question about rims and speedometer

[g222leav]

i just got some 15" rims from a friend and put them on to replace my 14" rims. i know that this messes up the speedometer, but how?

i mean if i'm travelling at 55 mph (i'm a good driver:D ), will my speedometer say i'm goin faster or slower....

or conversly,

if my speedometer says i'm goin 55, will i be really goin faster or slower...

now i haven't had a speedin ticket since i was 17, hoping to keep it that way. anybody know? it seems like my speedometer is reading 5mph faster than it normally should...am i right?

[Jihforce]

Or you could drive 5 miles under the speed limit :P

[g222leav]

thanx guys, i think the circumferance of the new tires are just a tad bit larger than the old. i think i'm goin to wing it though (for now)..wish me luck from the MAN!:angry:

[g222leav]

man, i thought i was just about to get pulled over. on my way home, right before the toll, there as a sherif just chillin, i was crusing along at like 65 (i think, who really knows, i don't) and some idiot comes flying by on my left. well, he got pulled over. it's funny cause the cop waited till after he paid the toll.

[shaggymatt]

What was your previous tire size, and what is your new tire size? As in 185/65/14.

[johnnymk]

If you put larger tires than what you originally had, the speedometer will read slower than your actual speed.

Simple formula for percentage difference at all speeds:(Example)

Original diameter of tire= 24"
New tire diameter= 26"

2" difference/ Original diameter

2/24=8.3% difference for all speeds;

So if the speedometer is reading 60 MPH, with the
new tires, you are actually going 60 X 108.3%= 65MPH

Where did that face come from? Must have escaped from the Twin Towers and entered here.

[rollhard]

dont make that big of a difference. also depending on your tire sizes. the apexi rev/speed meter has a function to compensate for tire and wheel size changes. it lets you set your aftermarket wheel and tire size and it corrects the speed

[brainsmile]

You'll be going faster than the spedometer.

Think of it this way. A vinyl record: The outside of the record has to travel faster to make a revolution than the inside.

[shaggymatt]

Here's a good site to calculate the differences: http://www.venommotorsports.com/howto/wheeldiameters.htm

I used to have 195/65/15 on my car and upgraded to 225/45/17, my overall tire height only changed by 0.01", so my speedo still reads the same. And from the factory the speedo reads 4.2 mph high on both of my cars ('99 & '00 Passat).

[Blu]

Originally posted by johnnymk
2/24=8.3% difference for all speeds;


Keep in mind that speedos usually show higher error as you go faster...

[g222leav]

geez, you guys got all technical on me, but thanx!

[g222leav]

well, here's my plan.....i'm just goin to wing it. i've noticed i've been driving a lot faster lately, or so the speedo says. anyways, i'm just guaging my speed in comparison to other cars on the road. i should be ok.

on a side note, i got pulled over tonight by university police. i thought it was for speeding but it was for expired plates. i had a temp sticker in my glove so it was ok. i got let off...hehe, i was scared though, the first time i was pulled over in like 3 years.

[rollhard]

this is my speedo solution. it cot about $100 and i can program it for whatever tire size i have and that i will get.

http://www.rollhard.com/rpinterior2.jpg

its the top meter. allot more accurate than the analog one cause cars tend to get off on the speedo as you go faster. paticularly after about65-70mph. they always do. anyhow it can easily be corrected and you dont have to keep trying to calculate and figure out how fast you are going.

[johnnymk]

If you want to tell how fast you are really going, borrow a friend's GPS unit. They are very accurate. I have a cheap unit and it works great!

[g222leav]

i think i finally fixed the problem.

a friend and i were both driving (separate cars) and we compared speeds (via celly phones) he was goin 68 mph, i was goin 70 mph, considering the high[er] speeds and allowing for error, id say that my speedo is alright. i guess i've just been driving a lot faster with the new rims.

thanx anyways guys

[rollhard]

lol, if both cars are messed up both of you guys will get a ticket together = ) man only way to tell is digital reading.

[g222leav]

yea i thought about that too, cause he's the one i got the rims from..hahaha, i can just imagine us being pulled over simultaneously....