..As you may guess from the title, I put new 315s on my truck, and already got a ticket. Actually my brother, I was in the passenger seat laughing Cop jumped the interstate and was following us, has the cruise set on 75mph (speed limit of Colorado). Stayed on us, finally pulled us over. CLOCKED AT 86!!! Explained it, but the jerk-off still gave us a ticket. I expected it to run 2,3,4 mph fast, but 11mph????

 

you should be able to get out of that one, just show a recipt from your larger tire purchase and a pic of your truck to the judge if it even goes that far. good luck and let us know how it goes.

 

Sucks about the ticket. My truck came stock with 265s and speedo always read a little fast when checked with a gps. Went with 285s which trued the speedo up the near perfect. Now I have 315s on and my speedo is off a little over 11% with the gps. I.E. 50=56, 60=67.5, 70=79.2. I drive with that in mine and so far so good.

 

I've got to know, what exactly did you say to the cop to try and get out of the ticket?

Seriously though, the truck needs a pretty accurate speed signal for a few of it's modules (PCM, Tranny, ABS) to work right. Your local dealer can probably re-program your PCM for just a few bucks.

 

Here is a good tire calculator. Plug in your numbers. standard tire VS new tire to see the speed difference. http://www.miata.net/garage/tirecalc.html

 

Buy a SuperLift TruSpeed deal, install and callibrate and never worry about it again. Probably the best hundred bucks I've spent on the truck

 

So, what are you whining about? You changed tires... you KNEW this would change the speedometer... and you got a ticket. That tire swap shouldn't have bumped you 11mph... I suspect you were sitting on 78 or 79, relying on that small buffer zone, and with the addition of the tires, you got popped. The officer had no reason to extend any leniency to you, and you won't see any mercy from the court.

You screwed up. Pay the ticket, do defensive driving, whatever. Quit whining, quit insulting the officer by calling him childish names, etc. In short, deal with it.

Oh, and get your PCM recalibrated. It'll save you more tickets and keep you from burning anything up on your truck.

 

Are you trying to get rid of this guy? I know he has only 35 posts on here but really. He made a mistake and he is doing the right thing by posting on here to warn others as to what might happen. Thinking you are doing 75 or 78 mph who cares. Its the guys doing 90 in a 75 mph zone that should get the ticket. Jonredbird, thanks for posting about your delima. I am sure there is a least one guy who has changed the size of his tires and has now learned from your mistake. seee ya

 

I agree with TVSJR, just not so vehemently. The guy knew it would mess with his speedo, got bagged and then referred to the police officer as a jerk off for doing his job. Spent 10 years as a state trooper - heard it ALL - the guy was doing his job and if your wife or daughter were stuck on the interstate, and the officer rendered assistance he'd be a hero but for now he's a jerk off for doing EXACTLY what he should.

I had 295's on my 2003 and got bagged exactly the same way - 78 in a 55 - I sucked it up and admitted I made a mistake - AND WAS LET LOOSE - but it wasn't expected and I thanked my luck because I was wrong - I never once looked to blame someone else for my lack of judgement - I messed up, simple as that.

You can disagree, and I found the original post amusing because it hits close to home, but if you really want to get down to it most times when assigning blame, a simple look in the mirror will suffice - for me as well as anyone

 

http://rockymountainsusp.com/TrueSpeed.htm

 

Correct me if I am wrong, but calibrating the PCM will not correct the speedometer. It will correct the ABS function and tranny shift points, but I beleive the Speedo is calbrated from the VSS. I don't think the Ford engineers are interested in giving out codes to change speedo function since some may want to mess with them to add up less miles, etc.

At least this is what I know from having my truck "calibrated" after installing 325/70r17 tires on my 06 F250. I watched the tech go through the PCM calibration and put in the correct size, but as soon as I left the dealer and drove a few miles, my GPS showed me about 5 mph off. I'm used to it now, and if I got a ticket because my speedo was off, its not the cop's fault.

I just put aome take off 265/65R18s on for a long trip and my speedo was back to almost dead on.

 

TVSJR,

I think we all know what happens when one "assumes". In your case, it holds true. You insult me for posting the results of changing tires, and then doubt the veracity of the fact that it raised the speed 11mph?? You "assume" we were driving 79......we were going 75 on cruise. Cop registered 86mph. Did I imply I wasn't going to pay the ticket/reconcile the action?? You imply that I am a liar, but want me to not call an officer (which you didn't meet) a childish name. Laughable. I am sorry to the readers that tvs has had to detract from the original intent of my post.

VZE,

.....No, I wouldn't consider the officer a hero if he helped my wife and daughter if they were stranded on the interstate. I'd consider him a guy that is doing his job. Personally, I don't get much into the cop/hero worship business. They have chosen a line of work that pays well for someone who most likely doesn't have a college education, can retire in 20yrs with a pension, gets freebies everywhere they go, can live in certain apartments for free, and is probably funner/more interesting than 99% of jobs out there. Sure, there is an element of danger to the job, but so is installing shingles. And a roofer makes the choice to put his life on the line just the same............I feel I am going to insult some law enforcemnt professionals, and I am sorry if that is the case. Not my intent. But I feel I have the right to refer to a police officer as a jerkoff without it being as serious of an offense as spitting on the Pope's ring.

OFF RANT--Point is, 315's WILL change your speed! Only thing I wanted to impart!!

 

It does correct the speedo. The VSS just sends a pulse. The PCM using your gears and tire Revolution Per Mile (RPM) calculates speed and then uses that to send info to the different modules and instrument cluster. There's a few problems though;
1. Tire sizes (the size listed on the sidewall) are an estimate, not a measurement.
2. Several things can effect tire size, air-pressure, load, and wear.
3. When the dealer re-programs for larger tires they often use the tires listed size, not it's actual RPM.
4. The accuracy of the speedo from the factory is often off from the get go, so adding larger tires may make the problem that much worse. It seems like whoever does the program for the PCM put in an average for all the possible tire sizes and that's what the truck comes with. When I tested my speedo with a GPS and stock tires it was off by -4 mph. at 70 mph. So when the speedo read 70 mph I was actually doing 74 mph.

So it's important that if you have the dealer re-program your PCM that you know the tires RPM at freeway speed, with a normal load, and proper air-pressure. And then have them use the RPM and not a stock size listed in the diagnostic tool.

It's also important to note that the PCM is limited to RPM's of 600 or greater, or about a 35" tire. Go bigger or use a 35" tire that is actually larger than 35" and it would probably be best to go with a TruSpeed from Superlift. The TruSpeed takes all of an hour to install and once calibrated can be very accurate. After a lot of tinkering, I was able to get my speedo and odometer within .5% of my GPS using the TruSpeed.

 

Enviro,

...Very informative, thanks. Gonna call the dealership, I hope they know as much as you do!!

 

Here's an article from Ford Doctor's website that discusses calculating RPM.
http://www.forddoctorsdts.com/articles/article-05-21.php