For you math type people

[Tiggeroo]

I drive a mustang gt. We've been having some gas consumption issues with it and had some work done. But we've been monitoring the mileage and gas useage.
While doing this things seemed odd. We found that when we drive a mile we are really only going 8/10ths of a mile, maybe less. Also if I ride beside my husband and he is going one speed my speedometer is quite a bit different. Now for the math stuff. I don't know if i'm going to put it in the shop over this so...
Does my car actually have less miles on it then it says? Is my actual gas mileage better or worst. Are my two 19yo sons telling the truth when they say they've been pulled over and clocked going faster then the speedometer says?
Can the wrong sized tires, smaller or larger do this to a car?
We've suspected the speed thing for awhile so it's likely been doing this since I bought it a year ago.

 

[Lisa loves Pooh]

Your tires are not the right size of the car.

I'm blanking on which is which.

Your car will make a set number of revolutions at a given speed to turn the wheel. A bigger tire covers more surface area in those revolutions than smaller tire.

I may have it backwards..

but sounds like your tires may be too small (less surface area covered in 10 revolutions for example).

It's late, I'm pregnant...so that may be backwards--but your tires are definitely the wrong size.

ETA: Yes it affects the odometer I believe...I'm not sure it impacts gas milage--that is correctable just by having the right tires. The odometer reading doesn't match what the engine effort has been.

 

[Lisa loves Pooh]

NOt that it helps you--but if you have the values to enter..here's a calculator to help you figure out proper tire size. No idea where you get the data to enter though.

http://www.csgnetwork.com/multirpmtirecalc.html

You could probably play around with one of the values to determine the impact on speed. Not sure which as I don't know much about cars other than what my physics teacher taught us about the wrong tire size on a vehicle.

Another calculator that might be more user friendly:

http://www.net-comber.com/tirecalc.html

Something else I found on some driving law website. Not sure what "one tire size" means--but found the delta important:

One tire size may increase speed 5%. (i.e. 55mph + 5% =
57mph)

 

[Tiggeroo]

I'm not wondering if it affects actual gas mileage, just my calculation based on miles traveled and gallons used. My tires might be a bit smaller then the suggested size. I"m not sure but i'm getting new tires this weekend and an allignment. I'm just wondering how this situation affects other calculations. I had a Jeep wrangler for awhile and it's common to put bigger tires on them. I knew this affected things, I just wasn't sure in which direction.

 

[Aidensmom]

Are you saying that you independently measured the mileage to be 8/10, but your car says its a mile? If so, then yes, your actualy mileage is less than what it says, but it really doesn't matter at this point because there is no way to prove what it really is.

I am thinking you should take it in to make sure everything is calibrated right.

 

[Tiggeroo]

yep we independently measured the mileage based on road side markers, several times. It comes up a minimum of 2/10ths of a mile short each mile. I'm going to have it looked at but i'm putting the new tires on it first to see if that fixes it.

 

[Lisa loves Pooh]

HEre's a physics explanation--they eventually reach the point--but someone posted a somewhat similar question and he eventually received a satisfactory answer..
http://www.physicsforums.com/archive/index.php/t-3071.html

I like the cart explanation.


I would plot your and your hubbies car side by side if possible. Start nose to nose---go a small distance at a lower speed (will be hard to calibrate and won't be perfect)...see who goes farther at the same speed.

If you have the proper tires on your car, it will make relatively little difference.

Acceleration would be the only problem though.

 

[Tiggeroo]

ok that answer made my head hurt. I have absolutely no math skills and physics, well hahaha. There's a reason i'm a history major. That's why i posted. I can't figure these things out for myself.

 

[Mysteria]

Are my two 19yo sons telling the truth when they say they've been pulled over and clocked going faster then the speedometer says?

There may be a billion different things wrong with your car but I'll bet my life the answer to this question is no, they aren't telling the truth.

Get it in and have it checked at a reputable Ford dealer. PM me of you have further problems. My cousin shows and races Mustangs and could likely come up with a reasonable explanation for you.

Myst

 

[Lisa loves Pooh]

ok that answer made my head hurt. I have absolutely no math skills and physics, well hahaha. There's a reason i'm a history major. That's why i posted. I can't figure these things out for myself.

Take two buttons (one large and one small)--roll them with your fingers..stat at the tip and stop when run reaches the base. Do them at the same pace.

The bigger button will reach the bottom of the finger first. It covers more distance with less rotation. Even at the same speed.

Gears are a fun thing to look at as well. All at the same pace--but the bigger ones take longer to get one full revolution and the smaller ones are speeding on through like no tomorrow..but they are all traveling at the same distance.



I know it is confusing..but it sounds like your wheels/tires are too small.

Of course that is IF the answer is that simple.

The only other thing I found on line was about underinflated tires.

 

[salmoneous]

In case the math part of this isn't clear so far...

What I hear you saying is that when your car *thinks* it has gone 1 mile, it has actually only traveled 8/10 of a mile. If that's true, then both your car, and your sons, are in even worse trouble that you thought.

After buring one gallon of gas, the car thinks it has gone, say, 19 miles. It thinks it is getting 19 miles to the gallon. But, in reality, it's only gone 15 miles. So you really only got 15 miles per gallon. Your actual MPG is less that what the car is telling you.

When it comes to the speedometer, after driving an hour, the car thinks it has gone 60 miles, so it tells you your speeed is 60 MPH. But it really only went 48 miles, so it tells your actual speed is 48 MPH. Taking the flip side of this... if the police radar correctly clocked the car going 60 MPH, the speedometer inside the car would have been telling your boys they were driving 75 MPH.

 

[Tiggeroo]

no when I check my odometer and drive next to the mile markers I travel what is one mile according to the mile markers at this point my car odometer indicates it has traveled at the most 8/10ths of a mile and that's being generous. I definitely don't have tires that are too big on it.

 

[Lisa loves Pooh]

In case the math part of this isn't clear so far...

What I hear you saying is that when your car *thinks* it has gone 1 mile, it has actually only traveled 8/10 of a mile. If that's true, then both your car, and your sons, are in even worse trouble that you thought.

After buring one gallon of gas, the car thinks it has gone, say, 19 miles. It thinks it is getting 19 miles to the gallon. But, in reality, it's only gone 15 miles. So you really only got 15 miles per gallon. Your actual MPG is less that what the car is telling you.

When it comes to the speedometer, after driving an hour, the car thinks it has gone 60 miles, so it tells you your speeed is 60 MPH. But it really only went 48 miles, so it tells your actual speed is 48 MPH. Taking the flip side of this... if the police radar correctly clocked the car going 60 MPH, the speedometer inside the car would have been telling your boys they were driving 75 MPH.

Great explanation...my head knew..but I just coudln't explain it right.

 

[salmoneous]

no when I check my odometer and drive next to the mile markers I travel what is one mile according to the mile markers at this point my car odometer indicates it has traveled at the most 8/10ths of a mile and that's being generous. I definitely don't have tires that are too big on it.

Then it is exactly the opposite of what I said. If the car thinks it is getting 15 MPG, it is really getting 19 MPG. And when the policy radar clocks the car at 75 MPH, the speedometer would have only read 60 MPH.