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I've a Hyundai Santro Xing eRLX car and currently it has 155/70 R13 tyres on four wheels and all tyres almost crossed 40000kms. I've recently replaced both front tyres with brand new tyres. I've two spare tyres which are of size 145/70 R13 (took from Daewoo Matiz car), which are pretty good condition (10000kms mileage). I wanted to use these two matiz tyres on Santro. I'm completely clueless about what are implications using two different tyre sizes for front and rear wheels. Also not sure what is the issue if I downgrade these tyres to 145/70 R13.

Can somebody explain me pros (if any) and cons and provide some details why this is a bad/good decision?

Wheel info of this car: Wheel Type - Pressed Steel Wheel Size - 13 Inch Tyres - 155/70 R 13

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2 Answers 2

First you need to under stand what the numbers mean.

  • 155 - tread width in millimeters
  • 70 - tire sidewall height expressed as a percentage of the tire width
  • 13 - rim diameter in inches

With this in mind, as your tire width becomes smaller, the tire sidewall height becomes smaller as well, so the overall tire height becomes shorter. This will make the rotations per mile (RPM - yes, this is confusing when you think of engine speed) become less as the tire gets smaller. Less RPM, higher tire/wheel speed at the same rate of vehicle travel.

So, how does this affect the vehicle? Many of today's vehicles have anti-lock braking, traction control, and stability systems which require that each wheel be measured for speed and then is compared to the other wheels to figure out if there is a difference. When you put two dissimilar tires front and rear, this may have the ill effect of messing with these systems, thus throwing a check engine light (or antilock break malfunction light) on the dashboard, which in turn disables these systems.

I don't know if the minor difference you'd see in these two tire sizes is enough to freak out the vehicle computer, but I'd suggest there is a good chance of it. The larger the difference, the greater chance this could happen.

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My car doesn't have ABS, so apart from that you say there is no other issue related to fuel consumption or alignment or vehicle stability at higher speeds or turnings? –  JPReddy May 15 at 14:31

A rule of thumb used by some tire retailers is to not deviate from the official sidewall height by more than 3%.

So the sidewall height of your original tire is:

155 * 0.7 = 108.5

If you go for a 145/70:

145 * 0.7 = 101.5

This is a reduction of roughly 7%. The rule of thumb recommends against 145/70 in this case.

However, the same rule of thumb suggests that 145/75 is much closer to the original sidewall height and should not be a problem.

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