I expect next they will shorten your warrenty by another 10K just because you installed larger tires...

 

Shhhhhh!!! Ya never know who's watching...

Later...
Brewster...

 

The speedometer may be off, but the odometer is right. I went through this when I went to 305's. If the speedometer is right, the odo is short. It is the way the speedometer is designed, and while some cars may be spot on, many vehicles have a 1 to 3 mph difference depending on speed. It's generally accepted that cars in the US are off by that much intentionally.

 

Both my 1995 F350 and 2001 F250 are showing 2-3 mph faster than my GPS.... even shows this on a handheld GPS too.
If Ford is knowingly doing this....how many $millions are they saving on Warranty work they should be honoring for customers.

 

Most vehicles I've owned err on the side of caution and run a bit high. This way you don't speed (and the factory gets the benefit of shortening your warranty).

Moving to 285s has almost corrected it and my error is around 1% now.

Just so you're not fooled, your fuel gage does not give the most accurate reading either! These are all psychological ploys to make you feel better about your purchase and your feelings towards the manufacturer...

 

It may be a stretch, but you could check with an Off Road or 4x4 aftermarket shop and see if they have a programmer that will adjust the speedo for tire size. Off road guys run large tires and change speedometer readings all the time. They may do it for you at a lot less than Ford will.

 

apparently some code readers can change the speedo for different tire sizes. I just ordered the autoenginuity unit that supposedly will do it, will probably get here next week. If anyone is in the raleigh area and needs a larger tire size programmed we can try it out. just PM me

 

You guys sure are a bunch of conspiracy theorists.

If your truck is within ±5%, that is about all you can expect. Consider the fact that most tires have 1/2" of usable tread, which means a tire will change diameter by about 1" during it's life. A 33" tire wearing down to a 32" tire, is a 3% change.

And that doesn't even factor in any differences in tire pressure between users.

 

Thanks Chris. I was under the belief that there's a way to correct this by changing the speedo gear in the tranny to compensate for larger tires. I also was informed that there is a setting in the computer to account for different size tires, although the results were reported to be mixed.

I would have thought that the speedometer and odometer readings are relational and proportional. So, with this line of thinking, it would be counter-intuitive that the speed would be fast but the odometer is correct. That's unless, as bpounds would call it, the conspiracy theorist's theory that Ford wanted to fool us into driving slower than the speedo was actually displaying.

That said, I agree that tire thread has a factor as well as speed. The faster the tire spins, the greater the rotational forces will change the contour of the tire, slightly enlarging the circumference. Manufacturer, material type, and build construction will affect how much of a factor. That said, just how much both of these will affect the overall accuracy of speedometer is unclear.

As mentioned, on both my Acura's (I have the CL and an MDX), the speed is pretty spot on comparated against the GPS. Both have varying degrees of tire wear. The tires on my EX are relatively new. If thread depth was a significant factor, then the more they wear down, the further off (i.e. faster) the speedo will be.

I could do some quick calculations later to look at changes in the diameter and how this would relate to the speedometer...

 

C = π d where C is the circumference, π is 3.14 and d is the diameter
C = 2π r where r is the radius
C = 3.14 * 35 = 109.9 where the diameter is 35”.
C = 3.14 * 34 = 106.76 where the diameter is 34” due to ½” removed from each side due to wear.
109.9 / 106.76 = 1.029412 = 2.9% rounded
On a smaller tire this would be slightly less and, conversely, on a larger this would be slightly greater.
This does NOT take into account flattening due to vehicle weight, etc., etc., etc.
SO, given a 3% difference explained above, this leaves a +/- of 14%... 93% on the low end to 107% on the high end. Do I have any volunteers to pay for my warranted repairs between 93k and 107k mileage?

Within 10% gives a 20% range. That’s an awfully big range. I would think Ford could do better and I would think all the Blue Oval fans would think Ford is capable of this also.
<O
I don’t think that this is so much a case of “conspiracy theorists” [i.e. the gov’t is responsible for the 9/11 attacks] as much as people expecting to get what they pay for or at least questioning for what they are paying [i.e. $3400 for a 4yr./48k warranty means either: A) 4 years as opposed to 3 yr.6mo. and days, or; B) 48k not 43k and change, or; only costs $3060 as opposed to $3400.]
<O
Later…
Brewster…

 

Speedo shows 2-3 fast but computer is dead on if you monitor it with a scanner or put the cluster in test mode. The computer controls the odometer.

 

Actually, 1" of wear makes a greater difference on smaller diameter tires, less difference on larger tires.

30"/29" = .034 (3.4%)
35"/34" = .029 (2.9%)

Not all tires will have 1/2" of usable tread per side. Most all-terrains will. Many highway tires won't. Mud terrain may have even more, but those are never OEM tires. But I just used 1" total as a point of reference.

You totally lost me here. How did we jump from a 3% difference, to a 14% difference?

I think we would all agree that they should do better. I think they do actually do better than the minimum they require of themselves. Mine have always been less than 5% off, and probably right around the 2-3% error.

As for conspiracy, I was thinking of the speedometer being purposely offset from the odometer for the purpose of defrauding us of our warranty. Sorry, I don't buy that.