Lower Profile Tyres

[Skyliner34]

Hi,

My car has 60 profile tyres back and front. What difference will changing the front tyres to 55 profile do?

I know the ride will get harder, but is there anything else? Someone said the speedo will be innacurate, if so how much?, will is speed up, or slow down?

Thanks in advance

Sky

[chaos386]

If all you're changing is the sidewall ratio from 60 to 55, and you're keeping the same size wheel and same tire width, then the tire+wheel will have a smaller diameter. This means that at any given speed, the wheels will be spinning faster than before, making your speedo overstate the speed and make your odometer run faster.

There are two ways to fix this. You can get larger wheels and lower profile tires to match so that the overall diameter is unchanged (more wheel, less sidewall), or you can adjust your speedometer for your new tires. On our family's '92 Mercury Grand Marquis, there is a gear in the transmission which drives the speedometer and odometer, and changing the size of this gear will change what speed is shown. Perhaps your car has a similar mechanism?

[Skyliner34]

What will happen if I just stuck 55 on the front?

It's a front wheel drive car, same problem?


Thanks for your help so far

[chaos386]

I'm pretty sure the speed is measured from the drive wheels, so yeah, same problem.

[fbc]

Originally Posted by chaos386

There are two ways to fix this. You can get larger wheels and lower profile tires to match so that the overall diameter is unchanged (more wheel, less sidewall), or you can adjust your speedometer for your new tires.

You could also get a wider tyre so the extra tread width offsets the lower aspect ratio, meaning the sidewall height is the same as the 60 series tyre - but there's obviously a limitation as to how much wide a tyre you can fit on the wheel.

[un-dee]

Uhm, and it also should be illegal where you live. At least it's here. Changing the wheel diameter really isn't a good idea.

[fbc]

^ Eh? Changing wheel diameter isn't a problem provided the wheel offset & overall track width aren't changed from the manufacturer's spec for that car - i.e. like putting M5's wheels on a 520i for instance.

[un-dee]

Wheel as in rim + tyre

M5 wheels have the same diameter as the ones on a 520i, because the rim is larger but the tires have a lower profile.

[fbc]

^ I see, thought you only meant wheels, so yes - overall diameter is very important, but the only mention of changing wheel diameter has been:

Originally Posted by chaos386

You can get larger wheels and lower profile tires to match so that the overall diameter is unchanged (more wheel, less sidewall)

[thedguy]

Originally Posted by chaos386

I'm pretty sure the speed is measured from the drive wheels, so yeah, same problem.

VW bugs are measured at the wheels, most cars are measured at the trans.

If you don't change anything but the tire profile your speedo will be off (it'll read fast). If you change to a larger rim (any combo of diameter or width), and account for it by changing profile it won't change.

Theoretically a smaller sidewall will handle better due to better sidewall stability. If thats your ultimate goal, just get tires that have a stiffer sidewall, they will probably have a better compound to grip better anyhow. And yes your ride will suffer.

[GraemeH]

Like everything it's a balance. Sidewall flex gives more grip but less feel, and the opposite. It's just another thing to balance for the type of driving to be done. A 1mm profile is going to handle like dogshit. Like replacing your suspension with concrete.