Will this fuck with my spedometer?
 

I have the 2004 civic, and I have been looking at differant modifications, I think the orig tire size is 195/60 R 15s

The wheels I want are these rims

So with the tires it comes with I have two questions... Is it going to make the car look lifted? And is it going to mess with the speed on my gauges?

Changing tire sizes does change the speedometer accuracy.

But doesnt it change what it reads? By a regular percentage?

There are formulas to calculate the changes but changing the tire size will change speed at a given rpm and rpm at a given speed. Without recalibrating the speedometer the speedo will be off. Here's an off road site that explains the concepts and provides some formulas. Obviously 4x4s are more affected than cars because the size changes are greater but the formulas and concepts will still apply.

http://www.offroaders.com/info/tech-...ng/mph-rpm.htm

On a car site, I'd just simply answer. Here, I feel like I should first philosophize; am I a paranoid maniac? So first you have to hear my speech ;) : some famous woman once said this way back when women were real smart: "here, take this apple and taste" ...errrr that's the wrong story ..."give a man a fish and feed him once, give a man a fishing pole and feed him for life".
This is why men go fishing all the time instead of doing what they are suppossed to be doing.

Here's how to figure it out:
The total tire outside diameter is directly proportional to the circumference, which is directly proportional to speedometer reading. So if you figure the percentage +/- difference in total diameter between the two sizes, that would be the same % difference in the indicated speed. The "60" or "40" numbers are aspect ratio, which is the ratio of tire sidewall height to tire width.

So if you have a 195/60 X 15" setup, the sidewall is 195 X 0.6 = 117mm; you have two sidewall heights plus the wheel diameter adding up to the total diameter (keep all units the same, inches or mm); 117mm = 4.606"; 4.606 + 4.606 + 15 = 24.21" total

If you have a 205/40 X 17" setup, the total is 23.46".

23.46/24.21 = .969

So your real speed with the 17" setup will be about 3% lower than the 15" setup. Assuming your speedometer is correct with the 15", if your speedometer says 150mph on the 17's, you will actually be going 145.5; at indicated 50mph, you will be going 48.5mph.

The above does not factor in how the wheel width effects the sidewall height of a tire, since the same tire width on a wider wheel usually results in a slightly lower sidewall height. But this is not usually very significant so I ignored it.

Lower sidewall height gives you less rubber to absorb road shocks, so it will probably feel a little harsher but even the 17" you show have quite enough sidewall in my opinion for normal comfort. IMO, most cars look way better with bigger wheels, up to a reasonable point. I'm not a fan of 24" setups on civics.

Nor am I a fan of 24". Thanks for the explanation. Pretty much what that means is the speed change is negligable for my purposes, because when winter hits its back the 15s anyway.

I could reasonably get 18s under there, but well... I just dont want to pay for 18s. Therefore, the 17s it is.

Those are some damn nice rims....:thumbsup:

BadNick pretty much nailed it, not much else to say. Your speed difference will be a constant pecentage.

Here is a really cool site i googled real quick : http://www.csgnetwork.com/tireinfo4calc.html

real quick rundown of the info

Old tire circumf. : 76.07 in
New tire circum. : 73.69 in
Difference in cirfcum. with new tires : -3.12% --> You are actually moving to a smaller tire

That means that your speedo will actually be off by 3% - reading faster than your actually going. So with the new tires, you;ll hafta read 67mph on the speedo to actually be going 65mph. So dont mention your change in tire size if your trying to fight a ticket :)

superlift makes a module that can correct for tire size changes, or you can hook up a programmer to your computer and have someone dial it in.

Here's a link that will tell you exactly how far you'll be off. With that particular tire size, you'll read slow, meaning an indicated 60mph would be an actual 58.1mph. If you used a 205/40R18, you'd be almost spot on.

my car has slightly larger tires then stock...
my spedometer is 10% slow...
so if it says 80 im really goin 88 ...its got the speeding built in :p

ive gotten used to it... i figured mine out by seeing how much farther the circumference was... and then i also had a friend pace me and the calculations were close enough

A lot of new cars (well F-Bodies anyways :) can be compensated in the computer for different tire sizes. 3% ain't much for speed difference but your waranty will run out 3% faster too. IE: if you have a 100,000 mile warranty you'll lose 3,000 miles of it.

Yup, and it'll mess with fuel mileage readings...everything. Some cars can have the ECM reprogrammed by the dealer to compensate for a new tire size. Not all, but some. I know the newer Saabs can, and some of the newer GM's can too. For a point of reference on that, my '97 Olds Cutlass Supreme can't be reprogrammed by the dealer for tire size.

That is a point I never thought of... Any idea how much it would cost me to have it reprogramed?

And while Is be right on with 18s... Damn man, I dont really wanna pay for that, :)