Calculate actual gas mileage when your speedo is wrong

[converted_to_datsun]

Hey so I know most of us have speedo's that don't read correct which makes your odometer off as well making your gas mileage calculation mean nothing.

 

I got tired of not really knowing my mpg's so I made up a way to calculate accurate mileage.

 

First you find a speed on your speedo I chose 55

 

Then you find your actual speed when it reads the speed you chose with one of those speed signs or a gps. Mine was 65

 

Then you divide your actual speed by your indicated speed 65/55= 1.18

 

Then when you calculate your total number of miles you times it by that number to get your actual number of miles

 

I thought I got 20.62 mpg but I actually got 24.28 mpg! Much better number!!

 

Anyway thought I'd share a solution to the problem allot of us have, hope this helps some of you

[herculesinwyoming]

GPS is what i have used, even on a brand new car they can be off a tiny bit.

[converted_to_datsun]

Yea I've only had a couple cars where the speedo was correct. Just got tired of not knowing my gas mileage haha I'd just use the gps to calculate but it'd have to record every mile I drove to work and that would suck. Now I know a simple number to times by to figure it out every time

[Dawa]

i just fill up my tank (approx 12 gallons)

and at around 250 miles or so i refill,

see how many gallons it takes to refill (9-10)

subtract that from 12, and then divide the number of miles ive driven by how many gallons

 

example 250 miles traveled using 10 gallons 250/10=25mpg

 

my speedo is FUCT. i have nooo idea how fast im going, i just cruise with traffic.

[bonvo]

i just fill up my tank (approx 12 gallons)

and at around 250 miles or so i refill,

see how many gallons it takes to refill (9-10)

subtract that from 12, and then divide the number of miles ive driven by how many gallons

 

example 250 miles traveled using 10 gallons 250/10=25mpg

 

my speedo is FUCT. i have nooo idea how fast im going, i just cruise with traffic.

 

how do you know how many miles you have travled its your speedo and therefor your odometer are off/ not reading?

[Dawa]

^ oh yeah, i forgot about that part (youre completely right.)

 

i had used the same method before i changed my wheel/tire combo (which is what now throws off my speedo)

and the numbers/mileage/mpg are fairly similar.

[converted_to_datsun]

Well you Calculate the miles normal like you usually would but then you times it by a number to correct for the percent your mileage is off to get the correct number of miles traveled.

 

I don't know why mine is off cause the tires are really close to stock and the rear is 4.11 gears like factory... Tranny is unknown tho but I'm doing a 5 speed swap that I know has the matching gears for my rear end

[ggzilla]

Make sure you calculate based on ODOMETER error, and not speedometer error. Usually the error is the same amount, but not always.

[converted_to_datsun]

All runs off of the speedo cable so should all be off by the same amount. I only used speed to find the percent of error so it transfers over well. Pluss I know in my truck I should get around 25 mpg's and I calculated 24.32 which is close enough to tell me I got it right

[ggzilla]

NOT a correct conclusion

All runs off of the speedo cable so should all be off by the same amount.

Check your odometer to be sure. The speedometer use a magnetic coupling so is not directly geared to the cable like the odometer.

[converted_to_datsun]

I'll mess with it at some point if the 5 speed swap doesn't correct my speed. ( 5 speed is from same parts truck as my new rear end)

[darrel]

Use the interstate mileage markers. 60 mph for time, distance and speedometer error.

 

I was always amazed after getting a new watch how accurate the odomEter was in the '68 ford wagon going to the lake and back. I must have been about 7 at the time.