the service advisor seemed to be against the idea. But that being said, it also sounded to me like he was a little prejudiced against the idea.

Am I looking to damage my drivetrain doing this? Or totally ruin my economy or performance? How big a deal is unsprung weight?

The service advisor is full of it. RX 8 rims should fit just fine. Unsprung weight can be an issue. As long as they are the same weight or lighter than the stock rims you'll be ok. Heavy rims will make for a harsh ride and give the car a sluggish feel. Check the offset, the RX 8 should be +50 so its ok there.

 

he hit the nail on the head, rx 8 wheels would look great on the 3 yes you want to weigh them and keep your overall diameter as close to stock (3 size not rx 8 ) as possible to keep weight and rolling resistance down so your ride, and fuel economy don't suffer (assuming you got the skyactiv one for fuel economy reasons) use a tire calculator to help you figure this out here is one: https://tiresize.com/calculator/ put the factory size in and your aftermarket size and it will tell you how much your off + or - in the diameter, width, sidewall and circumference , it also has an offset calculator and will tell you the speedometer error as well. but it wont eat up your drivetrain or bearings doing this. now if you where putting 22" chrome heavy big wheels that don't belong on anything on it, then yes your drivetrain will get shot to hell, you will eat up bearings etc. but going from a 16 to an 18 of similar weights and offsets wont mess with much, just make sure you have clearance in the fender, get a tire that makes the overall diameter of the new rim and tire close to or the same as the factory rim and tire so basically if you get bigger rims, get thinner tires to keep the diameter the same, or as close as possible. so basically in the calculator I linked you to, it will give you pics with measurments on the one with the tread facing you, you want to get a tire/rim combo that will keep the factory diameter on the 18" wheel if possible, the rim width and tire size options may prevent this, but if you cant get the same diameter try to keep it close so you have less speedometer error


and speed 3s have 18 inch wheels so you should look at their stock size and see if it will work on the width of the rx 8 wheels and probably go with that size if it works and is close on the tire size calculator. the service advisor is an idiot as a lot of 3s have 18" wheels and most come stock with 17" so putting 18" on wont mess up things already meant for them. also keep in mind the offset of the rx 8 and factory size as this is the clearance for the stuff behind the wheel.

 

Thanks for the feedback guys!

The rims currently have 235/40R18 tires on them. According to the tire calculator on 1010tires.com, that gives me a diameter variance of +2.1% from stock (205/55R16). (speedo error 2.06% slower than actual)

My next set of tires will likely be 225/40R18 to bring that number down to +.84%. Do you think I need to scrap the current 235's? Or can I use them up without major problems?

Also with regard to total wheel weight, how much heavier than a stock 16" wheel/tire combo can I get away with? (none at all, 1lb/wheel, 5lbs/wheel?) I realise lighter is better but where would you guys draw the line?

Again, thanks for the help!

 

The rims currently have 235/40R18 tires on them.

My next set of tires will likely be 225/40R18 to bring that number down to +.84%. Do you think I need to scrap the current 235's? Or can I use them up without major problems?

Also with regard to total wheel weight, how much heavier than a stock 16" wheel/tire combo can I get away with? (none at all, 1lb/wheel, 5lbs/wheel?) I realise lighter is better but where would you guys draw the line?

Assuming the rims are stock RX 8 18x8 +50s, 235s are ok but 225s would be a better fit on the 8" rim. Less sidewall flex, better steering response, a bit lighter too.
The stock 16" alloy wheel is about 19 pounds. That is already a bit on the heavy side. 5 more pounds would be quite noticeable.
So woud 5 less. :biggrin:

 

And thank you for the link to the new tire size calculator... it's much more convenient than 1010tires.com. Will use that one from now on.

 

So I just went out and weighed one of the RX8 wheels(with tire) and assuming that my scale is accurate it is about 43-44lbs.

Also, I should mention that I currently have my winter tires mounted on steel wheels not alloys. Manufacturer states that my winter tires(205/55/R16) come in at around 22lbs for the tire. I can't seem to find a reliable number for the weight of the steel wheels on this car, I've seen anywhere from 16 to 24lbs in my search. (I would like to avoid pulling a wheel off the car just to weigh it...)

Think I'll get away with the RX8 wheel @ 43-44lbs?

Thanks guys!

 

So that means the rims are around 21 lbs, the tires about the same.
Considering that the OEM 18" tire/rim combo is around 49 lbs, that's not bad. However, in the big picture, that is still heavy. My summer wheel combo is 36 lbs. :smile2:

 

i would say weigh the steel one with tire and see if the difference is worth the aesthetics of having the rx 8 wheels on it. but again the tires on the rx 8 rim may not be the lightest you can get perhaps google the tire and get specs for that size with weight. then you can go from there

 

I have them on my 2012 Sky as a winter set of wheels with Speed3 wheels for summer wheels. I wouldn't go any larger than 225/40/18. If I recall correctly 235/40/18 will rub. No problems with them even with a lowered suspension.

 

do you happen to know the weight of the SPEED3 rims? or could you weigh one with a tire on it so the OP can get an idea of how much extra weight it would have? I cant imagine rx 8 wheels being heavier than speed 3 wheels or not much anyways. I agree though the thinnest tire he can buy should be what he runs to prevent issues. 225/40 will have about a quarter inch shorter sidewall than 235/40 and has an overall diameter that's a 1/3 of an inch smaller so that maybe the difference in rubbing and looking great. if we can get him a weight on a speed 3 rim and tire or just rim and a good tire size to fit on the rx 8 wheels I think it would look great on his car and probably would not impact performance enough to outweigh the styling aspect of it. as he is running a skyactiv engine and its down about 110hp on the speed 3 so it will have a harder time making up for extra weight and it will mess up his speedometer.

just saw you have them on a skyactiv one as well, my bad. let the OP know how it has impacted gas mileage the speedometer and overall performance of the car, does the car feel heavier and slower with the bigger rims then stock, or negligible difference? how has handling been affected, steering, braking response all this would be great for the op to arm himself with to decide whether or not to proceed, I think his main concern is making it get bad mpg and making it handle bad so if we can confirm its still ok that would be great

 

Speed3 wheels weigh 24 lbs each, RX-8 are 21 lbs each. Both sets have 225/40/18. The stock 16" wheels that come on the Sky weigh 19lbs each, so the RX-8 rims are closer in weight to what came on the car.

Speedometer has had no impact with either. In terms of fuel economy, mine has suffered but I've also done a bunch of mods such as the OVTune, exhaust, GV front lip that could affect the mileage and aero.

 

Great info for our OP. yeah 19 to 21 pounds is not much when you consider 2 inch larger wheels that will net better sidewall flex as the tires will be thinner, better steering response and the appeal of rx 8 wheels on his car. I would take the mpg hit for that, I do not know if the OP would. the OP that weight difference is not bad, find some light tires you like and mount them on the RX 8 wheels and give er a whirl. Michelin pilot sport a/s3 cooper zeon rs3-a come to mind as both of those where some of the lighter tires whe I was looking.