Math aficionados in here please! - PowerStrokeNation : Ford Powerstroke Diesel Forum
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post #1 of 38 (permalink) Old 12-08-2008, 09:17 AM Thread Starter
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Math aficionados in here please!

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

2002 F250 Lariat SC 4x2
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- John Wood 4R100, AFE Stage 2, MBRP 4", 1.0 housing, WW, F5, V and B code springs, ITP's OBA and pre-pump kit.
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post #2 of 38 (permalink) Old 12-08-2008, 12:40 PM
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Re: Math aficionados in here please!

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

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Last edited by JonFord; 12-08-2008 at 12:41 PM. Reason: BS
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post #3 of 38 (permalink) Old 12-08-2008, 04:28 PM
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Re: Math aficionados in here please!

looks right.

Dan
96 Powerstroke: 6637, downpipe to dump exhaust, 140v IDM and a shift kit.

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post #4 of 38 (permalink) Old 12-09-2008, 04:59 AM Thread Starter
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Re: Math aficionados in here please!

Quote:
Originally Posted by JonFord View Post
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

Quote:
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

2002 F250 Lariat SC 4x2
- 38R, AC's, MBRP 4", AFE Big Boost, JW valve body, Riiffraff HPX soft line & CCV, ITP's OBA, Live Tuned by Jody

2000 4X4 Excursion
- John Wood 4R100, AFE Stage 2, MBRP 4", 1.0 housing, WW, F5, V and B code springs, ITP's OBA and pre-pump kit.
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post #5 of 38 (permalink) Old 12-09-2008, 05:00 AM
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Re: Math aficionados in here please!

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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post #6 of 38 (permalink) Old 12-09-2008, 05:03 AM
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Re: Math aficionados in here please!

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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post #7 of 38 (permalink) Old 12-09-2008, 05:06 AM Thread Starter
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Re: Math aficionados in here please!

Quote:
Originally Posted by bbbxcursion View Post
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

2002 F250 Lariat SC 4x2
- 38R, AC's, MBRP 4", AFE Big Boost, JW valve body, Riiffraff HPX soft line & CCV, ITP's OBA, Live Tuned by Jody

2000 4X4 Excursion
- John Wood 4R100, AFE Stage 2, MBRP 4", 1.0 housing, WW, F5, V and B code springs, ITP's OBA and pre-pump kit.
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post #8 of 38 (permalink) Old 12-09-2008, 05:07 AM Thread Starter
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Re: Math aficionados in here please!

Quote:
Originally Posted by TANK'S 250 View Post
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

2002 F250 Lariat SC 4x2
- 38R, AC's, MBRP 4", AFE Big Boost, JW valve body, Riiffraff HPX soft line & CCV, ITP's OBA, Live Tuned by Jody

2000 4X4 Excursion
- John Wood 4R100, AFE Stage 2, MBRP 4", 1.0 housing, WW, F5, V and B code springs, ITP's OBA and pre-pump kit.
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post #9 of 38 (permalink) Old 12-09-2008, 05:09 AM
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Re: Math aficionados in here please!

Quote:
Originally Posted by bbbxcursion View Post
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.
yup...we're saying the same thing...
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post #10 of 38 (permalink) Old 12-09-2008, 05:42 AM
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Re: Math aficionados in here please!

Quote:
Originally Posted by Stewart View Post
Mine is 5 MPH off at any speed.

Stewart



......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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