New Tires/Alignment/Handling Questions

[dexter]

Well it's that time again. I've got more wheel and tire questions.

A few weeks ago I ended up getting a new set of tires. I opted out of getting another set of the OEM 215/45/18 Potenza's and went with a 225/45/18 Nitto Invo.

The Invo's consistently got pretty good reviews in both wet and dry conditions and were a reasonable price. The day after the tires were installed I got my 72k service and a 4 wheel alignment.

In the weeks following I've noticed that while the tires seem to handle really well in individual corners and braking but in a series of lefts and rights the car seems to have a lot more body roll than it used to and the car has more of a 'floaty' feeling when lane changing, etc. The steering wheel just seems like it needs more input than before and the car seems less 'snappy'. My gas mileage has also dropped significantly from 9.6-9.7 to 10.9-11.1L per 100k.

I called about getting the alignment redone and they said to wait a bit more as the tires may need more breaking in. They have about 1,500kms on them so far.

So with said said:
Does it seems like my alignment redone?
Are the 225's a bit wide for the 7" wide rims?
Are the new side walls just softer and thus make handling differences?

Thanks in advance!

[speed77]

Does it seems like my alignment redone?
Are the 225's a bit wide for the 7" wide rims?
Are the new side walls just softer and thus make handling differences?

If the alignment was done right the first time, it shouldn't need to be redone.

225 is probably as wide as you'd want to go on a 7" wheel

Sidewalls are taller so that will definitely cost you some steering feel. Also keep in mind that you went from worn out tires to brand new tires. The extra tread will add quite a bit of squirm as well.

[zzz3]

your sidewall is too big. the correct profile is 40 series. mounting on a 7" rim, and the subsequent 'bulge' vs. square sidewall, isn't much of a factor. neither is alignment (as you already had one).

[dexter]

The sidewalls in both tires are a 45. So the sidewall height shouldn't have changed hardly at all. 96.75mm (215x45%) vs 101.25mm (225x45%). Is less than a quarter inch (4.5mm) difference in sidewall height going to make that big a difference?

Any thought on the huge change in gas mileage?

[zzz3]

that small change in sidewall is notable, believe it or not, and was probably compounded by going to a wider tread on a 7" rim. all-season vs. summer tire could be another factor (not sure which is what; original vs. replacement).

[Swerny]

New tires are too big as mentioned. Car now stands taller, higher centre of gravity, Full tresd is heavier. More rotational weight hurts mileage

[dexter]

Too big as in too big for the car or too big for the rim? Would a 225/40 be smarter?

I get that the car will stand a bit taller and that the full tread will be heavier but enough to have me lose 75kms on the average tank? That seems a bit excessive.

[Swerny]

too big as in the overall diameter is bigger which makes your car taller.

215-45-18 or 225-40-18 are better fits (closer to stock size)

http://www.miata.net/garage/tirecalc.html

Your size is 3.6% too big according to the calculator

By comparison, 225-40-18 is only 0.1% too big....so it's almost perfect

[dexter]

Ah, that explanation makes more sense. Thanks!

So much for the tire shop saying that it wouldn't be a problem going from a 215/45 to a 225/45.

[Swerny]

Ah, that explanation makes more sense. Thanks!

So much for the tire shop saying that it wouldn't be a problem going from a 215/45 to a 225/45.

usually within 3% difference either way is OK.

Your tires are slightly outside that....may explain some of your symptoms though.

[dexter]

Yeah, looks like the 225/40 would have benefitted me a bit more in that regard. Oh well, at least my alignment is good! Haha