Machine13

Hi everyone,

So, I’ve got the F-Sport rims and like them just fine. Not looking to spend the money to replace the rims with bigger ones.

What I am considering though is putting taller tires, when the time comes, for the purpose of improved ride quality and looks. I’m thinking that taller tires will fill the wheel wells a bit more, heighten the stance a tad bit, and look a bit beefier, and that the ride may become slightly smoother over the rough roads in Miami.

With respect to overall effect on gas mileage, I figure it may be a bit of a wash. I hear that since taller tires weigh slightly more that can cause the engine to have to work a bit harder to get their rotation going. In city driving, this may result in slightly decreased MPG’s. But once the heavier tire’s inertia is up, plus considering the larger diameter of the tire, gas mileage may improve slightly especially at highway speeds.

So, does anyone have any experience substituting for taller tires or have any thoughts or recommendations on the matter? If you did use taller tires on stock rims, what size did you go with?

Stock tires are: Front 225/40R18 – Rear 255/35R18

I’m considering Front: 225/45R18 – Rear 255/40R18

Also, what about it's affect on using the spare tire? Can people with taller wheels still use the spare on any of the four corners, and without damaging the differential in the back?

Thank you,

LeapFrog

http://tire-size-conversion.com/tire-size-calculator/

cp2

iTrader: (1)

you bring up some good points, but some are off.


as far as going to a bigger tire, your gas mileage will be affected. just because someone goes to a bigger wheel and lower series tires, that doesn't mean it's comparable to the stock set up respectively. I have 19's and they are lighter than the stock 18" wheels. with the tire size that I have, the over all difference in size does not affect my mileage in a negative way. the speed reading is off by less than 1 mph. going to bigger tires on stock rims will affect speedometer reading as well. though going to bigger tires will fill tire gap, I don't think it'll look any better...but that's my opinion. the spare is only a donut and isn't meant for long term driving. should only be used long enough to get a shop; shouldn't go over 50 mph or so...maybe even less.

LexRuger

Instead of going up in the aspect ratio, go up in the width. That'll make the overall height slightly taller and it won't make much difference on the speedo.

For example, a 225/40/19 is 26" tall. A 235/40/19 is 26.4" tall

If you went to a 225/45/19 it would be 27" tall. That might be a bit much IMO.

Noeh

iTrader: (3)

Best way to improve ride quality is to replace our ultra high performance tires with grand touring tires. Probably better mpg too.

Spagolli94

What's wrong with the ride quality? Compared to my old 335i, my IS 350 is ultra smooth and quiet. Even when in S+ mode, the ride is firm and planted but never harsh or crashy. In contrast, my 335i would bump and bang over every little crack in the road.

I live in Philly, home to some of the worst roads in the country. They are crumbling and when the fix them, the patches are never smooth either. Yet the IS 350 doesn't a great job isolating me from all the harsh stuff without feeling totally disconnected from the road.

What tire pressure are you running?

Noeh

iTrader: (3)

The OP wants to improve ride quality. I know the suspension in the 3IS is well balanced, but tires still make a noticeable difference.

Sphinx007

you'll rub when turning....I tried that for my winter tires....no good...had to get the stock tire sizes

Machine13

Thanks for this link, the Tire Size Comparison calculator is neat.

Now I know going from the 35R18's to 40R18's is about a 1" increase in tire diameter and about 3.8% decrease in tire revolutions per mile. Neat info.

Spagolli94

Perhaps he might consider just lowering his tire pressure a bit. The door jam says 36 PSI on all corners (measured cold). I tend to run 36-38, depending on ambient temps. The ride is definitely smoother at lower PSI in the 32-34 range.

Might be worth a shot before messing with the tire size.

Machine13

Thanks for the suggestion, I didn't know that. I looked up several models of grand touring tires and also found this nice short article that summarizes the benefits and helped me to understand better how tread pattern designs can affect acoustics.

http://blog.tirerack.com/blog/colint...quietest-tires

Machine13

Interesting, do you really think a 1/2" increase in tire radius might cause rubbing when using stock rims and suspension? Hmmm, I'd be astounded if it caused rubbing.

Machine13

Right, my IS 350 is the nicest, smoothest and quietest car I've ever owned. No qualms at all. But the roads around my work are so rough, even a Lexus suspension gets banged around a lot.

My goal is to improve the ride-smoothness from awesome to ludicrous, if it's possible.

Machine13

Thanks for the suggestion, it makes sense regarding softening the ride, but I guess may cause slightly more noise and worse MPG's. I keep my tires at 36 PSI (cold) and would rather run the tires within their recommended spec I'd think. When the time comes to replace my tires I want to get ones that achieve as soft a ride as I can find.

Machine13

Thanks for all the input fellas... Learned something today... I'm going to do a bit of tire shopping now...