Thought I share some info that might save folks a couple bucks. Just got back from my local Ford dealer to get my speedo recalibrated for my taller tires. I was watching the tech as he was trying to reprogram the computer. The lowest rev/mile the on-board computer would accept was 605 rev/mile. The BFG 315/75R-16 I am putting on state 599/mile. Since this is to much error for my taste, I will go ahead and install a Truspeed module from Superlift. The dealer charged me 33$ for the tech's time to plug in and find out we could not set it any lower than 605.

2002 F-250 CC Short box
V-10 Automatic 4.30 gears
Banks Power Pack w/chip
Transcommand
6" Full-Traction Lift kit
Soon to install the truspeed unit
315/75r-16 BFG AT's
16x8 Mickey Thompson Challengers

Regard: JDP2

 

Yeah, I wish someone would've told me that before I learned the hard way. Took mine in to adjust for 285/75-16's, they told me no problem. When I got back they presented me a bill for about $200 and told me the best they could do was "get it close" because Ford tightened up the adjusment range for 2002. Well, I did a calculation using road markers, and they did absolutely NOTHING. I was pretty torqued off, but they wouldn't do anything for me. Should have taken it to the dealer I bought the truck from, problem is he's an hour and a half away.

Waxy

 

Whew!!! That was close. I thought it read 2002 SD speedo recall. What a difference one L makes!!


2002 F-350 Crew Cab Lariat Short Bed 4x4, V10, 4:30 LS, Black w/ Med. Parchment Leather, Off-Road Pkg, Snow Plow Prep pkg, Heated Captain's Chairs, Towing Pkg, Privacy Glass, Rear Sliding Window, Power Telescopic Trailer Tow Mirrors, Auto Shift-On-The-Fly, Fog Lamps

Mods: Westin Nerf Bars, Front Side Windows Tinted, Bed-Liner, Tonneau Bed Cover, 16x10 Mickey Thompson Classic II Rims (not installed yet)

Future Mods: 6" Skyjacker Suspension Lift & 315/75 R16 BFG Mud-Terrains (coming soon), 1" Track Bar...

 

All, I wanted to upgrade the tires on my 2000 PSD to the 285/75 config. My question is, can I have calibration sucess with the
speed-O on my 2000?? Please advise.. Jeff...OUT

Some call it a six-pack, I call it a support group.

 

[updated:LAST EDITED ON 30-Jul-02 AT 02:06 PM (EST)]Mmmmm,I think you are pretty picky. How far do you think you are off?

Let me give it a try, the lowest available number is 605. Which is perfect for a 33.3 inch tire, which is ~104.5" in circumference.

And you need 599, which means your tire is about ~105.8"

correct me if I stray....

that means at 70MPH with a 33.3" tire you are spinning at 42,397 Rev Per Hour. Where with the taller tires you really will be spinning 41,893. Which says when your odo is tells you 70MPH (your tires) you are getting 42,397-41,893=503 more turns than you want.

still on track, steer me back if I am not?

So that means (503*105.8")/12=4439 feet. (less than one mile)

so at 70MPH you are only off less than 1 MPH. did I miss something.

My truck stock was off 2.5 at 70MPH

Let me know if there is flaw in my math....285 should be a easily calibrated.

dc

 

I figure the difference is about 5-6% between the stock 265/75's and 285/75's.

Here's how I figure it.

Circumference = 2(pie)r

Diameter (=2r) =
265- ~31"
285- ~33"
305- ~34.5"

Circumference=
265 - 97.3"
285 - 103.6"
305 - 108"

63360"/mile therefore
265 = 651.2 revs/mile
285 = 611.6 revs/mile
305 = 586.7 revs/mile

285 - 611.6/651.2 = 0.94 or 6% low
305 - 586.7/651.2 = 0.90 or 10% low

Therefore the 285's will cause the speedo to read ~6% slower at all speeds, so 100 is 106.

Not all that severe, but enough to get you ticket if you thought you were doing about 5mph over the speed limit and were actually going about 11mph over the limit.

Also, I'm not sure what effect this has on the transmission shift points/calibration.

On the upside, 10,000 on the odo is actually 10,600.

For Ford however, this is bad, so they severely limited the adjustability in 2002 to try and prevent mileage fraud and therefore prolonged warranty periods.

Like I said I had mined "calibrated" and they got it to about 5% off, if they did anything anything, well worth the $200.:-X23

Waxy

P.S. I'm no mathemetician, so feel free to correct me if I'm wrong.

 

[updated:LAST EDITED ON 30-Jul-02 AT 07:40 PM (EST)]Logic is right, andt C=2Pi, however, I think the numbers aren't. My 265s are barely 30.5 (stock firestones) and my 305s (Toyos) are 32.5 loaded and 33.2 sitting on the rack. So the %s you calculate aren't right. Unless you got way taller tires than I got. ? Anyway % delta wasn't the question.

The statement in the beginning was that the lowest number for 2002 trucks was 605 and it was way off, for JDs liking. If you check out my math, I am saying if you use 605 on a 315 (which when I measured was 33.8" ish) the delta at the speedo will be a nit.

later, dc

 

I based the diameter of each tire on manufacturers specs plus rim diameter.

So, 265/75 - 16

Tread width = 265mm, 10.4"
Tire sidewall section is 75% of 265mm, or 199mm - 7.8"

Therefore, 7.8" x 2 plus rim diameter should give overall tire diameter. You get 31.5". For ease, and because it's kind of a standard, I went for 31".

If you do the math, the 285's come out to ~33's, the 305's are 34's.

315's work out to roughly 34.6" by this way of thinking. 108.6" in diameter, doing 583 revs/mile.

If you go to some tire manufacturers websites you'll see these same numbers, or numbers very close to them, as the their specs for overall tire diameter. I don't think measuring the tire's diameter while it sits still is terribly accurate. Ever see the diameter of a top fuel dragster's tires change when they leave the line? An extreme example I know, but it shows my point.

darth, now to the real point of your post, which wasn't lost on me. My post was to clear up or try to make sense of the effect that different tire sizes have on your speedo vs. stock tires.

Max. adjustability is 605 revs/mile. So given that by my calculations with 285's you're getting about 611-612 revs/mile, gives you a fudge factor of about +1.2%. I find this hard to believe, there's NO WAY that my speedo is reading fast. By darth's calculations my speed should be right on. I have done testing with my odometer vs. a ten km highway speedo/odo test section, and I'm consistently about .5 kms off over a 10km strecth, or about 5%. I guess a GPS would be the real ticket, I wish I had one.

With 315's, by my calculations, you're speedo is ~3.5% low if your adjusted to 605 revs/mile, 10.5% if you're adjusted to stock tires.

jdp2, where did you get the 605 revs/mile number? This could be important for me to know now that I've done some more math. The tech at my dealer told me that prior to the 2002's he was able to adjust completely for 295's, but that as of 2002 he couldn't even get close to 285's. If you're right my dealer's got some explaining to do for my $200 bill and a speedo that is still way off.

I don't want to beat this to death, but I'd like to get it straight, I'm still torqued off about the $200+ I wasted, or was possibly screwed out of.

Waxy

 

Interesting observations, I don't drive a dragster...yet... but I guess the tires will stand up a bit more, but not sure how much. My dealer told me my tires,305s, would be 33.3-33.4, so those are the numbers I went from. Also their 315s were spec'd at 33.8-33.9.

Let's keep beating on it, I am going mull over some of the thoughts, but I got to get some work done or they will take my computer!!

Borrow a GPS, they are great as a speedo check.

d

 

So I think your numbers are right when you are comparing 31" to a 34.5" tire. My calcs confirm that it is 10% off both in tire circumference and all the way down to MPH. Your proof is a handy one, and much shorter than my excel spreadsheet. I say this in inches and not tire size because I do not believe a 305 stands 34.5". Mine again are only 32.5 loaded and 33.3ish on the manufacturer's spec. Also I pack a tape measure and have been checking out other trucks, and this measurement has run true on 4 different brands, BFGs included. It would be interesting to hear how the technicicans decide what number to plug in. I will be learning this soon as I might be doing this soon, once I check my delta MPH versus GPS soon.

The programmed number for a 31 should be about 650. A tire standing 33.3" should be spot on with a programmed number of 605 (min for 02). If you really got a taller tire that requires 599 @33.7" (my calcs say this is a 33.7" tire). The delta here using any measurement (my approach or yours) is less than 1% or less than a MPH between 605 and 599 settings for a 33.7" tire.

So my comment to JD? is you are very very picky, but we all have that right don't we, or we would be driving Chebbies or worse Dudges.

Again, my truck form the factory was off 2.5MPH at 70MPH, but that is a rounding error in my book.

QUESTION: What is the prammable range for the 99.5 SD?