Couple New Tire Questions

[galaxy]

Wheels and tires are going on this weekend and thought of a couple questions...

Speedo aside, does anything need to be done for the variance in diameter on the rears? I'm dropping .7" in diameter (27.4 to a 26.7). Minimal, I know, but thought I'd ask to see if it will affect anything (other than just the speedo reading obviously).

TPMS. They should be just plug and play, correct? (such was the case on my '16 Focus ST) No learning or programming or anything?

Thanks guys.

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[fpa1974]

Wheels and tires are going on this weekend and thought of a couple questions...

Speedo aside, does anything need to be done for the variance in diameter on the rears? I'm dropping .7" in diameter (27.4 to a 26.7). Minimal, I know, but thought I'd ask to see if it will affect anything (other than just the speedo reading obviously).

TPMS. They should be just plug and play, correct? (such was the case on my '16 Focus ST) No learning or programming or anything?

Thanks guys.

As far as TPMS goes it will automatically pair with the new sensors - nothing to do from your side.

 

[ahl395]

Speedometer and odometer will be affected by the change in tire size. Otherwise you wont have any issues.

New TPMS sensors will automatically relearn to the car after a few miles.

 

[GT_Dave]

I changed from 305/35 to 325/30 in the rear and my speedo reads 2 MPH faster than actual at 60 MPH. To me that is a safety factor, I don't intend on reprograming the car for the new tire size. Had no issues with the new TPMS sensors.

 

[galaxy]

I changed from 305/35 to 325/30 in the rear and my speedo reads 2 MPH faster than actual at 60 MPH. To me that is a safety factor, I don't intend on reprograming the car for the new tire size. Had no issues with the new TPMS sensors.

Exactly what I’m doing. Thanks for the tips guys. I read somewhere about someone changing diameter, had DTC fault or something and some engine bogging in 4th gear or something until he did some kinda reset, but now I can’t find that post/thread. Oh well. Can’t imagine .7” would effect anything (other than the speed).

 

[Crashspeeder]

I changed from 305/35 to 325/30 in the rear and my speedo reads 2 MPH faster than actual at 60 MPH. To me that is a safety factor, I don't intend on reprograming the car for the new tire size. Had no issues with the new TPMS sensors.

What size tires do you have on the front? And are you still running the stock wheels?

 

[GT_Dave]

What size tires do you have on the front? And are you still running the stock wheels?

I am running 305/30's on the front on 6GR wheels with the R width and offset.

 

[galaxy]

I also went with the 305/325 30's.

 

[Tank]

Exactly what I’m doing. Thanks for the tips guys. I read somewhere about someone changing diameter, had DTC fault or something and some engine bogging in 4th gear or something until he did some kinda reset, but now I can’t find that post/thread. Oh well. Can’t imagine .7” would effect anything (other than the speed).

The DTCs are triggered when the tire change is programmed via Forscan. See the last page or two in the Forscan sticky for this and the reported 4th gear boggling.. There’s also links there to research further..

 

[tdzee]

.... Can’t imagine .7” would effect anything (other than the speed).

I do not know how the nannies on the 350 work, but they usually work via the anti lock braking system/sensors to detect tire slippage (and other sensors). On my C5 Z06, this was an issue as I changed to track tires. There is a working deadband in these systems to accommodate a small change. On the C5 forum, it was suggested to be only 5% wide. Therefore, a tire diameter change, esp at front or rear of car and not both, will have the wheels turning at slightly different rates than the computer expects. This may cause the nannies to engage sooner than usual (or worst case all the time), because it interprets the wheel speed differences as tire slip.

Sorry for vague answer, but the answer is yes, it does probably matter. I try to stay withing about 0.1 or 0.2 inch of tire OD. 0.7 seems like a lot. How much bigger is the tire circumference? That will tell you the change in tire RPM vs vehicle speed which these sensors will see. Hope that helps.

 

[nastang87xx]

I do not know how the nannies on the 350 work, but they usually work via the anti lock braking system/sensors to detect tire slippage (and other sensors). On my C5 Z06, this was an issue as I changed to track tires. There is a working deadband in these systems to accommodate a small change. On the C5 forum, it was suggested to be only 5% wide. Therefore, a tire diameter change, esp at front or rear of car and not both, will have the wheels turning at slightly different rates than the computer expects. This may cause the nannies to engage sooner than usual (or worst case all the time), because it interprets the wheel speed differences as tire slip.

Sorry for vague answer, but the answer is yes, it does probably matter. I try to stay withing about 0.1 or 0.2 inch of tire OD. 0.7 seems like a lot. How much bigger is the tire circumference? That will tell you the change in tire RPM vs vehicle speed which these sensors will see. Hope that helps.

It's not that bad. I tracked on 305/30's on stock wheels and I never had odd behavior with my traction control. That being said, about .7" is about as big of a difference as I would go just in principle too though. Obviously the closer to stock the better but that won't do anything that us normal mortals will notice.

 

[galaxy]

So what would a mere mortal do if I did want to address it? I don’t have a forscan or anything. I also drove about 100 miles on the new tires yesterday and noticed absolutely zero I’ll effects. In fact, the positive changes were quite noticeable. I was actually caught off guard at the improved feel and performance from the weight savings and effective gear ratio change (from the shorter tire). Funny too, these things all but eliminated tramlining from the stock Michelin’s. I made no alignment changes either and drove the same roads.

 

[tdzee]

It really shows up on track when operating close to the handling limits and less so on the street. If you have ever tracked a car with these systems but thought the nannies were a little slow in responding, this may exacerbate that effect. Or it may induce the nanny effect prior to it actually being needed (since the computer is seeing a small difference in wheel rotational speeds and interprets this to be slippage and activates). If you only street it, you are probably fine. Good luck.

 

[galaxy]

Going from 305/35 to 325/30 is a smaller diameter change than going to a square 305/30 or even the R 315/30; manufacturer variations considered, obviously. I’ll watch it for a while. Drove all day again today. What a difference, wow!

If I did want to correct, do I just go to the dealer and have them reprogram something? What specifically would I need to ask for; just tire diameter change?

 

[tdzee]

Not sure a dealer will agree to reprogram anything, but you may have a good dealer and relationship. Should not hurt to ask and tell them you want to compensate for the different tire sizes.

If you get the Forscan OBD dongle and the free code, you can do it yourself. Plus a bunch of other cool stuff.

Good luck.

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