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Discussion Starter #1
I am shopping around for some used tires. My EX Sedona has 235/60R17 tires. I found a really good price on a set of 235/55R17 tires. I realize that a smaller aspect ratio will make the overall diameter less. Currently, my Sedona's speedometer reads fast. When it reads 70, I'm actually only going about 67. I'm just wondering if by going with these tires, would my speedometer actually read more accurate? I know a lot about tires, but I'm not good with all the math involved. Thanks in advance!
 

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Cerato S Hatch Auto
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55s will be 3.4% smaller OD, so will spin faster at any given speed. e.g. travelling at a real 67 MPH, your speedo will show 72 instead of 70 MPH.
 

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Discussion Starter #3
So I'd be better off with a 65? It would probably give a little less harsh of a ride, too.....there is a noticeable difference between the 225/70R16s on my LX. I can easily find 55s and 65s, but for some reason the 60s are nearly non-existent.
 

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I disagree with that calculation.

The engineers would have calibrated the speedo for a certain diametre tyre, knowing its roll out, or distance covered per revolution.
Based on this, the rpm of the tyre at 70mph can be calculated, and based on the gearing the speed of the output shaft of the gearbox can also be calculated.

Changing the diametre of the tyres does not affect (effect?) the rpm of the tyre/gearbox output that the speedo is sensing at 70mph. The car is traveling less distance per revolution of the tyre - less distance per minute - which will give a slower speed at an indicated 70mph.

Hope this made sense - its been a long day.
 

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Discussion Starter #5
So the 55s WOULD be the better option? I'm rather confused. I can see how both arguments could potentially make sense.
 

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Cerato S Hatch Auto
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I disagree with that calculation.

Changing the diameter of the tyres does not affect the rpm of the tyre/gearbox output that the speedo is sensing at 70mph. The car is traveling less distance per revolution of the tyre - less distance per minute - which will give a slower speed at an indicated 70mph.

Hope this made sense - its been a long day.
That's actually the same reasoning. Slower actual speed at an indicated 70 MPH. -or faster indicated speed at the same original speed.

65 series tyres will be 3.4 % larger OD, so driving at the same actual speed of 67 MPH, the speedo will read about 68 instead of 70 MPH.

This sort of puzzle can easily be solved by Reductio ad Absurdum: Imagine if you fitted really tiny wheels, the speedo might be indicating 70 MPH but you would be crawling along and hardly moving.

The preliminary discussion about engineers and gearing has nothing to do with it. Yes, you can measure the rolling circumference and do some calculations. I've done it a few times to calibrate the tachometer. Effective circumference is 97% of nominal calculated.
 

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Cerato S Hatch Auto
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While we're here, I'll explain the calculations:

100km/h is equivalent to 1667 metres/minute.
If driven wheel has effective circumference of 2 metres, it's doing 1667/2 = 833 RPM @ 100 km/h.
Typical final drive is 3.5 to 1 and if 6th gear is 0.75, engine = 833 x 3.5 x 0.75 = 2200 RPM.

Measure wheel circumference by applying a dab of tyre black and rolling along smooth driveway. Calibrate speedo by stop-watch or GPS and you can then derive a tacho calibration by driving at (actual) 100 km/h in each gear. Simple.

The effect of different tyre size is implicit in this sort of calculation; I've done it in practice years ago. My ute had speedo that was 4% optimistic; larger tyres brought it into line.
 

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For various reasons Mfg's purposely make speedo's read a few mph/km higher than car is actually going...

In today's cars, very few have a "mechanical" speedo cable. Everything is via sensors so engineers could calibrate speedometer to be "spot on" very easily.

So far as the OP's question:

Going from a 60 to a 55 series IF the tire is the *SAME width will:

1: Cause the speedo to read faster, as a 55 has more revs per mile/km.
2: Cause increased ride harshness.
3: Result in increased wear= Vehicle.
4: Result in increased wear= Tires.
5: Deceased fuel mileage *Depending on how car is geared, average cruising speed, road conditions.
6: Will result in more miles "showing" on odometer.

*Going from a 60 to a 55 series, SAME Width. IF width is increased on the 55, ratio /revs per mile will decrease.

.
 

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For various reasons Mfg's purposely make speedos read a few mph higher than car is actually going...

In today's cars, very few have a "mechanical" speedo cable. Everything is via sensors so engineers could calibrate speedometer to be "spot on" very easily.
Most cars that I've driven recently read about 3% high, the correction being linear, and I always compensate for this when I'm cruising. A rather conservative friend once expressed genuine alarm that I was doing this. I could never understand his logic; he probably put me down as a speedster.

The Australian Design Rule (ADR 18/03, 1 July 2007) requires:

Speedo must not indicate a speed less than the vehicle’s true speed or a speed greater than the vehicle’s true speed by an amount more than 10 percent plus 4 km/h.

-I presume other countries have something similar.

The intention being that the driver is always given "safe" speedo readings, but I query whether it is really very safe to be tootling along the 110km/h freeway at an actual speed of 96km/h. A 14km/h error like this would be unlikely, but it is quite common to see drivers maintaining a fixed speed 5km/h less than the limit. This is just slow enough to annoy others, but too fast to permit safe passing without exceeding the speed limit.

Before speed limits were strictly enforced with cameras, it was usually much safer to maintain a good speed and avoid hold-ups with a strategic burst of acceleration. I have seen more potential problems these days with processions of semi-comatose "safe" drivers sitting dead on 95 km/h.
 
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