Hi everyone,

On my truck I have a problem with my spedometor. For example, it says I am going 30 mph when really, I am only going 26 mph. I know this because in my town we sometime have those cop spedometors on the side of the road saying your speed. I know this is accuarate because I have tested this in a couple different towns.

Anyways, I was wondering how you figure out my milage. Is there like a way to figure out what percentage slower I am really going. When I calculate my milage at the pump it says I am getting about 20 mpg when I know I am not getting that good becasue it is saying that I went more miles than I really did. Anyone help me with this?

Thanks alot

 

Well, if you want to keep it simple, just take your mileage multiplied by the difference.

26 / 30 = 0.8666...
.8666... x 20 = 17.333....

So you got a little over 17 miles per gallon. Someone correct me if my math/logic is mistaken.

EDIT: Of course, you can make it even more accurate by taking the miles times .8666 and then dividing by the gallons of gas used.

 

So what about when I'm going 60 mph. I am really still only going 56 mph right? Because if you do that same math problem, it comes out to be i think 0.93333 or somerthing like that. So if i did the rest of the math then it would come out as different milage right? I'm not sure, I'm completely lost.

I think yesterday I had like 96 miles on the trip, and put in 4.2 gallons I think. That comes out to almost 23 mpg. So if i took the .933 would that still be accurate? I just did it and it told me a bit over 21 mpg.

I dont need to be too accurate, I'm just curious how many miles to the gallon less I am really getting. I know it cant be too too much less. Maybe from like 1-3 mpg less I would think. But thats what I'm trying to figure out.

 

No. If you were doing 26 and the speedo said 30, that means when the speedo says 60, you are doing 52. Its all about percentages.

Your speedo is off by almost 13 and 1/3 percent, roughly, according to the numbers you gave me.

96 miles x .8666... = 83.2
83.2 miles / 4.2 gallons = 19.8 MPG

Still really good for the 5.2.

 

Even if you don't take your truck in and have it fixed, you can figure your dollars per mile. How much does it cost you to go x amount of miles? Go to gasbuddy.com and sign up for free. Start keeping a fuel log. You will probably find you truck is not costing you as much as you think.

 

wow!! how did you do that?... how fast were you going.40 mph in OD?..lol

 

I usually take mine down to tire revolutions.

my truck is callibrated for 245's which are 30.5"

circumfrence= 30.5(pi)=95.8"
100mi=528000'=6336000"/95.8"=66137.79 rev's

I have 285's on there which are 32.8"

circumfrence=32.8(pi)=103.04"

103.04"(66137.79 revs)=6814837.88"/12=567903.16'/5280=107.56mi

So for every 100mi my odometer says I travel, I actually travel ~107.5mi

It works out roughlyto a % where tire diameter is concerned.

 

Yeah I thought it had something to do with percentages. I just didnt know how to figure it out.Oh and oops...I forgot to mention. This is not on my dodge, this is on my other rig, my ranger [sm=smiley26.gif] Thats ok though, dodge, they dont ever have problems.

Thank alot for all the help!

 

Oh and also, doI always use the 0.866 when trying to figure out my milage for now on? That number will always be the same right? Thats basically the percentage right?

 

Unless you recalibrate your speedo or change your tire size, yes, it should be the same.