Speedometer reading with larger tire

Sorry if this has been covered, but does the digital dash speed pull from an analog source that feeds the speedo? Or does it pull from the GPS on the Nav? This is on a 19 with entune.

I'm bumping up tire size and wondered what I would see on the dash. It's only a few MPH at Highway speed but that's enough to get the smookies mad sometimes.

 

Not gps or nav. Tire size in calculationd in ecu. The tire size setting can be changed with the right tools.

 

There are tire size calculators online that can tell you the difference in speed with change to tire size.

 

Saw those, just wondered if I'd be doing math in my head on the fly ;-)

 

What size tire are you going to? I’ve heard that our trucks read a little fast on the speedometer. Bumping up in size might just put you right on or reading just slightly slow depending on the size tire you go to. I think 33’s put it right on the money if I’m not mistaken.

 

Just saying I’d see where you’re at comparing your speedometer to your GPS. Get the new tires and see where that puts you before you decide to put money into having it corrected.

 

yeah. I went to 33's and my speedo matches what my gps shows.

 

33’s are fine. I had 35x12s and used a hypertech device to change my speedo, setting the hypertech to 35s but that went too far the other direction.

 

Difference will not be linear. The difference increases as speed increases. ex. .5 off at 35 mph but 2 off at 70 mph. if you can remember a few reference spots (mph) you can stay under speed limits. Also, by reading low, you are putting less miles on your truck.

 

It’s really simple. You just need 2 baselines memorized. Street speed and highway speeds

 

Same here, noticed from the day I got my truck I felt like I was going slow when set at 70. Sure enough checked with gps and I was 2mph under. Switched to 275/60R20 and now it’s dead on

 

I just changed from the OEM tires (31.9") to a 34.2" tire. The OEM was about 1 or 2 MPH under and now I am about 2 miles over at 70 MPH highway speed.
GPS shows 72 MPH when the truck shows 70. The way I typically drive (not 70) the difference would be irrelevant by the time they catch me. LOL.
But, yes, at 70 there is about 3 MPH difference at highway speed. I am not running the GPS app all the time, so I have a mental note to stay at 3 MPH slower on Highway and say 2 MPH in the City if there is any suspicion of speed traps. I am not going to pay the $70 or so to have it recalibrated.

 

Re calibrating the speedo is really more important for trans shifting than the mph number. Once you go to 35+ it really messes up your shifts

 

I have to get a Hypertech to program right? And will that void warranty?

Not concerned with speed but what you said, having correct shift points and mileage