"dont get bigger tires... it will throw off your speedo", i beg to differ Options

[lagos]

so, for years ive always had people say to stay with low profile tires, because anything bigger will throw off your speedo. im sure we've all seen the online calculators and countless threads. ive even had tirerack call me back and try to convince me to change my order from 225-45-17s to 205-40-17s because of this..lol ...

well anyway, i just wanted to say that all that, is mostly a myth. i recently got a gps for my car. my speedo is dead on with the mph reported by the gps. i was a bit shocked because i expected it to be off by at least 1-3mph. so go ahead, get bigger tires and enjoy the ride quality..your speedo will be just fine

This post has been edited by lagos: May 24, 2007 - 9:25 PM

[airfron]

i learned that with my GPS too

[Coomer]

With my 215/45/17s on my Celica (AT200 tranny), my speed is off by approximately eight percent. And this is according to a police radar.

I don't see why your speedometer isn't off. Are you using the factory S54 tranny?

[lagos]

With my 215/45/17s on my Celica (AT200 tranny), my speed is off by approximately eight percent. And this is according to a police radar.

I don't see why your speedometer isn't off. Are you using the factory S54 tranny?

yeah factory s54.

you cant really go by the radar gun, cause its hard to remember the exact mph you were at, at the exact second he shot you with the gun. you could have slowed down or sped up for a second and not realized it.

[Defgeph]

I have a set of BF Goodrich G-Force KDW2s 225/45/R17, They should be here tomorrow.

[devilsden97]

car companys actually set there speedo 2 to 3 mph higher then what your going, thats y cops get u for 5mph+....ever see a ticket for 3mph over? I learned with my gps that on stock wheels i can go 50mph on the speedo, but im actually doing 47mph on the gps....gg

[GreenyST204]

I had smaller tryers on my old car and they put the speedo out by 10%
at 110km/h i was realy traveling at 100km/h
alot faster take off by the way

also with bigger tyers you should be able to fix this with some resistors in the speedo wire, shouldn't you.
but can i ask why you want bigger tyers just for looks.

[lagos]

I have a set of BF Goodrich G-Force KDW2s 225/45/R17, They should be here tomorrow.

so now when your going 150+mph, youll know the cop had a right to write you that ticket

[Defgeph]

Shoot, thats if that chump can catch me

[lagos]

car companys actually set there speedo 2 to 3 mph higher then what your going, thats y cops get u for 5mph+....ever see a ticket for 3mph over? I learned with my gps that on stock wheels i can go 50mph on the speedo, but im actually doing 47mph on the gps....gg

you might be onto something. i noticed that my moms matrix, with bone stock wheels and tire size, was slightly off compared to the gps. so it could be that getting bigger tires actually puts your speedo back to where it really should be.

[devilsden97]

car companys actually set there speedo 2 to 3 mph higher then what your going, thats y cops get u for 5mph+....ever see a ticket for 3mph over? I learned with my gps that on stock wheels i can go 50mph on the speedo, but im actually doing 47mph on the gps....gg

you might be onto something. i noticed that my moms matrix, with bone stock wheels and tire size, was slightly off compared to the gps. so it could be that getting bigger tires actually puts your speedo back to where it really should be.

ive noticed it in my celica, and a 2005 toyota matrix (both on stockers) also when i had my aftermarket rims and tires on the celica, i passed a "how fast your going, this is the speed u should be at...SLOW DOWN" police sign, the road was 30, and i was going 30 says the police thing, and my speedo said 30 on the dot...so...idk who cares neway its like 3 mph...no ones gonna write u a ticket for 3mph over neway and if they do...ill pay it.

[stephen_lee]

i've seen a ticket for 5mph over...
i got a warning for going 11mph over

[bman]

some local town police on long island will give you a ticket for a few mph over... its their primary source of income lol

as for the speedometer accuracy - it is a liiitle bit off according to gps - the faster i drive the more 'off' it goes

[devilsden97]

i've seen a ticket for 5mph over...
i got a warning for going 11mph over

lol ive gotten 10 over....but ive never seen 5 of less.

[tonytutino]

here its impossible to get a ticket for five over because unles you're going 6+ you wont get a point off your license... aka its a waste of time.
i learned this in traffic school lol

[Defgeph]

Cops will only pull you less than 5 over if there is something else they are looking for. Drugs, alcohol, or suspicion of other nefarious activities.

[Supersprynt]

I was curious about this same thing. As odd as it sounds, my 18'' wheels with 35mm tires are millimeters bigger than my stock 15s & stock tires.

[Racer_J]

I was curious about this same thing. As odd as it sounds, my 18'' wheels with 35mm tires are millimeters bigger than my stock 15s & stock tires.

It's the rolling radius of your wheels that effects your speedo....

Say a Celica is supposed to have 195/55/R15 tyres

Its rolling raduis will be:-

195mm Tyre width
Profile 55 - 55% of tyre width (107.25mm)
Rim 15" or 381mm Dia (190.5mm for radius)

Rolling Radius = 190.5+107.25 = 297.75mm


My setup is 215/40/R17..

215mm Tyre width
Profile 40 - 40% of tyre width (86mm)
Rim 17" or 431.8mm Dia (215.9mm for radius)

Rolling Radius = 215.9+86 = 301.9mm

That's +4.15mm.


So for 215/35/R18's....

215mm Tyre width
Profile 35 - 35% of tyre width (75.25mm)
Rim 18" or 457.2mm Dia (228.6mm for radius)

Rolling Radius = 228.6+75.25 = 303.85mm

That's +6mm.

[bindertch]

The spec when setting a speedo is if you want it to display 20 mph then you can set it when the it is reading anywhere between 18 - 24 mph. So yours might have been close to correct but your tires have put you into the upper range. Also the speed readings from a gps aren't exact.

[azian_advanced]

if it really is a concern, you can always get it calibrated... this one shop 5 mins away can do it. i think it's about $60...