*I wasn't sure where exactly to post this, so if it's in the wrong spot MODs feel free to move it.*

Okay, for the longest time I had 18" wheels. About a month ago I went back down to 15" wheels and ever since, my speedometer has been waaaaay off. I'm talking like 7mph - 10mph off. On the freeway my speedometer will say I'm going 90mph when I'm traveling with the flow of traffic. So unless EVERYONE else on the freeway is doing 90mph, something is off.
I don't know what could be causing this? I did notice that my car is lower than it was with the 18s. (I'm on S-Techs). I also have to get my camber corrected as well.

Any thoughts? Thanks guys.

 

Do you have the right tire sizing that matches OE diameter specs?

 

You are probably doing 85 when it says 90 - what is the tach reading?

 

I'm running 195/50/15 tires. So basically to solve this problem I just need to run the stock size tires? I guess that's no biggie. I'll just have to live with the issue until it's time for new tires.

 

If you're not within OE specs, smaller or bigger, it would generally be off. You could leave it be unless you care about mileage, your readings are gonna be off until you go back to OE specs.

 

search the speedometer has been off since day one and how did you verify spped?

 

You might call your local Toyota parts dept. to see if they sell replacement gears for the speedometer. It's a long shot, but some manufacturers have them.

 

our wheel speed sensors is on the half shafts by the hubs...

 

It's because of the tire size. 50 series on a 15 is way smaller than stock...
Use this to visualize it.
http://www.miata.net/garage/tirecalc.html

 

yup tire size is 1 inch in diameter smaller (22.7" vs 23.7") causing it to spin 918 revs per mile instead of 880 revs per mile or about 5% faster add to that the speedo error of a couple or more mph and you get an easy 5-6 mph speed faster than you are going at 85mph actual speed.

 

Nothing is wrong with the speedomoter. Like already stated, you need OEM sized tires. Some 185/60-15, 205/55-15, or 225/50-15 tires will get your speedometer back to where it belongs. Your current 195/50-15 tires are too sort, throwing off your speedometer.

 

stock size shouldn't matter. it's when you upgrade to larger rim diameters that you ge the deviation. so my 15x?( cuz I don't know the width anymore) read the same as my 17x8's. mine reads exact to what the radar trailer shows me when I drive by it.

more interesting is the misc. speeds everyone was getting on a trip to Iowa using me as the base. I was doing 70. xB2 with 19's said I was going 65 and onthoer on 20's said I was doing 60.... it's all about rollout. theres a formula for that i just learned in a tech math class i'm taking. somethin about Pi radians.....

 

Um, yeah, it does matter. Not meaning to be a D___ but as posted above, as the OP stated he is on 195/50/15 they are 1" shorter in overall diameter from the OEM set-up. Tire sidewall will affect speed and it all depends on how close to spec you are when you purchase aftermarket wheels. Some set-ups can match spec and you will have no difference and vice versa. I had 17's with 205/40 and had no difference in speedo based on observation from nav system. When I stepped down to 195/50/15, my speedo was off by approx +5.2 MPH.

Also, the speedo is calibrated to bB spec, not the xB. The OEM size for the bB is 185/65/15 and the US OEM spec is 185/60/15, so you have some discrepancy to the speedo right off the bat. The diff between these two tires is approx 1-2 MPH.

Unfortunately, there is no way to recalibrate the speedometer so you're limited to two options to view actual MPH speed:

option 1: Get a navigation unit that displays MPH
option 2: Get a Scangauge II (You can adjust for tire size +/-)

 

I've got a navi and I believe it always shows as being atleast 5mph slower than what the speedo says.
Next time I get tires, I'll probably get the bB size.

 

Overall tire height matching the stock ones is key to maintaining speedo correctness.