Figuring out your Actual MPG

I searched to see if this was posted and couldn't find anything. My math could be wrong, so post any corrections you may see.

I do the math almost every fill up, and defiantly a few times after any maintenance to see if their is an improvement. If you have any tire size other than stock, your speedometer and mileage will be off.

So here is how you figure out your actual MPG using information from the Tire Size Calculator. I am going to use my variables as an example.


from the online tire size calculator

As you can see from the "Revolutions per mile" there is a -6.6%, That means your traveling farther and the wheel is rotating less to go that distance.


from the tire calculator

If you do the math on Speed Display VS Actual Speed, you come up with the following.

20 * 100 / 21.4 = 93.45
100 - 93.45 = 6.55 . (Almost 6.6)

To figure out your MPG, you need to do the following.

If your Trip meter displays 260 and you put in 17 gallons of gas, your MPG is 15.29mpg. (260/17=15.29)

Now taking those same numbers and do the following math based on info from the tire calculator (6.6%), you get;
260*1.066=277.16/17=16.30mpg
(Miles*Miles+6.6 Percent=actual Milage)

A difference of 1.01mpg more that your actual trip meter/speedometer will show.

I know its not much, but I feel a little better know I am getting 1 more mile out of a gallon.

 

Cool... now, please tell me why the stock 265/70-16 tires that come with the Off Road TRD 4WD Tacomas are giving 'wrong' mileages? My GPS (I assume is balls on accurate) says I am going slower than my speedometer reads. ie. When my speedo is around 68 mph, my GPS is showing 65 mph... If I put a tire that is an inch taller on, will that correct the too fast speedo?

 

What about stock tire weight and new tire weight?

 

Sure weight affects MPG, more rotating mass = more fuel to get it rotating.

All I am showing is how to see your actual mileage based on stock calibration speedo and non stock tires vs new amount of actual rotation.

 

I would try more than one GPS unit, and if it is still an issue, take it to the dealer and have it calibrated, or you could try going up tire size. I am no expert, so I am not giving any advice. Just showing some math formulas.

 

Ok, I see what your saying, great point!

 

I have used three different GPS units and have had 3 Tacoma Off Road TRDs... all three the same... about 3 mph off. The speedo says 68 when the GPS says 65. I guess that is a 5% error. So, I guess I need to subtract 5% from my odometer for true miles traveled... which makes the gas mileage figure worse... oh joy.

The stock OR TRD tire (265/70-16) is 30.6" in diameter.
A 275/70-16 tire is 31.2" (1/2" taller).
A 265/75-16 tire is 31.6" (1" taller), adds 1/2" ground clearance.
A 275/75-16 tire is 32.2" (1 1/2" taller), adding 3/4" ground clearance.

What I think I would like to know is what tire will slow down the speedo 5%? Good thing is that it will be a taller tire!

 

Really... ?! How (and why) are the two not connected?

 

I thought we were discussing truck mileage; 2WD ass dragger is not a truck!

 

The odometer should be perfectly accurite. (unless you have different tires) The odometer takes the digital signal and converts it into miles traveled. The speedo takes the digital signal converts it to analog which slightly distorts it. Generally they try to make the distortion to the drivers advantage so they don't get tickets. So a Speedo reading 68 and a GPS reading 65 is normal. Mine is off by much more than that.

 

The speedometer is an analog representation, so that alone has some fudge margin.

In addition, speedometers are set back a certain percent to comply with UN regulations concerning speedometer settings (speedo must not show slower than the actual speed). Since the Tacomo has several stock tire sizes, the speedos were calibrated for the largest tire size minus x% due to other factors (like inflation, tire wear, atmospheric pressure, etc). The one hard rule is that if someone is going the speed on their speedometer, they should never be guilty of speeding, so it is calibrated to show a faster than true speed.

The UN allowed margin of error above speed is 10% + 4km/h. The US allowed margin of error looks to be 5%.

So, to answer your question, the speedometer is purposfully showing a little slow in order to comply with UN regulations, while the ECM is receiving/recording a closer to true speed to track distance. Now with larger tires, it throws things off a bit more, as described earlier in this thread.

 

GPS systems used today can be off by up to 15 ft at any given time. Extrapolate that error percentage out over the distance of your trial...

 

The math says 1.4+ Diameter tire would show correct display. (give or take for proper inflation)
68 x 100 / 65 = 104.61

so if a stock tire size is 30.6, you want to increase by 4.6%.
30.6 * 1.046 = 32.0076

I am not an expert, a math wiz, or a super mechanic, so please check with others before taking anything I type as 100% true and correct.

 

Stop already dammit!! Math is faulty. Everybody has failed to factor in the coriolis effect/force. Remove your pencils from your pocket protectors and start over.....geeeeez!

 

The odo is off by almost 4% on 4cyl 4X4 Access Cabs. Toyota is aware, they have acknowledged it, and say it is within tolerance. My 2010 OR is off by 1%.
The speedo on the 4cyl was off by 6.5%, the speedo on my OR is off by 2.5%

 

GPPS may be off by 15 feet at any one time, but that is a 15 foot radius, over time it will almost all cancel out, so the error won't be cumulative, it will stay within 15 feet over a trip of any distance.

 

I am talking about a GPS measuring speed, not finding a geocache hidden in the brush.

 

My MPG progressively with new tires:

265/65/17 - 18
265/70/17 - 16
285/70/17 - 14