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Never Fry Bacon NAKED!
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367 Posts
Discussion Starter #1
What is the formula to figure the correct MPG if a speedometer is reading 5 MPH faster than the actual speed of a vehicle?

Thanks.

Stewart
 

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An Ancient view on life
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314 Posts
Miles driven=M Fuel used=F
M / F = MPG

Now if at a true 70 mph but showing 75, divide the real mph (70) by shown mph (75). 70/75= .933333333

Then multiply MPH by .9333333 or what ever number you have and wala.

Example

600 miles

11 gallons

600/11=54.54

75 speedometer reading 70 real MPH

70/75=9.33

54.54/.933=50.909 real mpg

Now all that said, you really need to check the miles driven (actual) versus what your odometer is showing. I do that by checking with the freeway mile markers versus that my odometer shows. Not perfect by close.

Typical mpg of my 2003 VW Jetta.

JonFord :ford:

Also you need to know that after 2 years of College, the Dean wouldn't let me go back and I wouldn' go back.
 

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2,448 Posts
:whs: looks right.
 

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Never Fry Bacon NAKED!
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367 Posts
Discussion Starter #4
Miles driven=M Fuel used=F
M / F = MPG

Now if at a true 70 mph but showing 75, divide the real mph (70) by shown mph (75). 70/75= .933333333

Then multiply MPH by .9333333 or what ever number you have and wala.

Example

600 miles

11 gallons

600/11=54.54

75 speedometer reading 70 real MPH

70/75=9.33

54.54/.933=50.909 real mpg
You have a couple mistakes, but I get what your saying.

I'm sure you meant for me to multiply Miles Per Gallon (MPG), not Miles Per Hour (MPH) by .933.

And in your example, I know you meant to post your example as:
54.54 x .933, not 54.54/.933, right?

Just curious though, when I multiply 54.54 by .933, I get 50.88582

Now all that said, you really need to check the miles driven (actual) versus what your odometer is showing.
Why?

I knew my true MPG would be less than the 15.54 MPG I got with my last tank, I just didn't know how to factor in the 5 MPH difference, that's why I asked for the formula so I could get my actual MPG.

I don't understand what your last quoted paragraph has to do with the formula.

Stewart
 

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ARMY bound
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6,990 Posts
If it's reading different because of tires and such, it's a fraction of speed rather than a solid 5mph. Just something to keep in mind.
 

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King Dingaling
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9,386 Posts
you can get close,...but you still won't be accurate.

when your mph is off,...it's not a standard "5 mph"

if you are 5mph off a 70,..then you're about 7mph at 80mph...etc etc etc
 

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Never Fry Bacon NAKED!
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367 Posts
Discussion Starter #7
If it's reading different because of tires and such, it's a fraction of speed rather than a solid 5mph. Just something to keep in mind.
Why would that be? If the difference is from the size of the tire, the size doesn't change with speed, it's constant. Shouldn't the factoring difference be constant also?

Stewart
 

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Never Fry Bacon NAKED!
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367 Posts
Discussion Starter #8
you can get close,...but you still won't be accurate.

when your mph is off,...it's not a standard "5 mph"

if you are 5mph off a 70,..then you're about 7mph at 80mph...etc etc etc
Mine is 5 MPH off at any speed.

Stewart
 

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green member
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297 Posts
Mine is 5 MPH off at any speed.

Stewart



LOL ......rethink what the others have posted and what I quoted.



If you are right your speedo reads 5mph when in Park, if they are right your speedo reads 0 when parked.;)
 

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green member
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297 Posts
He's forgetting something......tire size doesn't change but the number of times they go around and around in an hour does.:doh:
 

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Never Fry Bacon NAKED!
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367 Posts
Discussion Starter #13
He's forgetting something......tire size doesn't change but the number of times they go around and around in an hour does.:doh:
LOL! I guess.

I'll be more specific. My speedo shows 5 MPH faster at 55, 60, 65, and 70 per a calibrated radar unit.

Stewart
 

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Never Fry Bacon NAKED!
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367 Posts
Discussion Starter #14
He's forgetting something......tire size doesn't change but the number of times they go around and around in an hour does.:doh:
Ah, ok...that's the variable with regards to the tire size. That I understand, and is why I questioned it. It didn't just make sense immediately.

Now, I never said the cause was my tires guys. I have no clue what it is. I just got the truck and I'm checking things out as they become apparent to me.

Stewart
 

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Stimulated
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1,430 Posts
Mine is 5 MPH off at any speed.

Stewart
There's only one way this can happen. If you take off your needle and put it back on wrong. That happened to me on my Crown Vic. If you have bigger tires, it will be off by a percentage. Here's how:


31" tire- 31x3.142 = 97.402" circumference

there's 5280 ft in a mile, or 63,360 inches

divide 63,360 by 97.402

that gives you 650.499 revolutions per mile

35" tire = 109.970" circumference = 576.157 rev/mile

650.499/576.157 = 1.129 or 112.9% of the original indicated speed

you are now off by 12.9%, giving you an indicated speed of 79.03mph when your actual speed is 70mph

The same coefficiant, 1.129, would apply to distances as well. Take your indicated distance, multiply by your number, and you have your true miles.
 

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Never Fry Bacon NAKED!
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367 Posts
Discussion Starter #16
I tried telling him....LOLLOLLOLLOL
No, you didn't

"Telling me" implies explaining to me. I asked for an explanation and I get ridicule?

"I tried to tell him"

Please.

Keep your supposed "help" to yourself if you lack the intelligence to explain to someone on the forum a direct question on why something is.

Stewart
 

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King Dingaling
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9,386 Posts
There's only one way this can happen. If you take off your needle and put it back on wrong. That happened to me on my Crown Vic. If you have bigger tires, it will be off by a percentage. Here's how:


31" tire- 31x3.142 = 97.402" circumference

there's 5280 ft in a mile, or 63,360 inches

divide 63,360 by 97.402

that gives you 650.499 revolutions per mile

35" tire = 109.970" circumference = 576.157 rev/mile

650.499/576.157 = 1.129 or 112.9% of the original indicated speed

you are now off by 12.9%, giving you an indicated speed of 79.03mph when your actual speed is 70mph

The same coefficiant, 1.129, would apply to distances as well. Take your indicated distance, multiply by your number, and you have your true miles.


Wow....all that came from you,....your actually a tard in person....:poke:;);)
 

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Never Fry Bacon NAKED!
Joined
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367 Posts
Discussion Starter #18
There's only one way this can happen. If you take off your needle and put it back on wrong. That happened to me on my Crown Vic. If you have bigger tires, it will be off by a percentage. Here's how:


31" tire- 31x3.142 = 97.402" circumference

there's 5280 ft in a mile, or 63,360 inches

divide 63,360 by 97.402

that gives you 650.499 revolutions per mile

35" tire = 109.970" circumference = 576.157 rev/mile

650.499/576.157 = 1.129 or 112.9% of the original indicated speed

you are now off by 12.9%, giving you an indicated speed of 79.03mph when your actual speed is 70mph

The same coefficiant, 1.129, would apply to distances as well. Take your indicated distance, multiply by your number, and you have your true miles.
Thank you Matt for EXPLANING to me why there is a variable factor if the problem is from the tires.

I have no clue why my speedo is off at present time.

I never said it was from my tires guys.

Stewart
 

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King Dingaling
Joined
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9,386 Posts
No, you didn't

"Telling me" implies explaining to me. I asked for an explanation and I get ridicule?

"I tried to tell him"

Please.

Keep your supposed "help" to yourself if you lack the intelligence to explain to someone on the forum a direct question on why something is.


Easy there tiger,....I WAS trying to help,...sorry if you took it any other way.

you can get close,...but you still won't be accurate.

when your mph is off,...it's not a standard "5 mph"

if you are 5mph off a 70,..then you're about 7mph at 80mph...etc etc etc
 
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