Did you measure the exact height of the tire?

 

35s arent actually 35". You need to measure them and put the correct size in.


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^X3

 

Are you sure the GPS is 100% accurate. When I've programmed in my tires, I've noticed the GPS will be off by a few mph at certain speeds, but when I pass one of those roadside mounted speed monitoring signs it is dead on.

 

As long as you're using a real GPS, it should be more accurate than a road mounted radar sign. A normal GPS is accurate to within 1/10 mph. Radar is accurate to with 3mph.

 

your 35s are probably closer to 33s.

 

x2 there are only a few 'true' 35's out there...km2 runs the smallest, goodyears are close...toyos are almost dead on...check n see the actual size then put that in the programmer

 

This has been asked on the Superchips website also. They couldn't come up with a reason that the ProCal is more accurate than the Flashpaq. Well, they did come up with some reasons, but nothing that would actually explain it. You just have to start with the actual measured height, and then play around with the height numbers until you get it somewhat close.

Actual measured height vs gps worked great for me with the ProCal, I gave up trying to find a Flashpaq height that was as close. Just settle for semi-close.

 

Isn't that pathetic though? We spend all this money on the SC and they can't get the speed right? I remember when I had my SCT XCalII for my Escape before my Jeep. Got new tires...programmed in the revs per mile of the tire...and it was DEAD on with my GPS. This measuring shit still leaves soooo much margin for error.

 

I had programmed my tires to their original size, 20,000 miles ago. Well, after checking it with my GPS, i found that my 34.5" measurement was not the required 33.96".

Do you have a real GPS? Garmin, TomTom, etc? If you do, turn it on each time you drive, before you move an inch, on a fresh tank of gas. Reset one of your trip odometers to 0.0 mi also. Once you need to refuel, take the total miles driven by the GPS, the total miles driven by the Jeep trip odometer, and the tire size you put in. Then PM me.

 

It is not a problem with the superchips unit guys come on....like others have said not ALL tires are a "true" 35" when I tried entering 35" when I got my SC my speedo and shift points were all screwed up so I went out and measured my actual tire height and bingo !! they were 34.25" high once I entered that number everything fell into place and I have verified it with two different GPS units.

 

I know that it's not a true 35" tire and I didn't program it that way. But I know that it doesn't matter where I set mine at...it's never dead on. I wish it were just cuz I'm anal to have things perfect with my rigs...but i guess it's something I have to live with

 

Yes, you're correct.

Maybe he does have 4.10 gears in it? Although the 07s did come with 4.10s if you got the tow package.....so i'm inclined to think thats correct.

 

I would believe your GPS if you hold a steady speed for say 10-20 seconds or more; then do the math for the tire size to adjust the speedometer.

 

You are right, 2 door jk, if my tires were less than 35 inches, then my speedo would show higher than my actual speed, not lower. Does anyone else not catch this? I understand that many 35" tires aren't 35's but something smaller.... but if you're programmed for 35 and runing something smaller you will actually be going slower than what your speedo says. My problem is the exact opposite. Any other feedback besides measuring my tire? Seems like SC website says, "use trial and error". I find this unacceptable.

 

I'd check that 4.10 ratio, 3.73 x 1.1 = 4.1. That could be your 10% error.

I don't know about the '07, but the 2011 Sahara w/tow comes with the 3.73 gears according to the jeep website.

 

07 Sahara's WITH the tow package come with 4.10's. All other years with the tow package come with 3.73's (except Rubi's).

Also the gear ratio in the programmer has absolutely no effect on your speedo you can set it to 5.38 or 3.21 and it will not change a thing on the JK. Our speed is measured at the wheel (ABS Tone rings).

You may need to unplug your ABS module and start the Jeep, then shut it off and plug the module back in. I had to do this on mine before the changes would take affect. You'll get a CEL but just use the programmer to clear the codes and your good to go.

 

If you have a superchips and a GPS, I would just change the tire size until the speedo and the GPS match... thats what I did. I believe my half worn 35" km2's dialed in at 33.25" if I'm not mistaken. So far, the error is minimal and I am very happy!

 

Tire height error

Another explanantion is that the measured tire height doesn't give the effective diameter. "It's only flat on the bottom" will skew the measurements.

Calculate the diameter by marking the tire and ground, rolling a complete rotation and measure the distance traveled, giving the tire circumference. Calculate the diameter by dividing by pi (3.14). I eventually did that and the speed was dead on.
This method takes into account the tire flexing during rotation and gives a larger measured length - hence less error.
A variant method is to measure the height from the ground to the center of the wheel (the radius) and multiply by two. The error is much greater though and is only slightly better than measuring the tire height.

 

Factory tire sizes are measured without any load against them. Once vehicle weight is added their measurements change.

I have always recommended using a tape to measure the tire under load on a flat surface like concrete. The actual height is at the peak of the tire which in most cases is in the middle so you have to eyeball it somewhat.

This method has resolved all of the issues brought to my attention.

Scott

 

Wanted to chime in to say that you are not crazy. I have the same year JK with 4.10's/Tow Package, new 35X12.50X17 Nitto Trail Grapplers (which are damn close to being a true 35), and Superchips and I am still trying to get my GPS and speedo to sync. I started at 35" and now 34.75" and may continue downward until it's closer. Still not sure that it will ever be perfectly synched though.

 

Here is where I ended up on my '08 with Goodyear 35 inch MTRs (I am 2008 Rubi on Superchips):

Tire size entered to get 60 mph on my GPS with 60 mph on the speedo was 33.25 inches.

Now, this did not quite agree with the tire diameter I measured by marking off one full revolution of the tire on the garage floor and dividing by pi. That measurement was 33.75 inches. There is probably a slight dynamic difference at 0 mph vs 60 mph.

So, the question becomes 35s aren't 35? Well, the tire measument off the Jeep was 34.75 inches, by once again measuring one revolution on the garage floor and dividing by pi.

In conclusion, the tires are pretty close to 35s, unloaded. Superchips works.

 

Measure the one revolution distance again with the tire on the Jeep.
Its very hard to believe that your measuring a 1.5" difference (33.75" vs 33.25"), except when talking about loaded versus unloaded tire diameters.
I'd bet you a sixpack that on the Jeep the distance comes out at 104.4" rather than as 106.0".

 

new 35" pitbull rocker radials

On the way to get them I measured 100km/hr on GPS and 99km/hr on the speedo

Superchips reset the size to 35" and 100km/hr GPS was 108 Speedo

Increased to 36" and achieved 100km/hr GPS and 104 Speedo

Haven't tried upping it to 37" yet. wondering, does anyone not trust the GPS speed on an iphone?

 

Don't go by sidewall numbers because once under load the height changes.

Get to a flat surface and use a tape to measure actual height. The peak of the tire (middle) is where you want to measure to so a little eyeballing is in order but you should be dead on with that technique.

Scott

 

So the consensus is to mark the floor and tires on the Jeep, drive one full rotation and divide that distance by pi? I'll try that tonight. My speedo is under by 5-7mph on the highway when comparing my my Garmin & android simultaneously.

update:
I have 315/75R 17 Duratracs. Goodyear's site says they are 34.4" tall. On the Jeep I measured them as 33.75" and one rotation travelled 105.5". I divided by pi and got 33.58".

My programmer was set to 34.75" and I changed it to 33.75", gears were correct at 4.10. Going 35mph my speedometer is spot on with my Garmin, Android and my brother's stock car.

When my speedometer read 65 both the Garmin and 'droid said 62.8 and my brother was "below 65."

I reprogrammed to 33.5" tire height and all my speed checks were spot on. Math is awesome.