Okay I have a question that I am stumped on. I have heard that gas mileage depends alot upon the accuracy of the speedometer. I have 315/75/R16's on my truck and bought the Superchips 1715 Tuner a while back. I measured the exact tire heigth and they are just a little over 34" tall. So I changed my tuning settings to a 34" tire heigth. Problem fixed....so I thought. I recently had a Pioneer AVIC-D1 installed and it has the vehicle computer thingy in it which is hooked into the speed sensor wire. I was told that this unit would be pretty accurate in regards to the speed it displays. Well when I am driving, somewhere around 60mph, my navigation system will say that I am running 56mph and my speedometer will say 60mph. Which one is correct? I would think the navigation would be the same as the speedometer because they are both connected to the speed sensor wire. How should I go about fixing this? Thanks for any help.
Bradley

 

Depending on where the head unit is getting its speed from, I'm more inclined to say trust the speedo. The head unit might be displaying the uncorrected speed.

 

Are you sure the Pioneer unit is hooked into the vehicle speed sensor? Although I am not familiar with the Pioneer unit in question, a quick online search seemed to indicate that it incorporated a GPS unit. If its speed indication is GPS-based, then it should be far more accurate than your speedometer.

If you have (or could beg, borrow, or steal) another GPS unit, that would be the easiest way to confirm which of your systems is more accurate. Like I said, if your Pioneer unit is GPS-based, then I am betting on it.

If you confirm that the Pioneer unit is in fact accurate, then fine-tune your speedometer by "trial and error" by tuning the vehicle computer for 35 inch tires, then 36's, 37's etc until you get it dead on.

 

Yes, the Pioneer unit is a navigation unit, but it is hooked up to the vehicle speed sensor wire. I would also say that the unit is far more accurate than the speedometer, but the performance computer on the unit did not work at first. When I took it back to the place I had it installed, they did not have a good connection going to the speed sensor wire. After that was connected, the computer worked. I would think that the unit works off of the GPS, but Pioneer told me this, "The AVIC-D1's incorporated vehicle computer runs strictly off of the speed sensor wire it is connected to. The vehicle computer has nothing to do with the GPS unit." I got this today, as a reply from an e-mail I sent them. Thanks for all the help though. I still appreciate it.
Bradley

 

Hmmm, Ok, in that case I would tend to trust the speedometer more. Both your vehicle computer and the Pioneer navigation unit are getting a raw signal from the Vehicle Speed Sensor. Using your Superchips tuner, you have programmed the vehicle computer to interpret this signal correctly, using the new tire size. I assume the Pioneer unit is not interpreting the signal correctly, since it doesn't know about the new tire size. This could pose a problem for the navigation functions, since it almost certainly uses the signal for distance calculations also, and right now it looks like you have roughly a 15% error.

Is there any way to adjust or calibrate the speed indication on the Pioneer unit?

 

That is the best idea. If the Pioneer and your speedometer are receiving the same bad input, both will be off.

 

Alright guys. Thanks for the replys. I don't know what to do. I could probably either follow somebody to see which is correct or get a local county cop to radar me at a certain speed. That would definately tell me which is off. I also thought about something else. I installed some different gauges awhile back before the nav system and I had to remove the needles. I am almost certain that I put the needles back on in the correct positions, though. Guess the best option is to have a cop radar me. That should definately work. As for re-calibrating the nav sytem, I'm not sure. I'll have to read up on that in my manual. Surely Pioneer would allow the consumer to do this in some way.
Bradley

 

i know that when i put my larger tires on it messed with the speedo but then when i got the AVIC N2 installed it shows me the correct speed.

 

I too installed bigger tires and a GPS. Even with the tires measured and entered into my 1715 tuner, the speedo was always 3 to 4mph faster then the GPS. I had to reenter a 1 inch smaller tire size to get the speedo to within 1mph of the GPS.
I would trust the GPS above the speedo. Note that having the speedo read a tad higher than real life can help you in avoiding the donut speed tax payments to you local JBT.


Fritz