I am learning more than I ever wanted to know - but now have another question.

I read someone say to recalibrate the speedo when you get bigger tires. REALLY? I got bigger tires 9 months ago and nobody mentioned it to me. I did not get bigger rims - they are the same as what came on my 1999 XLT 4.2L - just upgraded the tires. I'd go down and get the size for you, but we are have a lightening storm - so it will have to wait.
What could go wrong if I don't? And how much does it cost if I need to?
If you need to know the size, let me know and I will get it after the weather lets up.

 

Rim size doesn't really have all that much to do with it, especially when you get into SAE sizes instead of metrics. What matters is the overall tire height. These trucks use a formula to calibrate the speedo and odometer. This is based on a revoultion per mile. When you go up or down in tire size that changes the rev/mi value and will make the speedo and odometer read slow (with larger tires) and fast (with smaller tires). The implications for this are you aren't getting an accurate speed and mileage reading any longer. The biggest is if you have an auto instead of a manual. These are computer controlled and the shift points are somewhat based on speed. Since you are now getting a false speed reading the shift points are thrown off resulting in a bad shifting trans and IMO this also causes some long term damage. This isn't as bad with a manual trans as you control the shifting. The best thing you can do would be to adjust the speedo to make it read right. I noticed going from 255/70/16 to 265/75/16 that the shift points were off. Then going from 265s upto a 285/75/16 it was worse, and then finally with the 315/75/16s I had on at the end it was VERY bad. To adjust things will depend on when your truck was built. 97/98 F150s use a VSS gear on the output of the transmission/transfer case (4x2 and 4x4 respectively). If you have a 99+ then it uses a computer controlled speedo and it requires the use of a microtuner (about $300-400) or a trip to the dealer to have them use their scanner to readjust the rev/mi value to that of your new tires (typically about 1 hrs worth of bay time).

 

I will look into getting it to the dealer nect week. I have a 99 auto. In all honesty I don't remember what the stock size was. But the new tires are taller and fill the well nicely. I teased that my boy grew up, because it is definately sitting higher now.
And now another question -
The mechanic swears that my truck has to be running rich to break the cat at 117k. I did a full tune up 15k ago, so would not let him change the plugs and wires again already. Could this have caused it to run rich and overheat the cat?

 

That doesn't sound premature to me. I replaced the cat converter on my Jeep at around 100,000. Most of the Jeep owners I talked to started having problems at arount 80,000 miles.

 

To find the stock size just look on the sticker on the door jam and it will list the size that came on the truck. Since you have a 99 that doesn't really make any difference. All you need to know is what the rev/mi value is of your new tires. Most manufacturers will list that value for each tire they make, though I'd go about it another way. This other way takes into account variations in the tire molding, air pressure, and what truck they're mounted on. Simply mark the rear tire and the ground. Then drive 1 full revolution of the tire. Then mark the ground again when the tire mark touches. Measure that distance and divide that number into 5280ft. This will tell you how many rev/mi that particular tire has. Then when you go into the Ford dealer tell them how many rev/mi your tires are. After that your speedo, odometer, and shift points should be dead on. Now some dealers will not reprogram the speedo for the reason that on occasion there will be an error and lock things up. When this happens they have to reflash the computer taking more bay time. This is somewhat of a rare occurance, but some dealers just don't want to take the chance that it may happen. If you come across a dealer that won't do it just find another one. This will take about 10min to reprogram the value and will cost about $60-100 most likely.

 

Thanks, I will try that. I will call before I take it in and make sure they'll do it. I appreciate the help!

My new CAT is ticking (see engine forum) so I may have to wait until that gets fixed first. Lucky for me I have a friend at Hertz and can rent cars for cheap!!!!