my dealer told me 90 bucks to flash the computer. Worth it in my opinion.

 

The dealer can reprogram the computer for a maximum diameter of 28" which is the size of the OEM 215/65/17 tire.

 

that is correct. They can only correct it for any of the EOM tire size.

 

I put 215-75R15 tires on my Patriot which are oh so very close to the diameter of the largest stock diameter tires. The dealer reset my computer for free when I had the oil changed. It is still off a speck, but say I'm going 65 MPH by the GPS, the speedometer needle is touching, but not directly over the 65 mark on the dial. I calculated that I'm only off about 100 feet in a mile.

I was told, the computer can only be programmed for the diameters of the stock tire sizes. If someone has found the speedometer calibration to be on a sliding scale rather than preprogrammed for the 3 stock sizes please post information about how it is done.

 

Correct me if I'm wrong, but this is only needed for a diameter increase, not a width increase, right?

 

Yes. Diameter effects the speedometer. Width does not. However, a wide tire on a narrow rim will be larger in diameter than stated in the specifications.

Width only effects how sharp you can turn without rubbing the fender well. My 215 tires do not rub under any condition I have encountered.

 

I would just stick with it the way it is. 10% off? so if your speedo says 65, you're really going 71.5. at 70, you're really travelling at 77. easy to remember.

as for your MPG's? just factor that in when you calculate. just add 10% to the number of miles travelled.

I had the same issue on my Wrangler a few years ago. not a big deal. actually, your odo will show 10% less miles than you've driven too, so keep that in mind when you're servicing your Jeep.

just go with it and enjoy the new ride and the new look! :smiley_thumbs_up:

 

I am 10% off also with my tires, so I use my GPS for MPH and also for MPG calculation. Also, since I don't have the lifetime warranty, my 100 000km warranty will last me a bit longer

 

You can also pick up a scangauge II and set one of the "gauges" to MPH. They have a way to correct the speed in the scangauge II, plus you get all of the other gauge info and an OBDII code reader. Worth the money IMO.

I bought mine from http://www.scantool.net/scangauge-ii.html

Their prices are the same as everyone else's, but they have free shipping and for $1.00 more you can get an extra cable which comes in very handy. I have one cable permanently installed and routed in the jeep, and use the other cable to pull codes from cars I'm working on, or to lend it out without having to undo the jeep cable/routing.

 

Thanks all. Good info. My commute is 150 miles hiway daily so i always seem to guage my driving habits using the AVG MPG mode ($$). i'll try your suggestions. My Safari had the real-time MPG. i found it amazing how much of a gas savings you could get with some careful accelerator manipulations. Yes - my commute is boring. Thanks!

 

Why not just leave it the way it is ? Years ago when I had my first truck lifted I was pulled over for speeding at night. My speedo said I was doing 80km/hr but the cops radar said 102 km/hr. lol So he and I did a couple passes just to check the difference ( between 20-25 km/hr) On the other hand, it's a lot better for you when your reselling your vehicle and it has 10-25 % lower mileage hahaha Peace

 

If I were you I would do everything in my power to get the correct reading from the car's own speedometer while using the wheel and tires you want. Anything else is just selling out and settling for 2nd best. It still bugs me mine is several hundred feet off. Less than 2% but it is still wrong and wrong is wrong. If it were simply mechanical like my other cars I would have several good solutions for you. However, since the Patriot uses electronics to run the gauges, you will have to research programming techniques to fool the car's systems into telling you the right speed. Like I said, telling the car that is has the biggest factory wheels/tires does get you very close. The limitation of the strut tower in the front wheel well is the great equalizer for all larger wheel and tire questions.

 

I don't know if you've gotten this issue resolved yet, but here are the actual differences between the 2 tires sizes.

 

so i guess if i got 18" rims and 225-55-18 tires (28"), they could set it to be accurate? Also, i called my dealer and asked if i went from the 215/60/17 size to 215/65/17 if it needs to be reprogramed and they told me no, only if i go bigger than 215/65/17, which they did not recommend obviously. Was he correct?

 

Go here to find tire chart

Go to the Discount Tire website. Enter your vehicle info, and once it gets to the tire size, click on the tire calibration link on the left of the page.

Once there you can enter what tire size you have now, and what you want to run and it figures the diameter differences and speedo difference for all sizes of tires for you.

I tried to post the link here, but I haven't posted enough yet. Hope this helps. Also road kill customs site has a good wheel conversion chart.

 

Try this site to see the differences between tire sizes and speedo differences. Tire Calculator

 

Dave, that's a handy link; thanks for posting it.
From what I'm reading in this thread it looks like the 28" tall p215/65/17 is an authorized OEM tire size upgrade. Should that mean that any size tire within this height should work without a lift? Does anyone know if adding a lift and going to a bigger tire effects the warranty?

Thanks: Charlie

 

I think I found a calibration solution. Check this out and lemme know what you guys think who are more educated on this.
http://www.transmissioncenter.org/speedometer_gears_for_dodge_and_jeep.htm

 

New vehicles don't use speedometer gears in the tranny anymore. It goes off the drive shaft revolutions and uses a speed sensor that sends the info to the computer( whatever module handles it in a specific vehicle )now. The revolution rate is coupled with the tire size programmed in and then that is used to determine the speed.

Changing speedo gears used to be how you did it back in the day( muscle cars and pre 90's or so +/- )when changing tire sizes. You would have to know how many teeth you needed( used to be a way to figure it but it has been so long I forget )and even the color of the gear played into it at times. There was a cable from the gear housing in the tranny that went to the speedometer. As the gear turned it manually moved the speedo. Modern speedo's no longer operate that way though and it is all electronic now( that I know of ). Only way to correct a speedo these days is through programing.

 

Or you could just use a Nav system.

 

so how can the speedo be corrected? I already have the GPS to know how fast I am going but want to know if there is a way to fix this. I know the one guy who is getting his ECU unlocked (was thinking it may be possible) if I unlocked it.

 

You go to the dealer and they hook the vehicle up and reorogram the computer using your new tire size and data. I believe you need the StarScan tool to do it. Just unlocking something won't do anything. You need to enter tire data so the computer knows what is going on.

On some vehicles they are limited to only changing to OEM sizes offered while on others they can enter various stats which can program it to no OEM sizes. I honestly don't know what can be done on the Patriot at this time( I am going to find out if I change tire sizes ).

I know on my Dodge Ram's you could program for non stock tire sizes. The dealer needed the tire size, overall diameter, and the revolution p/mile rate for the tire. That info was programmed in and then it corrected the speedo.

For vehicles with aftermarket support you can get handheld performance tuners( Superchips, Hypertech, Diablo, etc... )that allow for tire size changes as part of their features. You can also buy specific Speedo Calibrators these days too( Hypertech offers a popular one )but they are vehicle specific. Unfortunately, there are no such aftermarket programmers I know of available for the Patriot.

The only option you have is to have the dealer do it unless you have the software to do it yourself and know how to program.

 

the dealer did try to change it to something like 235/??/18 and told me it should read 2mph faster. I got in the car for a test run and it read 6mph slower at the same speed which was originally 4mph slower at 40mph. so I dunno what happened. It didnt work so I just gotta wait or see what other options are available.

 

Any news on if they can reprogram for tires larger than 28"? I was looking at getting some at ~28.5". Maybe the newer models have support?

 

Try calling ProCal. I bought one of their units to reprogram the speedo in my Wrangler when I went to 33 in tires, from 29's. The unit plugs into the obd port. I havent seen one advertised for a Patriot, but they would be the ones to ask if their unit would work with a corporate cousin.

 

Here's another link to a tire size calculator: http://www.miata.net/garage/tirecalc.html
If you have a Garmin, mine will tell me my actual speed which is oh-so-close to the dashboard speedo reading. I go between 205-65-16 for winter and 215-70-16 for summer. The winter tires are a tad slower than the speedo, the summer tires are about 1 mph faster than the speedo. Do you need to get any closer than that?