I run an old car which i've had a few years. It came with later alloy wheels. I'm considering returning the car to original wheels and tyres these are thinner (165 against 185) I expect to have less grip, anything else?
Alfaboy.
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You don't mention whether there is any change in wheel diameter involved. You will need a higher profile to retain the same rolling radius. You will probably notice an improved ride!
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Thanks Mike, There's no change in diameter or profile only width I couldn't think of any downside to this apart from grip but I don't drive hard enough for this to become an issue, I think.
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Suprisingly grip may not be an issue, it may even grip better in the wet. I slapped a set of wider tyres on a Fiat once and it was worse in the wet.
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I would be surprised if the same profile was used for 165 and 185 tyres - one of them would cause a dramatic, and illegal, error in the speedo reading as the rolling radius would be very different. You really need to check the spec in the handbook & confirm the profile for the differing tyre sizes. Assuming the 165s are higher profile youll get a better ride and, as RF says, probably better grip as the footprint on the road will be smaller - caused by greater pressure per square whatever (centimetres or inches depending on your age).
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The tyres on the car are 185 by 70 by 13 the original spec is 165 by 70 by 13 so i think everything should be ok, one other thing, it's not power steering so should be easier to park!
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You might want to consider talking with your insurance company as you are modifying the car by putting smaller width tyres on and you could end up compromising the safety and handling of the vehicle.
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No alfaboy.
You're missing the point raised by Mike H.
A 185/70/13 tyre has tyre width of 185mm and tyre height of 70% of 185mm, which is 130mm.
A 165/70/13 tyre has tyre width of 165mm and tyre height of 70% of 165mm, which is 115mm.
If you change the size of your wheel / tyre combination by 15mm diameter you will change the gearing and speedo calibration.
This is a relatively significant change.
Hope this helps,
Sean
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Sorry,
You are changing the radius by 15mm and the diameter by 30mm.
As stated, this is significant.
Sean
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Thanks Sean and everyone, Am I glad I asked! I thought there must be something i wasn't picking up. I thought 70 was a measurement not a percentage. I must be running the wrong tyres now! I won't say how long I have been.
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Unfortunately you would have been going faster than you thought!! Though you may not have been going faster than the speedo reading, since these are usually set to +5% or so.
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But if the 165's are the original fit, the speedo will have already been calibrated for them, and will be slightly out with the aftermarket 185's, but negligible!
Slightly wider tyres means slightly more road contact = slightly more friction, so economy suffers. However, you might notice that the car handles worse...swings and roundabouts!
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I cannot see 15mm making that much difference,after all there is a good few mils difference between a new tyre and one that is nearly worn out,speedos are not that accurate anyway.My MOT man was not botherad about me running 65ar on one axle and 70ar on the other.
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It won't be 15mm. The bottom of the tyre is "distorted" by the weight of the car. The rolling radius has to be taken into account. Bear in mind also that the tread depth will vary as the tyre wears, probably by about 7mm or so depending on the tyre and how worn they get before being changed.
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I cannot see 15mm making that much difference,after all there is a good few mils difference between a new tyre and one that is nearly worn out,speedos are not that accurate anyway.My MOT man was not botherad about me running 65ar on one axle and 70ar on the other.
For the purposes of the MOT, there is nothing to stop you running 10 inch mini wheels on one axle, and the huge wheels of an SUV type vehicle on the other axle.
Having different sizes of tyre on the same axle is not allowed by the MOT rules.
The tread wearing on an original fit tyre is making the speedo reading more conservative;- i.e. the car goes slower than indicated because the rolling radius is reduced.
The tyre change suggested by the original post is, however, an increase in rolling radius. This change may or may not lie within the acceptable tolerance band for the speedo reading to remain "safe".
number_cruncher
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