I took my first trip since the purchase of a Tom, Tom GPS. I noticed that the MPH on the GPS was about 5 MPH higher than the spedo. I have a trip dog tuner and have matched the tire size to the tuner.

Not only do I think this would throw off my hand calculated MPG, but more importantly, when I have my cruise set on 76, my GPS say's 82. That kind of make's me wonder. 76= they look the other way, 82= big-ole-ticket.

Thank's in advance.

Jeff.

The tire size recalibration on the Triple Dog might be off by a little. Try changing the size to something a little bigger and checking it with the GPS. If this sets it off more go with a smaller size. You should be able to get it close if not right on.

Trust the GPS. Then verify on a stretch of highway with the calibrated distance signs. They should match nearly exactly.

The tire size recalibration on the Triple Dog might be off by a little. Try changing the size to something a little bigger and checking it with the GPS. If this sets it off more go with a smaller size. You should be able to get it close if not right on.

I have this same problem with my GPS and truck speedo. My GPS says about 4 mph faster. I thought of doing what you are suggesting but what if the GPS is what is wrong? I often wondered how accurate the GPS speed is.

I trust the GPS more than the speedo.

When I had the stock 245's it read about 2mph high on the interstate. This was verified by GPS and one of those portable "YOUR SPEED IS" signs that the state troopers use in construction zones on the highway. The radar and the GPS read the same.

I will give the tire size change a go.

Thank's

I have the same issue. Mine is 3-4 miles off also. I am trusting the GPS speed also. Better to be safe then sorry. I can't really afford a ticket right now.

This is verified in my duramax truck also, i have new 245 at's, and the GPS i ran to GA and Back to WV consistently read that at interstate speeds i was 2mph slower than what the speedo read... at 72mph on speedo i was running 70mph on the GPS,

however, on my gasser 07 HD, with 285's on it... i will be running at 72 mph on the speedo and actually with GPS be running 75or 76 mph.

Both trucks have no electrical or programming recalibrations.

GPS will calculate the speed accurately than "picking a menu number on programmer".

In very very simple terms GPS is constantly doing calculations for distance traveled in relationship to satellites being tracked. Only time GPS will be inaccurate on this type of calculation is when satellite reception/lock is intermittent like in a dense forest.

I have this same problem with my GPS and truck speedo. My GPS says about 4 mph faster. I thought of doing what you are suggesting but what if the GPS is what is wrong? I often wondered how accurate the GPS speed is.

The GPS is actually very accurate. The way that they pick up on different satellites and track your position is how they can produce a speed reading, basically by you jumping from one satellite channel to another as you move across the ground.

GPS will calculate the speed accurately than "picking a menu number on programmer".

In very very simple terms GPS is constantly doing calculations for distance traveled in relationship to satellites being tracked. Only time GPS will be inaccurate on this type of calculation is when satellite reception/lock is intermittent like in a dense forest.

That is why i gave the explanation above... to verify this.... :):D