Larger Tires and speedometer
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Junior User
Joined: Feb 2011
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Larger Tires and speedometer
I recently had a lift installed and put 37x13.50x20 tires installed. I took it to the dealer and had them try to correct the speedometer for the tire size. The only thing there computer offered for tire size was the available factory tires. How has everyone else dealt with this problem?
Lead Driver
Joined: Sep 2003
Posts: 5,844
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From: Albuquerque
strokin -- Wow...that is unbelievable. I was under the impression they could correct tire size all the way up to 40" by using the manufacture ratings of rotations per mile (rpm). Since not all 37 or 35 inch tires are the same. Maybe they were looking in the wrong section on their computer system??? 
IMHO...going with a tuner is a high risk proposition. Might take that...and not honor warranty repairs...even if you leave it in stock tune.
Hopefully a FORD tech will chime in here to offer details...
biz
IMHO...going with a tuner is a high risk proposition. Might take that...and not honor warranty repairs...even if you leave it in stock tune.
Hopefully a FORD tech will chime in here to offer details...
biz
Senior User
Joined: Jul 2011
Posts: 181
Likes: 0
Strokin is correct. Ford computer will only allow for stock tire sizes. To add to that it will only allow for tire/axle ratio combos that are avalible from Ford on any given truck. When I went to bigger tires on my factor 18" rims the best match for rolling diameter was one of the factory 20" wheel/tire combo that only came with the 3:55 axle and since I have the 3:31 ratio the computer kept throwing the CEL until we changed it back. What I ended up with was setting the computer at the largest tire that came with the 3:31 ratio and I am now within 1mph at 60mph.
Senior User
Joined: Mar 2011
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I have 37"s on my truck and checked the speedo with an aftermarket gps. Mine is out about 7 mph at 70 mph. The only good thing is your truck is registering less miles than your actually driving. When you figure your mileage out by hand you need to account for the lower miles.
Lead Driver
Joined: Sep 2003
Posts: 5,844
Likes: 6
From: Albuquerque
I have 37"s on my truck and checked the speedo with an aftermarket gps. Mine is out about 7 mph at 70 mph. The only good thing is your truck is registering less miles than your actually driving. When you figure your mileage out by hand you need to account for the lower miles.
YES...absolutely that is going to happen.
What I am more concerned about is the potential transmission shifting issues that could arise. It was very common with the previous 2 trans to shift incorrectly...and cause shutter...which would then cause excessive heat buildup.
IMHO...this is pretty ridiculous from FORD not to allow (YES allow) its customers to run larger than OEM sized tires...and want to preserve the trucks shift characteristics...by changing the tire size with the computer accordingly.
Bad...bad...bad... Just not cool.
biz
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