Two part question.
It's a 08 F350 6.4L. Just put in an H&S Mini Max a couple months ago. LOVE IT! Good amount of power, less turbo lag, cool sound and I think I'm getting about 25% better mpg (have not done an accurate reading yet). Which isn't overly exciting since I was only getting about 12 before. Still happy about it though.
But I'm having a problem with the accuracy of the speedo. It's reading about 5% off. I've adjusted for my tire size within .25". Yes I measured the diameter of the tire with a tape measure. I've re-programmed it twice now and still about 5% off. I've double checked my measurements and I know I'm within .25". So what could my problem be?
My solution for now will be to enter a tire size that will make the speedo read correctly. Which means I will enter a tire size that is about 1.5" larger than what I have. Just don't understand why it's off. Could the tire be "growing" as it gets up to speed due to centrifugal force? I could see that happening by maybe .25-.5" but not enough to throw the speedo off by 5%.
Thoughts?
Second question: is there any reason I can't run the truck without the Mini Max itself plugged in? Other than monitoring the engine. But I'm not very hard on it. Is there a reason I need to have it plugged in and on every time?

 

I have the same tuner. I think we can down load and then put it in the console or leave it home. But, why would you want to give up all that info?

Hav to wait for someone else on the speedo. Good luck.

 

curious how are you measuring the tire. i think they recommend measuring the tire from the ground to the center of the rim also you say to get the speedo accurate you would need to enter a larger size than what you are measuring? I know i had to play with mine a few times when resetting it and comparing the speedo to gps. the fact the you are saying you need to input a larger size kinda puzzles me. i could see if you had to enter a smaller size if you had been measuring the full tire height or possibly using a tire size calculator.

 

So your speedo is showing your traveling at a slower speed that you actually are?

I doubt there's any type of centrifugal force that large. Especially in the higher rated tires these trucks require. I wouldn't stress it too much. Either leave it and know your 5% variances or change it to the bigger size to make it accurate.

 

What size tire? Are you measuring from the ground up or from the horizontal plane? It could just be error in your measurement. Do you know you are measuring square to the tread surface. If you are in the 35"+ tire size it would not take a lot of error to translate in a larger/smaller circumference to translate to a 5% error.

 

If your speedo is off by 5% through any speed, centrifugal force is out of the question. Faster speeds would increase the variance.

How are you measuring your "true speed?" Smart phone? GPS? Has the device been calibrated properly?

 

They are 35's. They measure about 33.25 from the ground. Never really thought about that. But I'm not sure that would equate to a 5% error. That would equate to the tire being about 39.75". Not sure it's compressing that much. But still a good thought.



I've measured three times. Twice with the mile markers (different highways) on the side of the road and once with GPS. All suggested about 5% off, +/-1%.

 

1.75" (Variance from 33.25" to 35") divided by 35" is 0.05 or 5%.