If I want to replace my 17" wheels with 18", how do

You only have to do that if the outside diameter of the tire changes. You can increase wheel size but install a thinner tire and have no change in the outer diameter.

whats your section width and aspect ratio? larger wheels shouldnt factor in with the proper rubber

>>16477669
Nigguh you gotta be trolling. Your driving wheels got a drum with equally spaced holes and a sensor that reads rotation to figure out speed. Changing rim size won't affect speedometer reading.

>>16477693
How do i get a thinner tire?
Can I judge by the tire code if it is thinner like you can figure out the size of the wheel?

>>16477713
>How do i get a thinner tire?

There are places that sell tires.
>Can I judge by the tire code if it is thinner like you can figure out the size of the wheel?

There are online calculators that will show you how to read a tire and calculate the outer diameter.

>>16477669
if you were riding on the wheels without rubber,at 60mph an 18" wheel will read 3.5mph slower than 17"

>>16477712
you're failing to see that a larger diameter tire travels more distance per rotation than a smaller diameter tire.

>>16477713
Go to tirerack.com
shop for tire and wheel packages
plug in your year/make/model and they'll know the factory wheel/tire stats
then choose an 18" wheel, proceed to tire choosing, and see what size they choose for you. It should have the same outer diameter as your stock setup and not affect speedometer.

ITT: No-one actually read the OP. He asked how to re-tune the speedo after changing tire size. He did not ask how to avoid needing to do so, he asked how to do it given the predicate that it becomes necessary.

>>16477873
FYI: Retuning the speedo in most cars is not possible, and or it would be cheaper to purchase properly sized wheels/tires than getting a dealer to lube your ass up just to "tune" the speedo.

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>>16477873
ITT: No-one actually read the thread
>>16477787

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>>16477873
Wait. There's someone who can actually read here?

>>16477886
It's actually not really hard at all. There are little devices that read the pulses from the speed sensor, multiply them by the conversion factor for your gearing change, then send corrected pulses to your speedometer.

>>16477817
I know it travels faster. However that doesn't change speed readings because as I said there is a drum behind brakes that rotates with tire and abs speed sensor that reads the rotation speed. That speed sensor triggers traction control and abs breaks if it senses one driving wheel spinning differently from the other. Changing rim size does nothing to speed readings you dumbass. I don't know why I even come here.

>>16477903
Arguing Semantics, that's not really tuning the speedo, and furthermore thats not what OP asked, nor does it seem like OP would be capable of doing that himself.

>>16477908
The computer is assuming stock tire size in that equation. You just changed.a variable in a math equation and expect it not to give a different answer

>>16477908
are you retarded? It doesn't matter that the speed reading is taken from the wheel. The tire size changes how far the wheel travels per rotation. Watch the first couple of minutes in this video if you don't understand. https://www.youtube.com/watch?v=BVS3cdSGkCM

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>>16477669
>>16477787
>>16477873
>>16477894

>>16477928
http://www.healtech-electronics.com/products/sh/
It's exactly what OP asked for. This device can changes the input to the speedometer (and odometer, since it's input is the same) to get an equivalent output as if the tire/wheel/gearing had never been changed.
>couldn't do it himself
It's just a couple plugs and entering the conversion factor.

>>16477873
He actually asked how to re-tune the speedo after changing *wheel* size. I assumed ( because of how he asked ) that he bought a package deal for his car that included tires which would retain the outer diameter of the original tires.

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>>16477669
Smart enough to know speedometer won't be correct if changing the diameter of wheels.

Stupid enough to not know about different tire sizes.

How?

>>16477982
>implying OP knows how to pull his dash apart and rewire shit.

That was my point.

>>16478173
OP here and I agree. Car is old so I'm ok with messing with anything short of the dash.

Shit will come apart and not come back together

>>16478073
Why intergra side ways?

>>16477903
Is that a supported operation? I would assume there are "ethical" implications since you could have your car read 30% fewer miles if you just "tuned" it to read 30% slower.