Will the placement of larger rims (i.e. 17") make my XB turn lower rpm's at the same speed? Let me clarify... I once had a Chevy full size bagged on 17"s and not only did it throw the speedo off but also the odometer. Here's how it relates... I love my new XB but the gears are very close together, as you know. I live in San Diego, CA and if you're not going 65-70mph on the freeway, you're going to get run over. So I travel about 40miles a day on the freeway averaging 65-70mph, my XB is running around 3500-3800rpms. To me that seems kinda high...If I put 17" rims on it, will it lower my rpms at that speed? Sence its geared close, will it also give me a greater shift range? Any info would help and sorry for the novel. Thanks, 2GoBox

 

I'm on 18's and RPM is 4k on 80 mph

 

the diameter of your wheel will affect your final drive ratio - but not the gear ratio (that's a constant)

so ya, a bigger wheel will have to turn less times to go the same distance, so it will lower the rpms on the freeway, but with 17's they are almost exactly the same diameter as the stock wheels, so you wouldn't notice a difference

with 18's or 19's - you may see a little change, but no more than 3% or so

- Brian

 

The overall diameter of the wheel/tire combo is what changes *effective gear ratio.*
Smaller than stock = quicker acceleration, higher cruising rpm's.
larger is the reverse.

http://gs.tolan-hoechst.com/tirecalc.htm

 

Well if you throw off you gear ratio you will throw off your speedo also. For you to make a significant effect on the gear ratio you would have to put on some huge tires like 44" (JK) or some smaller ones. You'd have to be rollin on like some 13in daytons w/ a 35 series tire(JK)

3500-3800 isnt bad at all, at about 80mph my dads BMW 330 runs about and its a 5spd auto. my bros s2000 at 80 does at least 4500rpms.

Dont know if this is part f the question but you have to plus size correctly in order to keep gear ratios, speedo and odometer readings the same.

Plus if change your gear ratio to make rpms lower at highway speeds your car wont accelerate worth a crap.

 

try this site
http://www.miata.net/garage/tirecalc.html
may be helpful for you in choosing your tire size.
sid

 

It would depend upon the tires you use.

The important thing is the revolutions per mile of the desired tire. Go to Tirerack.com and look up the revolutions per mile of the stock tires and of the desired tire. The ratio of the new tire to the old tire multiplied by the old RPM at a given speed will give the new RPM at that speed.

RPMnew = RPMold ( REVperMInew / REVperMIold )

IMO, the Scion could do with a little reduction in RPM. Toyota overdid it when the lowered the final drive ratio to make up for the added mass of the vehicle.

 

Most people who upgrade wheels do so with low profile tires so that the overall diameter of the tire/wheel is very close to stock. If thisis the case, then RPM's, speedo, etc., aren't affected.