smaller tires, faster rotation and braking gas mileage

[solaracanada]

stock was 17x7.5, new is 19x8.5. Stock tire size was 215/55/17, new tire size is 235/35/19.

[panic]

For comparison, the "official" (not exactly accurate due to rim widths, tire pressure, etc. but the usual method) diameters:
215/55-17 = 26.31"
235/35-19 = 25.48"
Change in speed at same RPM: 96.85% (old 60 = new 58.1)
Change in RPM at same speed: 103.26% (old 2,000 = new 2,065)

This should have no effect on your mileage or braking distance that you can notice. The extra tread width may reduce cruising mileage due to rolling resistance, but not much. Mileage under light acceleration should be about the same, because the shorter tire gives you more leverage against the car's weight so for the same G force less torque is needed.

As to the inertia effect?
This one is tough. The most important weight is at the OD, and it's more with the 19 (both the actual wide surface of the rim and the tire), but at a smaller radius than the 17. I can't tell which way it goes.

The leverage of the caliper against the rotor is of course unchanged.

In short, you shouldn't have the symptoms you report - there's something else going on.

[solaracanada]

For comparison, the "official" (not exactly accurate due to rim widths, tire pressure, etc. but the usual method) diameters:
215/55-17 = 26.31"
235/35-19 = 25.48"
Change in speed at same RPM: 96.85% (old 60 = new 58.1)
Change in RPM at same speed: 103.26% (old 2,000 = new 2,065)

This should have no effect on your mileage or braking distance that you can notice. The extra tread width may reduce cruising mileage due to rolling resistance, but not much. Mileage under light acceleration should be about the same, because the shorter tire gives you more leverage against the car's weight so for the same G force less torque is needed.

As to the inertia effect?
This one is tough. The most important weight is at the OD, and it's more with the 19 (both the actual wide surface of the rim and the tire), but at a smaller radius than the 17. I can't tell which way it goes.

The leverage of the caliper against the rotor is of course unchanged.

In short, you shouldn't have the symptoms you report - there's something else going on.

thx for the analysis, that sounds about right. I dont know why, but everything is fine now. It is still a little worse gas mileage but it is nowhere near the previously reported horrible mileage, it is really close now. As for BRAKING, it actually is way better now. I dont know what the problem was but I am happy that it has worked out now.

[Jon11582]

Maybe because the tires were new. New tires have a coating on them that helps the rubber separate from the molding at the factory. Your running on that coating for the first few hundred miles. After that, the coating is worn off and then they have the traction that are expected from a tire.

[panic]

I wonder if the ABS and traction control "accept" the new diameter automatically, or do you have to flash the ECM?