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Discussion Starter #1
The overall diameter of the 15" and 16" wheels are shorter than the 18" wheel. Anybody have any ideas why they did this? I love how the 18" wheel fills out the vehicle, but I didn't want that combo so I got the + model. What I am not sure of is if the sport and ! models have different speedometer calibrations, or if they are the same as the other trims and the 15" and 16" wheels use an incorrect calibration. Time to use the GPS.

Also, if they did some different things, I wonder which size we would need for 17" wheels? Normally, a 205/55-16 would call for a 225/45-17, but that's the same width and aspect ratio of the 18" wheel, so you're getting .5 inch increased radius. Strange option indeed. If the + and ! are using the same suspension, then I don't see any issue with using 225/45-18 size and going with a different aspect ratio for a 17", although I don't know of any 225/50-17 tires off hand.
 

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I'm thinking that the Sport and ! models have different overall gearing to compensate for the larger diameter tires. I also suspect that the smaller diameter wheel setups will accelerate faster and ride better than the models with the heavy wheel setup. Before I drove a + model today I was thinking about buying the Sport model for the stiffer suspension but trading for some 16" wheels and mounting 205/60-16 tires on them to get close to the diameter of the 18's. The + I drove handled very nicely though so I might just buy one of those instead.

Curly
 

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Discussion Starter #3
Well, I used some GPS software on my G1, and it appears the Soul was geared for the 18" wheels, as my true speed was around 2mph less than my real speed on the speedometer. Not a big deal, but I would like more people to confirm this. I might go on a trip to a nearby city this weekend, and if so I'll have some long stretches of road to confirm this.
 

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There is a tolerance built into the speedos.
You will typically (in a properly functioning speedo) read on your speedo that you are going faster than you acutally are. This is a precaution put into the design of the speedos. +2 to 5mph difference is common This way people can't claim their speedo is the cause of a ticket. Unless of course you go and alter the tire size drastically but still it typically wont alter it enough to make that big of a deal.
 

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I believe the correct tire downsizing from the 18's to 16's should be P215/60R16 = your speedo will be off by 0.7kph...speedo reads100kph (60mph)you are travelling 100.7kph (60.4mph)or P205/60R16 = your speedo will be off by 1.1kph...speedo reads 100kph (60mph) you are travelling 98.9kph (59.3mph)...these are the closest sizes to the P225/45R18 tires...
 

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they usually do a tire calibration built into that wonderful obdII computer onboard your vehicle you can take it to a dealer for updated calibration if drastic tire changes were made usually you should be off in a percentage about 1-2% but can be as much as 10%

now if only i could get my hands on one of their programing computers i'd be set
 

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the 1.6l base models 195/65-15" in 18" would be a 225/40-18, not the sports stock size of 225/45-18.. its a very small difference, but it is a difference.

I suspect the base model with the 1.6 has a bit different gearing to optimize the different motor... but thats just speculation.
 

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As long as you are within +/-3% of the original size.

This holds true for the way the car will track as well as the speedometer issue.
 

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This is the correct answer when you go from 15 (base model) to 18
should be P225/40/R16 is a difference from only 0,04%.
You go from 644,64 mm to 644,40 mm wheel diameter.
Is in inch 25,3795 to 25,3700 wheel diameter.
So Ebacherville your explanation is the right one.

Regards,

Crazy Dutchman RelenaRelena
 

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This is the correct answer when you go from 15 (base model) to 18
should be P225/40/R16 is a difference from only 0,04%.
You go from 644,64 mm to 644,40 mm wheel diameter.
Is in inch 25,3795 to 25,3700 wheel diameter.
So Ebacherville your explanation is the right one.

Regards,

Crazy Dutchman RelenaRelena
And Student driver you can better go for 225/45/17 = only + 0,35%
225/45/18 = -3,47%

Crazy Dutrchman RelenaRelena
 

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Keep in mind that the calibration of the ABS system is based on the diameter of the tires. It is recommended to stay within +/-3% of the original diameter.

If you wish to stray from the 3% limit I highly advise you to discuss your choice with a qualified technician.
 

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Excellent thread with excellent comments and advice! I rate this thread a 10!
I like it when the outcome is in percentages and not raw numbers.
 

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Have a few friends in the FHP and asked if one of them could verify the speedometer reading of the soul. One said he would be more than happy if we just happen to be going in the same direction. It happened the other day and up to 70mph it was dead on. Their cars are calibrated quite frequently. Naturaly, now I have no excuse if I get stopped for going over the speed limit. This is related to the fact I have 18's on my soul.
 
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