Any idea why when I put 35 into my superchips my speedo is still off when compared to my GPS. It is a bit low. When I'm going 60 mph according to speedo i'm going more like 65 according to GPS. Almost a 10% error. When I plugged it in and it read my VIN it said that I have 4.10 gears. I have an 07 auto sahara with the tow package. That seems to jive with everything I've read. Jive:mr-t:
Superchips tire size change speedo still wrong
rockclimbermd
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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.
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.
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87 Posts
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
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
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66 Posts
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.
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.
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32 Posts
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".
That is 0.50 inches, not 1.50 inches. You owe me a beerMeasure 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".
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32 Posts
Granted, but I don't deliver!That is 0.50 inches, not 1.50 inches. You owe me a beer
0.50" * pi() = 1.5"
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?
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?
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87 Posts
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
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
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899 Posts
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.
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.
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