speedo can compensate for smaller tire diameter?

[Tenacious G]

OK, I have 205/45-16s on my car. according to the tire calculator, my speedometer should read 3.7% or 3.9% too fast since the overall diameter is a couple of tenths of an inch smaller than stock.

so i decided to test that number out. on my way back from a meeting, i puposely hit the fuel cut off in third gear. i know in the 1/4 mile in stock form, i would trap at around 84 mph (slow, i know). that speed forces a 3-4 shift just before the end of the 1/4, otherwise you hit the fuel cutoff. so on my way back to the office, i hit the fuel cutoff at about 82 mph, about the same as when i had the stockies on.

so my question is, if the tire calculator is correct and the speedo should read too fast, my indicated speed should have been around 85 mph at fuel cutoff.

anyone know if the EP speedometer is analog or is the speed reading taken from a point where tire diameter does not affect it?

[anjapower]

speedo doesn't know that you're going faster or slower. Get an ep3 with stock wheels and tires to do 80 mph next to you. Pace it. You will be reading 4-5% higher or whatever your tire difference is.

BTW since your rolling diameter is smaller, you will actually be capable of LESS speed in each gear.

[Chinaboy]

bigger diameter tyres do reach more grounds than smaller tyres... but I thought the speedo will read correctly upon the amount of air and if your tyres are in tip top shape... because with 15's or 18's, you would still be able to hit up the whatever milage you got, but its juss the difference between stability or how much rubber you got on the ground. So isin't the more tire the better for the correct speedo reading?

[anjapower]

wha??

This is how its done:

My friends have 205/50/15 Azenis on their EP3s, a tire that is ~7% smaller than stock rolling diameter. If someone who didn't know drove their car, he would have no idea that the speedo was off. He would get in the car and drive 80mph, but he would actually be traveling at 75mph or so.

When I'm in my car with 225/45/16s...a size that is much closer to stock rolling diameter, I do 80 but my friends momentarily forget their tires are smaller and are afraid to do 85+ in their cars to catch up to me.

What this also means is that they can do less mph in each gear for they have to turn more revs to go as fast. I can do around 93-95 mph in 3rd gear with the Hondata flash and they can only do around 90. HOWEVER, on their speedo they see 93-95.

The difference can only be seen relatively....if you compare to a car that has correct calibration for the stock rolling diameter.

I hope this is making sense to you guys.

[BarracksSi]

13's or 22's, it doesn't matter -- the tire's outside diameter, and thus the outer circumference of the tire, should be the same as stock to avoid speedometer errors.

This is why you don't see 14" rims with 30-series tires or 20" rims with 60-series tires on cars (at least ones intended for the street).

[anjapower]

well technically it does since you have to add the wheel size at the end to get the rolling diameter...but yea you're correct.

[bloodzombie]

speedos can't compensate for much


http://www.hero.com/public/schwarzenegger.jpg

[oldschoolimport]

speedos can't compensate for much


http://www.hero.com/public/schwarzenegger.jpgdamn, dude, cut that shit out! :confused:

[mhx]

We should of matched at 80mph on sat night on the way home..

[Lucid Moments]

The way I understand speedometers to work is they track how fast your axle is spinning. When the engineers program the enging management they know the size of the stock tires. In our case a 195/60/15 tire has an overall diameter of 24.21 inches, or a circumference of 76.05 inches. This means that each time the axle makes one full rotation with stock tires your car moves forward 76.05 inches, or that the axle has to turn 859.1 times to move your car a mile.

205/45/16's have an overall diameter of 23.26 inches or a circumference of 73.07 inches. So, when your axle completes one rotation you have only moved 73.07 inches, or 2.98 inches less, or your axle has to make 894.2 revolutions per mile.

Your speedometer will not change so your indicated speed compared to when you hit the fuel cutoff will not change. What does change, is that you will be going slower than what your speedometer indicates. So if your speedometer indicated 82 mph when you hit fuel cutoff you should have only been going about 78.8 mph.

All of this assumes that we live in a perfect world. There will be slight differnces in measurements from brand to brand, and as tires wear and you loose tread the tire gets smaller so that can affect things too. The only truly accurate way to measure your speed is with a set of timing lights (like at the racetrack).

[BarracksSi]

Yup --

The reason why the speedo indicated the same mph for fuel cutoff is because it effectively measures axle rotations per second (or maybe per minute).

The engine speed wasn't any different, so therefore the axle speed wasn't any different.

Now, if we had a speedometer sensor that read ground speed, things would be pretty different.

[Tenacious G]

ah , i see where i was getting confused now. i didn't think about the smaller diameter tire traveling a shorter distance per revolution.

makes sense now. thanks for the explaination.

and thanks, bloodzombie, for the nasty visual.

[BarracksSi]

Oh yeah -- a quick way to reasonably measure speed is by timing how long it takes to go one mile.

Math warning* :D

Say that we drive one mile in 45 seconds, and want to know miles per hour. The expression goes like this, where m is one mile, s is seconds, h is hour, and M is the unknown speed:
m / s = M / h

Algebraically, flip it around to solve for M, and you get:
(h * m) / s = M

We drove one mile, so "m" doesn't matter -- h * 1 = h.

Now, you can't directly divide hours by seconds, they're different units. So, convert that one hour (remember, MPH = miles per single hour) to seconds, which is 3600:

3600 / s = M

And s was measured at 45 seconds, so our result is:
3600 / 45 = 80

80 MPH is the speed of a 45-second flying mile.

SO, REMEMBER THIS:

*** Time one mile, then divide 3600 by that number. ***

If you're really good at starting & stopping your stopwatch (or whatever), you can get a really accurate reading -- one second of error (a LONG distance at highway speeds) isn't even 2 mph different around 80 mph.

** Quick reference: 80 MPH should be 45 seconds over one mile. If it takes you longer, your tires are too small.