Speedometer variance with bigger tires

[Wanderlust073]

So just looking to confirm my thinking here...

I replaced the 285/60r20 Firestones that came on my 2500 with 295/65r20s - a jump from 33.5's to 35's. Every other vehicle that I've jumped tires up an inch or more has taken a hit to speedometer accuracy, and if I run the calculator on tiresize.com it looks like I should be rolling at 68.17mph when the speedo is reading a flat 65mph.

The last couple days I've been running a gps while driving at various speeds up to 85mph, and the truck speedo is matching the gps all the way. No difference between the two at all.

Someone HAD to have had 35s on this thing before and adjusted the speedometer for them, right? That's the only possible explanation?

 

[Jimmy07]

So just looking to confirm my thinking here...

I replaced the 285/60r20 Firestones that came on my 2500 with 295/65r20s - a jump from 33.5's to 35's. Every other vehicle that I've jumped tires up an inch or more has taken a hit to speedometer accuracy, and if I run the calculator on tiresize.com it looks like I should be rolling at 68.17mph when the speedo is reading a flat 65mph.

The last couple days I've been running a gps while driving at various speeds up to 85mph, and the truck speedo is matching the gps all the way. No difference between the two at all.

Someone HAD to have had 35s on this thing before and adjusted the speedometer for them, right? That's the only possible explanation?

Just so you are aware, if your phone is connected to the radio at all running CarPlay or Android auto, then the phone is not using its native gps anymore, it is using the truck’s.
Make sure whatever gps app you’re using, your phone is completely disassociated from the radio.

 

[Wanderlust073]

Just so you are aware, if your phone is connected to the radio at all running CarPlay or Android auto, then the phone is not using its native gps anymore, it is using the truck’s.
Make sure whatever gps app you’re using, your phone is completely disassociated from the radio.

Wow, hadn’t heard that before though I’m not thinking it would impact the numbers. The speedometer isn’t gps based.

 

[towpig]

Wow, hadn’t heard that before though I’m not thinking it would impact the numbers. The speedometer isn’t gps based.

I think he means that the gps speed that is showing up is actually being pulled from the trucks speedo and not a stand alone reading from your actual gps and no the truck doesn’t rely on gps for speedometer readings.

 

[Greenhills]

I didn't notice any real difference in the speedometer going from 33 to 35s. Going to 37's resulted in a major discrepancy.

 

[el_barto]

Just correct it with alfaobd or Tazer Ram ?

 

[Crusty old shellback]

Measure the actual size of the tire while on the truck and at the pressure you plan on running. You'll quickly find out it's not 35 inches.

 

[Wanderlust073]

Measure the actual size of the tire while on the truck and at the pressure you plan on running. You'll quickly find out it's not 35 inches.

Good call, Mr Crusty! I’m running them around town at a smooth riding 57psi front and back and they’ve gone limp

 

[Crusty old shellback]

I run 45/37 hot in town driving. Found out they go up about 10 PSI on the highway in 100 degree heat.

 

[Ctimrun]

With the stock tires my truck’s speedo reads fast. Was thinking when I go up a size or two the speedo will be right on. Did you ever check your speedo with stock tires?

 

[Shadowboxin20]

I went from stocks to 37s and my mpg went from 13.8 to 16 for mostly city driving.

Big swing

 

[Brutal_HO]

Good call, Mr Crusty! I’m running them around town at a smooth riding 57psi front and back and they’ve gone limp

View attachment 43815

Even that measurement isn't accurate.

Bottom of tire to center of hub to use for true and proper circumference calculations.