Switch from 20 in tires to 17 in tires on winter?

[SimmZ]

Hi!

When I purchase my 2019 RAM Classic, my specific model comes with the chrome appearance package, so I have the factory 20 inch chrome wheels with the 275/60R20 tires. Then my dealer gave me (for free) an extra set of steelies with some very good Pirelli 17 inch winter tires that match the factory size for the base RAM with the 17 inch wheels, the 265/70R17.

But when I calculate the overall diameter between the 20 inch and the 17 inch, the 17 inch are pretty much smaller than the 20 inch... check the chart :

Specification Sidewall Radius Diameter Circumference Revs/Mile Difference
275/60-20 6.5in 16.5in 33.0in 103.6in 611 0.0%
265/70-17 7.3in 15.8in 31.6in 99.3in 638 -4.2%

I was told several years ago by a tire shop that the tire overall diameter must match by more or less 3.0% between factory and aftermarket to prevent any problem like bad readings from the gauges as per the vehicle speed sensor and also some other vehicle variables that are driven by the speed sensor.

So, do I need to get my BCM reprogrammed on every tire (seasonal) change because I put 4.2% smaller tires during the winter? I assume that this is a very small operation that the dealer can take care of, however they will surely charge $$$ for doing that twice per year...??

All advice are welcome

 

[korby05]

All you would need to is buy a programmer like superchips or something similar. I have one for my truck and if and when I change wheels the programmer has a setting to reconfigure your computer for tire and wheel size.

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[SimmZ]

All you would need to is buy a programmer like superchips or something similar. I have one for my truck and if and when I change wheels the programmer has a setting to reconfigure your computer for tire and wheel size.

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I already checked that, but for some reasons the Superchip brand don't have any application for RAM V6 and it seems to be the same for other brands of programmers. They only offers application for the HEMI. I am pretty sure that the BCM is similar but not identical between the V6 and the V8.

 

[Elevated 2013]

Going with the smaller diameter rims but bigger sidewalls in the winter will help with the deeper snow for sure. As for the size difference, even a little change in the overall tire size with throw your speedometer off to some percentage. Do you HAVE to reprogram the PCM/BCM/speedometer for it?? You don’t have to, but it makes it easier on your mind to not have to mentally adjust the speedometer so that you don’t speed and get tickets when you weren’t trying to. You can also buy a cheap speedometer calibration tool but if you try to use a DiabloSport tuner to actually tune your PCM keep it in mind as it could cause issues if you don’t bring it back to stock before tuning it. It’s not always true but it’s a possibility.


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[korby05]

I already checked that, but for some reasons the Superchip brand don't have any application for RAM V6 and it seems to be the same for other brands of programmers. They only offers application for the HEMI. I am pretty sure that the BCM is similar but not identical between the V6 and the V8.

Oh I didn't know yours was a v6. I wouldn't think there would be much difference between the hemi and V6 in the BCM for just changing the speedometer but you never know though.

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[Firetruck41]

There is no need to change the BCM for this. Just put them on, remember there is a 4% difference in your speedo/odo. Thats it. Most speedometers are not that accurate anyway. 4% is 2.4 mph at 60 mph. There could be nearly that much difference between the same size tires made by different manufacturers. Not sure what the tire shop was saying about 3%, maybe they meant not to have that much of a discrepancy between tires that are mounted on the car at the same time? If you really care about it, you could use AlphaOBD to change tire size (maybe, not sure of 2019 capability)

 

[SimmZ]

Oh I didn't know yours was a v6. I wouldn't think there would be much difference between the hemi and V6 in the BCM for just changing the speedometer but you never know though.

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I just sent an email to SuperChip about the compatibility with the V6... just in case.

 

[2014greatwhite]

I have a 14 ram that came with stock 20” wheels. I tried to put a buddies 17” wheels on after I sold my stock wheels to get an inspection here in mass but the 17s wouldn’t fit. They were hitting on one of the suspension components and wore a groove into the inside of the wheel. Just was t sure if you actually tried the wheels on before you go ordering a programmer.


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[SimmZ]

I have a 14 ram that came with stock 20” wheels. I tried to put a buddies 17” wheels on after I sold my stock wheels to get an inspection here in mass but the 17s wouldn’t fit. They were hitting on one of the suspension components and wore a groove into the inside of the wheel. Just was t sure if you actually tried the wheels on before you go ordering a programmer.


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Well, based on what you are saying you probably had offset issues. Maybe your friend's wheels were aftermarket or from another RAM model, such as putting 2X4 wheels on a 4X4 vehicle, that usually mess with offset and wont fit. The 2X4 will have positive offset while 4X4 have negative offset, or maybe it's the total opposite, I am always confused between the two. But the bases remain the same; different offset wont fit.

In my case the 17 inch wheels are factory/genuine Mopar wheels, so they are guaranteed to fit my RAM. They'll also clear the brakes and any mechanical component as my specifics RAM model is coming with 17 inch from factory except if you have optional package like I have.

In both case, I have no concern about that because the wheels and tires are the responsibility of my dealer that are giving them with the truck, if they don't fit properly, they'll correct it for free as well. However the speedometer adjustment are my responsibility, if required.

 

[2014greatwhite]

Well, based on what you are saying you probably have some wheel's offset issues. In my case the 17 inch wheels are factory/genuine Mopar wheels, so they are guaranteed to fit my RAM. They'll also clear the brakes and any mechanical component as my specifics RAM model is coming with 17 inch from factory except if you have optional package like I have.

In both case, I have no concern about that because the wheels and tires are the responsibility of my dealer that are giving them with the truck, if they don't fit properly, they'll correct it for free as well. However the speedometer adjustment are my responsibility, if required.

Gotcha. Just making sure before you buy a programmer. The 17” wheels I tried were factory ram wheels as well. He had a single cab express with factory 17” and mine is a crew cab express with stock 20”.


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[Quyonmob]

Gotcha. Just making sure before you buy a programmer. The 17” wheels I tried were factory ram wheels as well. He had a single cab express with factory 17” and mine is a crew cab express with stock 20”.


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There is something else going on there. 4th Gen OEM 17’s fit a stock 4th Gen Ram that came with OEM 20’s.

OP- As mentioned, just run them, the tire size difference isn’t worth correcting if you are aware of it. My OEM P265/70/17 SRAs only measured 30.9” tall, replaced with P265/70/17 grabber AT2 that measured 31.9” tall. Variances like this are far too common.

 

[2014greatwhite]

There is something else going on there. 4th Gen OEM 17’s fit a stock 4th Gen Ram that came with OEM 20’s.

OP- As mentioned, just run them, the tire size difference isn’t worth correcting if you are aware of it. My OEM P265/70/17 SRAs only measured 30.9” tall, replaced with P265/70/17 grabber AT2 that measured 31.9” tall. Variances like this are far too common.

As I’m reading your post in just popped into my head which I’m feel dumb for not thinking about it before. I have a 4” zone lift on my truck so it was most likely rubbing on a part of the lift components. I apologize for creating any confusion to the original posters question.


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