Does tire circumference impact our car's tune?

[dale_gribble]

I went up a tire size today and that got me thinking a bit... I am assuming the car is taking into the account vehicle speed sensor and rpms as a factor into which load values the car is using at what rpms and at what speed. It would seem to me that changing the tire size would skew which load values from the load tables are used by the tune due to the RPM and vehicle speed change. For example, before I would be going 75mph at 3k rpms and now I am going 75mph at 2850rpms.

Is the mazdaspeed engine decoupled from this factor (tire rolling diameter of the tire) for the tune, or does it play a part? and what part does it play? Even though most of us change tires typically within 1" or so from stock, I am curious if there is any connection. Has this been discussed before? I couldn't find anything. Perhaps the +/- 5% tire size change is in the weeds and doesn't even make a difference. Maybe the tune doesn't even consider speed as a factor of the tune and only uses rpms?

This isn't discussing tire/wheel weight; purely the impact that a larger or smaller tire circumference may have on the car's tune.

[Vansquish]

Originally Posted by dale_gribble I went up a tire size today and that got me thinking a bit... I am assuming the car is taking into the account vehicle speed sensor and rpms as a factor into which load values the car is using at what rpms and at what speed. It would seem to me that changing the tire size would skew which load values from the load tables are used by the tune due to the RPM and vehicle speed change. For example, before I would be going 75mph at 3k rpms and now I am going 75mph at 2850rpms. Is the mazdaspeed engine decoupled from this factor (tire rolling diameter of the tire) for the tune, or does it play a part? and what part does it play? Even though most of us change tires typically within 1" or so from stock, I am curious if there is any connection. Has this been discussed before? I couldn't find anything. Perhaps the +/- 5% tire size change is in the weeds and doesn't even make a difference. Maybe the tune doesn't even consider speed as a factor of the tune and only uses rpms? This isn't discussing tire/wheel weight; purely the impact that a larger or smaller tire circumference may have on the car's tune.

I would think that the larger diameter wheel/tire would have to have an appreciable impact on the tune.

1) As you noted, the larger diameter has an impact on the wheel-speed. While wheel-speed is not generally a large factor in the tune...

2) RPM, obviously is. Moreover, much of the basis on which our ECU seems to perform its calculations is based on Load in relationship to RPM.

3) Load ABSOLUTELY CHANGES when you increase the diameter of the wheel. Basic mechanics teaches us that the larger the diameter of the wheel, (generally) the larger the inertia of the wheel.

This means that in order for our engines to turn larger-diameter wheels, they have to work significantly harder than when the wheels are stock sized or smaller. As a result, you may see significantly higher load values when you log after fitting taller/wider wheels and tires.

As a result, it's certainly possible that you could hit whatever load-cap remains in our ECU earlier with larger wheels than with smaller ones.

4) If memory serves, I think there are only a couple of tables that have anything to do with ECU calibration for wheel-speed. The most pertinent (and possibly overlooked) would be the gear ratio tables. If you calculate the change in wheel-diameter properly and can account for it in those tables, you might be able to re-calibrate your speedometer to account for the modification. This might (though I seriously doubt it will make much of a difference) change how the car's load calculations are run.

[easy]

changing tire sizes effects gearing ....not sure about a tune, possible

[dale_gribble]

All I know so far is my mileage improved about 10% since I put back on stock diameter tires. All of my baseline logs are at 70-90F before the tire change, and it has been cooler here so I haven't been able to run a comparative log run.

[Vansquish]

Originally Posted by dale_gribble All I know so far is my mileage improved about 10% since I put back on stock diameter tires. All of my baseline logs are at 70-90F before the tire change, and it has been cooler here so I haven't been able to run a comparative log run.

The interesting test would be to run some larger diameter wheels/tires, but to get a set that has the same amount of inertia. Basically, what I'm suggesting is a lighter, larger-diameter wheel with a narrower tire instead of a shorter, fatter, wheel and tire combo.

I'd guess that with such a combination, your mileage might improve, BUT, that would also hinge on how hard the engine is working (calc load) at lower RPM to maintain the same speed.

I know that when I switched from my stockers 18x7.5 with 225-40-18's to my RX8s (18x8 with 225-45-18), my fuel economy also decreased by ~10% (33mpg highway down to 30mpg).

[chuckms6]

Afaik load is a function of air pressure and volume for a given rpm as measured by the engine's sensors. The only impact wheels and tires could have is rolling resistance and inertia.

Tapatalking