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I had this argument with my dad the other day.

He said that it doesn't matter what the size of your tire is, it will only affect the speedo reading but not your cars overall mileage.

So if my tire was small and i was going 100 kmph on the speedo but actually going 95 in real life; then by the time my odometer read 100km I've actually travelled a distance of 95km. His argument was that yes I may have travelled 95km, but the engine and tranny have travelled 100km in effort trying to get there. So the wear on my enigine and tranny hasn't changed and the overall mileage of the car is exactly what reads on the odometer. So when my odometer says I've gotten XXX amount of miles on the tank, then it's right and i don't have to accommodate for the size of the tires.

Does this make sense?

 
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Your father is wrong. To use your example, if you are going on a trip to somewhere that is 100 km distance, when your odometer reads 100km you will have gone 95km and you will still be 5 km from your destination. At this point you will need to travel an additional 5.xx km so you odometer will read 105.xx km instead of the correct 100km. With the smaller tires your drive train has had to complete more revolutions to go the same distance.

When you start talking mileage it gets a little more complicated. The smaller tires in effect give you a different final gear ratio which will decrease miles per gallon but since the odometer is off as well, the two should offset and your calculated mpg should be the same but wrong. If you have to travel 100km to go to work and you fill your car every 5 days, your car will travel 500 km but read 525.xx km. If you use 10 gallons to go this distance, then you would divide the 525km by 10 gallons and you would get the same number as before but if you use the correct 500 km the mileage is less.

 
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Thats exactly what I said but he woudln't believe me! Thanks!

 
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hahah. you cannot argue with ur dad. he will always be right in his head.

he's prob thinking "who the hell is this young'in think he's schooling"

 
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Lmao, too true

 
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Parents arealways right. Hahahaha:P

 
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One other thing to keep in mind is the added weight of larger wheels/tires will make your mileage decrease. I went from the factory 14's up to 16" Rotas and my mileage dropped.

 
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Careful.
Heavier is harder to stop moving. It wants to keep moving, so if lots of hwy driving heavier is better. If too much stop-go, lighter helps but not by much

 
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so if im on 18'' rims and im going 60. am i really going 60?

 
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Depends on the tire size, not neccesarily the rims.

This site is awesome I think the stock size is 185/75 14. It'll tell you the speed you're actually travelling.

http://www.miata.net/garage/tirecalc.html

 
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hmm interesting. that tells me that when im going 60 im actually going 57.
that's exactly what i wanted to know. maybe that's why people are passin me on the highway. lol