Bass Mech

so I am about to pull the trigger on a set of wheels and tires, but my car has not arrived yet.
call me crazy but I am very **** about the speedo being correct, but I want the wider tires, so I need to make a compromise between the 2.
what I need before I can make any decision is to determine how accurate the factory speedo actually is with the factory tires installed.
my Tacoma for example always read high, so putting taller tires on it ended up making it dead accurate! (awesome!)

but I am even more concerned with the IS because it's digital, that will really bug me! my hope is it reads a little high already. discount tire said there are different tire sizes offered in different trim levels. the question is do they just calibrate it for something in the middle?

so here is the favor, can someone who owns a portable GPS drive the car at say 75 mph and give me an actual speed verses indicated? the faster the speed the more accurate this will be but 75 should be plenty.
if you could just report back your findings it would be much appreciated!

Skinnyfats

At work now but you can use this and you can play with tire sizes and get yourself close to what you want.

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

Bass Mech

thanks, I already know how the tire sizes affect the speedo and how far off ill be, but unless I know that the speedo is dead accurate to begin with it doesn't help me much.
I hope it reads a little high, that would make my tire size choice a little easier to live with.

enoren

I've seen a difference of 1 MPH at 50 driving speed: speedometer says 50 while a roadside radar detector clocked me at 49.

I've also seen a difference of 2 MPH at 80 driving speed: speedometer says 80 while the roadside radar clocked me at 78.

Mine is a 250 F-Sport, but I think the tires and wheels are the same.

Bass Mech

sweet! I would trust a GPS a little more because it's in the same car traveling the same speed and doesn't need to be calibrated.
but this is encouraging.

Skinnyfats

70 my car is going 69 according to my iPhone GPS and 75 I am going 74 according to my GPS. 350 F-sport RWD.

Skinnyfats

Also when you change out your wheels and tires you can take it to a shop and have your speedo calibrated.

Bass Mech

you have said this before, but I don't see how because to my knowledge the only way to program the speedo on a Toyota/ Lexus is to re-flash the ECU which has been un-crack able for some time.
but thanks for the information, this is exactly what I needed to hear.
by my calculations if I run a 275/30/19 on the rear that should put the speedo .5mph accurate at 75 mph
which is .5 mph closer to the actual speed than the stock speedo reads.

also there is some speculation here as the tire store says that for some time Toyota has been using ABS wheel speed sensors to read speed, not a sensor on the drive shaft output as most older cars have done since cable driven speedos.

if this is true it is quite possible the ECU actually gives an average of all 4 wheels and comes up with a value for the actual speed.

what I can tell you about Toyota is both my 1996 and now my 2009 Tacoma read high, (about 3%) the odometer was pretty close but it took a full 2 sizes larger tires on the Tacoma to get the speedo accurate, but the odometer now reads about 3% low which is fine with me, that only helps my resale value!
my Tacoma reads off the output shaft of the transmission through a hall effect sensor, same kind used for ABS at each wheel.

vbx

I doubt you can get your speedo calibrated for the new wheel size.

Skinnyfats

For a Tacoma you can buy something like this.

http://www.autoanything.com/performa...A3757A0A0.aspx

Going to check with some high end shops around the area and ask them. Right now my plan is to buy 20" rims with tires that get me close to stock so I won't have to worry about it so much.

Bass Mech

i actually have a speedo healer that i used on my Honda VTX 1800 it was about 10% off from the factory. it does the same thing.

but as it turns out i wanted a wider tire anyway on the tacoma. i am running 20" 285/50's i think and now the speedo is accurate so it worked out.
whats really strange is the speedo and the odometer don't agree. this means toyota built it that way since the speedometer and odometer are in the same gauge.

if i could give any logic to this it might be that toyota knew some people will put larger tires on the truck and compensated for it?
who knows..

Skinnyfats

Apparently this can't do your speedo but it can do other things. But there are ways to get into your ECU.

https://techinfo.toyota.com/techInfo...rog&_nfpb=true

Bass Mech

techstream is what is used to reprogram the vehicle computers, this is what is used by the dealer to set your lexus personal settings. it even was used to ref lash my tacoma when the SC was installed.
but you cannot alter the actual engine parameters. the only thing you can do is upload a toyota reflash or firmware update.
if it was possibly to use a tech stream to mess with the engine parameters, you would have seen a lot more toyota vehicles with superchargers than you do.
the only way to add performance goodies to a toyota is to run a piggyback ecu.

but heres the thing, the good news is the OE speedometer reads high. running the factory tire sizes or trying to stay within the same tire height as factory if fine with me because thats how toyota designed it.
but more to the point of concern, i would not want to put a larger tire on there and have it be more out of whack. seems you can go a size larger and actually improve the accuracy of the stock speedometer.
so no longer am i as worried about running a larger tire. the way i see it, running a bigger tire seems to solve the wider profile issue and the speedo accuracy for me.
so i really don't think even with 20's you'll have to re calibrate it.
personally i think 20's are too big and tires too expensive, more road noise from a shorter sidewall, and the tires are more expensive.
i think 19's are plenty! in fact if i could get the widths i want in an 18 i would stick with that size.

also where we live and the pot holes we deal with larger than 19" rims wouldn't last long here anyway.
but i do think that with a 20 series profile is likely what you'll be running, the different section widths won't affect the height as much as they do on a 19 or an 18

amoney805

iTrader: (1)

20" tires are cheaper than 19's.