Bad matching tires dimensions bring Limp mode


Funpilote

GT Owner
Jan 7, 2015
10
In France, we must get European certifications with CE tags on our sidewalls tires, but there is not a too much choice regarding dimensions, even less GT factory size who are not available anymore.
First choice has been 245/40X18 in front and 325/30X19 in rear (Michelin pilot sport cup 2) to not lose too much thickness, alas this choice was bad and the car entered in Limp mode, It took me a long time to understand where the problem came from🙄🙄.
Today, i swap the four wheels from another GT with Stock wheels and tire, go for a long road test in fact to be sure ; no limp go home problem anymore ! It was realy incorrect tires size who enter limp mode.
Ford factory sizes are 235/45X18 front = 669 mm diameter and 315/40X19 = 735 mm. The ratio between front and rear is 1.098 (9.8%)
GT friend's sizes are 245/40X18 front = 653 mm diameter and 325/30X19 = 678 mm. The ratio between front and rear is 1.038 ( 3.8%)
My personal GT sizes are 235/30X20 front = 649 mm diameter and 345/30X20 = 715 mm . The ratio between front and rear is 1.101 (10.1%)
Normaly, factories tolerance regarding ABS moudules are 5% up or 5% below, so 1.098 - 1.068 = 0.06 (6%) outside factory tolerance!
I know SCT tuners, who are able to correct speedo reading when you change tires heigth or differential gear ratio ,* I don't think they could resolve the problem because the speedo sender signal is not the same signal than the ABS wheels sensors.
Can anyone confirm or refute my reasoning ?
Thank you
 

06fordgt

GT
Mark II Lifetime
Le Mans 2010 Supporter
Nov 8, 2006
1,763
Toronto Canada
had no idea there was limp mode on the 05/06 cars!
 

Xcentric

GT Owner
Mark II Lifetime
Jul 9, 2012
5,019
Myakka City, Florida
I never heard of anyone running 20s front and rear. +2F/+1R wheels and small diameter tires? Why are you and your friend using tires that are shorter than stock? Are your GTs lowered a lot?

Check this thread (post #7) http://www.fordgtforum.com/forums/showthread.php/33886-2006-GT-shuts-down-on-acceleration. Guy had limp mode and thought it was ABS. Unfortunately, the real problem was never reported. But, a GT expert says "...the diameters could be too close of a ratio from front to back causing the ABS module to limit speed to around 50mph." You seem to have the opposite...too big a ratio. Perhaps that causes the same result, as you suspect.

Is that your GM station wagon pulling the trailer? How the hell do you fit through French villages? :lol
 
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Funpilote

GT Owner
Jan 7, 2015
10
I used this 20 rims size because as explained above we cannot find 13 year old dimensions with European certifications, and insurances companies will not cover responsability if accident occur. Also GT with 20 look very pretty.
Concerning my friend, he want to keep factory wheels, but no more tires available again, I gonna swap 245/40X18 by 245/35X18 in fact to keep in 5% manufacturer's tolerance.
I had already read the GT guy post, i agree
Regarding my Roadmaster, don't worry, i own this station wagon since 22 years and I really like to drive it through our pretty little villages! Anyway i don't have the choice, when i go to the track, i use slick tires they are strictly forbidden on open roads in France
 

2112

Blue/white 06'
Mark II Lifetime
Try Bridgestone Potenzas 265/40 -18" in front & 345/35 - 19" in the rear on stock diameter wheels.
 

Funpilote

GT Owner
Jan 7, 2015
10
Not availables in France
 

2112

Blue/white 06'
Mark II Lifetime
What do the Porsche and Enzo owners do?
 

GT101

*UK Support*
Supporting Vendor
Jan 20, 2010
273
Colchester, Essex, UK
We can easily ship you Bridgestone RE050 in 265/40R18 and 345/35R19 from the U.K.

As suggested these are the optimum sizes for stock wheels.


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