I know you guys aren't talking tires here but you seem to know alot about the computer systems.
I want to put larger tires on my 97 Expedition. Does the computer get it's tuning and shifting info from the speedometer reading? If my speedometer is off will it change my shift points electronically? I understand the larger tires will affect my overall gearing. Right now I have small 255-70R16 tires and want to go a few sizes larger. Can the dealership adjust my computer for the new tires if the overall diameter matches that of the largest factory tire option? How much does that cost?
I have a Banks Trans-Command that increases my line pressure based on RPM, speed and throttle position so I don't think I can install an aftermarket chip but I'm going to check on that.
Any help would be appreciated. I don't want to open up Pandora's box just by changing the tire size.

 

If the 97 Expy is like the 97 truck (which is most likely is), there is a gear that will need to be changed.

 

Hi Davet,

Dealership service departments can only change for a tire size that was originally offered by Ford on that vehicle, any other sizes you'll need to do the speedo gear change for.

For 1997 & 1998 trucks, you'll need to change the speedometer gear in the tailshaft of the transmission when you have a tire size or gear ratio that your dealership can't correct for, meaning a tire size or gear ratio that is different from anything that was originally offered by the factory on that same vehicle in that model year. This is a pretty easy 10-15 minute job to change the speedo gear, and the part costs under $20 at any Ford parts counter.

You'll need to know *exactly* how far off your speedometer is at a given speed, preferably what your speedometer reads at a true road speed of 50 mph since it's so easy to do the math (just double that number of how far off it is at 50 mph to get your percentage of speedo error), turn that into a percentage and then apply that percentage to the number of teeth on your *current* speedometer gear to determine the number of teeth you need on the new speedo gear, and then just order the gear and install it.

For example, let's say your speedo reads 46 mph at a true road speed of 50 mph. Convert that speedo error of 4 mph at 50 into a percentage (4/50=8%, or since it's the amount of error in mph at 50 mph, you could just double the number of mph it's off at 50 to get your percentage), and then add that to 100 (8% + 100% = 108%) to get 108. Since your speedometer is showing slower than what the vehicle is actually traveling, you would then divide the number of teeth on the current gear (and let's say that is, oh, 19 teeth) by that figure of 108 to get the number of teeth you'll need on the new speedo gear to correct that error (19 teeth/108 = 17.592 teeth needed) . In this case, the answer came up at 17.5, so you would have to use either a 17 or an 18 tooth gear.

Now if your speedo error of 4 mph have been the opposite, meaning if the speedo was showing too fast (I.E., showing 54 mph at a true road speed of 50 mph), then you would have taken that same 108 number (100 + the percentage of speedo error) and *multiplied* that times the number of teeth on the present gear to get the number of teeth needed on the new gear. In that scenario, starting with a 19 tooth gear, you'd end up needing 20.52 teeth (19/108 = 2052), so you;d use either a 20 or a 21 tooth gear.

There are numerous ways to do the calculation, this is just what seems to be the easiest (for me, anyway) to try to explain to someone else, so I hope that helps you.

Just so you'll know, if you'll make use of the search feature here, you'll find *tons* of info in how to recalibrate the speedometers in the 1997 & 1998 F-150 platform vehicles (which includes your Expedition and F-250 Light Duty trucks). The icon for the search feature is in the upper right area of your screen here, and it can be your best friend, giving you immediate access to the wealth of info from F-150 Online's 6 years of postings on these message boards. I have posted here many times the method for determining the number of teeth you need on the new speedo gear based on whatever speedo error exists, you can look up those posts too with the search feature, so you'll have all that specific information.

Good luck, & if you get stuck on this you can always give us a quick call, but this should get you thru this pretty easily.

 

Thanks. Can I also take a percentage of tire diameter increase and use that in your formula in place of speedometer percentage? Easier to get ahold of tire diameters from tire makers than trying to find a radar. Also, will the difference in speedometer reading affect engine and transmission performance besides the usual overall gearing effect? Just in case I don't recalibrate?
If my new tire size is the same diameter as factory 17's should I have the computer recalibrated or just get the gear?