never had larger than Lt 265's on it before I purchased the truck.

Thanks for all the info, I am going to check with the local diesel performance shop to see if he has the equipment to recalibrate the GEM or at least check to see what size tires it is currently calibrated for.

thanks again

 

http://www.dieselinnovations.com/tec...Procedures.pdf

Take that to a ford dealer and they will recal your speedo.

 

I had checked mine prior to larger tires and found it to off by approx. 4 mph, fast. Taller tires have a larger circumference which will slow down the speedo. There is a standard dimesion for each tire size as listed by Ernesteugene, but different manufactures usually alter it slightly so you need to what your specific tire diameter is if you want to figure out speed by rpm. I bought a Superchip Flashpaq programmer that has the speedo re-calibrate ability and now it is right on.

Good luck

 

I have 315's and GPS says 5 miles faster.

 

i would check with ford first... something is not right in Denmark lol

either a. Your truck had a very fast reading speedometer from day one
b. the GPS is not working right

I assume its a. but it cant hurt to try a buddies GPS

Kenny