Bigger tires VS speedo accuracy

[Randomrunner12]

Hey all,

Hoping that you big dawgs can help shed some light for a young buck.

I've got a 2012 2wd g56 truck, info in Sig. it came factory with 245/70r17's and I stepped up to 265's. According to my speedo when I am doing 78mph my phone shows 74mph. Is this correct? I thought that when you get bigger tires your going faster than what the speedo shows.

Does anyone know if this difference has an effect on the actual mileage of the vehicle either?

 

[AEdelheit]

Larger tires would make the speedometer show a slower speed then the vehicle is going and would reduce the miles on a odometer.

 

[MtnRider]

Bigger tire wold make your speedometer slower then your actual (GPS) speed. With that said 245's seem extremely small for a 2500. The new 265's should be about what it came with from the factory? (265/70-17) so it had to be way off with the 245's. Are they a low profile 265?


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[R.Taylor]

It's the speed sensor input to the computer which calculates speed/miles so I'm sure your mileage will be slightly incorrect.. Not sure which tuner to use to adjust tire size but the Smarty I used on my 06 had a feature to adjust it.

 

[Randomrunner12]

Yeah, I picked it up from the dealer with the 245's on it and swapped up to 265's little while ago. Using my phone GPS and Garmin GPS my actual speed shows slower than what the truck Speedometer states. Ive tried re calibrating the GPS units and still the same thing.

 

[lpennock]

Most likely the previous owner had been running bigger tires such as a 35 or 37 and had reprogrammed the computer for that tires size. You can have the dealer flash the computer to your tire size as long as it is a size that was sold as an option on a RAM. Better option is to get AlfaOBD then you can set the tire size on your own along with turning on options that my not have been enabled from the factory. Some examples:
1. Front windows down with double press and hold of unlock button on remote
2. 3 flash of turn signal with short press of turn signal. (Lane change notification)
3. Vehicle Theft alarm with flashing light on the dash.
4. PTO mode to allow you to get a high idle using Cruise control buttons. (Jumper has to be added to ECM connector for this with the G56)

 

[Randomrunner12]

looks like i might just get the alfaOBD.

 

[Ozymandias]

Most Speedos show more then actual road speed. 2mph is very common, sometimes even more.

 

[MtnRider]

looks like i might just get the alfaOBD.

Just be aware not all functions of AlfaOBD work for a 2012. It's mostly for 2014 and up. There is a big thread on another forum that goes into detail on what has worked with what years.


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