Upsizing to 18s, Speedo changes
 

I know there is a ton of info on this but I'm not sure why this is so hard for me understand....

I'm looking to get a pair of summer wheels/tires and I would prefer an 18x8 wheel. I am trying to find tires that will make that as close as possible to the diameter of the stock wheels on the stock tires for the speedometer's sake. It looks like 225/40/18 is about the closest I can get. I would love to step up to a 235/40/18 purely for the look of it and for better grip with FI in the future but it seems to make the speedo variance too high as tire rack recommends not going over 3% difference with a 1% difference being ideal.

http://www.1010tires.com/Tools/Tire-...5R17/225-40R18

http://www.1010tires.com/Tools/Tire-...5R17/235-40R18

On the calculator it shows that my speed variance is 1.87% too slow for the 225 and 3.08% too slow for 235.

Stupid questions:

This means that my actual speed is 1.87% and 3.08% faster than what the speedo shows right?

The confusing part is both of the tire setups I am looking at have an overall larger diameter than stock which I thought would cause me to actually be slightly slower?

Would you recommend 225 or 235 in this case? Rubbing/scraping on stock suspension/height?

EDIT: Pilot super sports are the same price in 225 and 235 so that is not a factor.

why is the width effecting the speedo? 225 and 235 should have the same diameter all other things being equal (ie 40R/18). honestly go 235 if thats the look you want.

Price might be, but weight will not be the same. Narrower tires also have to get less water through threads to not aquaplane. Maybe worth to postpone going wide for when after FI installment?
P.S.
Have you considered profile of 35? Imho that should get you into lower diameter/circumcision size, then stock, no? Check up with some online tire calcs entering stock size and playing with changing desired sizes for tires you want compare them with.
P.P.S.
There is also option of higher ratio FD installment to get back transmission ratio even with larger diameter tires. Though not sure if it's worth doing if you already decided on FI in future. Car also will be lifted a bit with bigger diameter tires (seeing most prefering lowering, not sure if it's wished result).

go a way1: sidewall height is given relative to width (in %), not absolute number like width (in mm). 40% of wider tire vs 40% of narrower one = higher sidewall profile/bigger overall tire diameter/circumsize, hence differences in speedo readings / transmission ratio.

The width affects the diameter. In 225/40/18 the 40 is not the height of the sidewall it is the ratio of the sidewall to the width of the tire.

225 x .40 (think of it as the height of the sidewall is 40% of the width) = 90

235 x .40 = 94

The 235 is ever so slightly taller.

Oh snap I had a brain fart lol I was thinking it was 40% of the rim size :bonk:

Correct.

The speedo is actually calibrated to read a little fast in most cases so a slightly taller tire will end up being almost exactly right when you compare it to a GPS speed.

I'm running 225/40/18 right now, well in the summer anyway, and it's a good size looks wise and works really well at the track. If you are going FI and want more grip just look for a grippier tire than the PSS.

I'll need to replace my tires early in the season and will be going either 235/40 or 245/35.

I would like to get the wider tires and new wheels now partially so I can leave my winter tires on the stock rims and partially to get ready for FI. I already have the turbo kit but I am just not installing it until I have all the supporting mods in place (ultimate test of patience :cry: )

Most of the configurators I messed with would not let me do a 235/35 setup for comparison.

Don't plan on changing FD as like you said it is not really worth changing for FI. I'm going to do very minimal lowering springs in the future as well so I'm not too worried about a slight lift from the increased tire diameter.

Thank you for the input and I'll see if I can find one that will let me do the 235/35 or I'll just do it by hand!

Issues of bigger tire size is not so much about speedo (it's just often brought up), but rather about 1) rotational weight increase (even if overall tire+weight stays somewhat same, due weight being further from center it needs more energy to spin up/slow down, and also even while one can get quality 18" wheels that are lighter then stock 17", good 17 of same model will weight less anyway) & 2) lowered gearing ratio (less engine rpm-s per same distance traveled).
Given relatively low power if staying NA it's best if you keep that rotational unsprung weight down to not get slight decreases in acceleration/braking (wheels are like some 4 flywheels. You spend extra engine energy to speed up them, and extra brake to slow their rotation down). & suspension quality (the less tire+wheel weights, the less mass/inertia it has & the easier for same suspension to get it down after hop, resulting in more grip, less unsettled ride). These reasons are more important then speed increase at same speedo reading (which as other noted, by car manufacturers agreement for new cars always is read lower even at completely stock).

I didn't read all responses but I have 225/45/18s stretched a little on 9.5s and my speedo is only off like 2mph at 55mph. I've seen down to 30 series tires but it may be hard to find somewhere to put on such a short tire. I'd just get the widest that will fit under your fender and drive 2mph slower(according to speedometer)

^^ am I the only one who noticed that? LMAO :lol:

Good catch. I missed it too