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Ron, ASE Certified Technician
Category: Ford
Satisfied Customers: 23821
Experience:  23 years with Ford specializing in drivability and electrical and AC. Ford certs and ASE Certs
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2008 Ford 350 4x4 6.4: 2008 Ford Super duty 4x4 6.4 Auto 39

Customer Question

2008 Ford Super duty 4x4 6.4 Auto 39 inch tires ,4:88 ring and pinion gears were installed and now the vehicle has run ability issues.Wont accelerate,wont drive above 20mph, down on power. Tone ring seems to be in place in rear carrier. Changed rear diff mounted sensor no improvement.Vehicle is equipped with a tuner and tire size/gear ratio values have been changed. Shut traction control off and the vehicle drives normally with no issues.When traction control is turned back on the symptoms immediately reappear.
Submitted: 2 years ago.
Category: Ford
Expert:  Dr. Hamman replied 2 years ago.
Hi, I am a professional certified mechanic, with an engineering background, and 35+ years experience. I will do my best to assist you. Also keep in mind I don't know if you are a pro or a novice, so feel free to add any additional info at any time.
There isn’t a straight forward definitive answer. I am thinking there are problems with what the computer sees as far as the front and rear tire rotational speeds. Also the 39 inch tires and 4.88 gears might be outside the computers ability to process, both are pretty steep alone. If you maintain the correct ratios, in drive shaft revolutions per foot traveled, you can input shorter tire, and higher gear ratios that the computer can understand many times. That is if the tire/ratio are beyond the computers program capability. I believe the stock tires are about 245 75 17? those are about 32 inches tall. You could do some math and get some creative numbers to explore that. The traction control and the anti lock system both read off of the wheels speed. If the front and rear report different speeds it will confuse the system. Do you have the equipment to read the front and rear speed sensors to make sure they have output, and next matching output.