I know I saw a thread with the formula for this, but if you guys could help me out that would be great.

I have 2000 GS300 w/ an LMS SuperCharger

the OEM rear diff is a 3.96..

I am running a 285/30/19 tire in the rear AND a KAAZ 4.27 2-way LSD (w/WPC plates, which I love because there is no more chatter noise at low speed)

anyways,

when I am on the freeway and my speedo says 80mph, am I actually going faster or slower than what I see?

THEN

I am looking to go to a 285/35/19, will that make my speedo error worse or better?

Thanks in advance for all your help.

Airek

 

the 285/30/19 pretty much has the same overall diameter as stock. so when it says 80mph, that's pretty much your speed

if you put 285/35/19, overall diameter larger. it will have more error. if it says 80mph, you probably around 84mph

the rear gearing shouldn't have any effect on speedo. it has no effect on the 400/430, i don't think it will have any on the 300. speed sensor is close to the wheel axle i believe

 

i have checked with two diffrent nav units and it only shows a diffrence of about 3 mph and i have 20s with 255s.

 

Here is a site the explains EVERTHING you could ever need on wheels/tyres/offsets etc etc:

http://www.carbibles.com/tyre_bible.html

(Wheel sizes are on page 2)

 

Isn't the speedo calculated off the front wheels's speed sensors anyways? So it's independent of the diff ratio and the tire sizes.

 

independent of diff ratio, yes. tire sizes, no..... with different sizes, distance traveled per revolution is different. speed sensor is just checking on the axle/wheel revolutions.

 

Well the size mentioned was 285/295 so I reckon it was for the rears, which shouldn't affect the fronts where the sensor is at, no? Oh wells, whatevers...

[/proceed]

 

umm, good question. i was under the impression the sensor is in the rear, but now you mention, i can't be 100% sure

 

this is the calculator i've used in the past...
http://www.miata.net/garage/tirecalc.html