Help do the MPG Math

Not another crying and throwing a fit my MPG sucks thread.

I recently moved up to 285/70/17 on a 2.5" Eibach lift and a three pack AAL with the Overload in place. And for those that will ask, minor trimming and no rubbing on my mostly pavement life.

Real question is trying to figure out a ball park MPG so I can plan for a road trip.

Is it as easy as this:

(displayed Miles since fill up + 10% that I am running slow)/ gallons of fuel used = Approx MPG

Pic for looks.. Because I like how it looks.

 

That is it, but the main question is how do you know how many gallons were used?

My math is generally:

21.1gal - ( Trip Meter A / Displayed MPG ) = Fuel Remaining

From there you can plan the fuel stop using the MPGs thus far or base it on the DTE showing plus a little as it is conservative.

I would not add in the speedo correction as the ECU is measuring the distance and the fuel flow. I.E., both the displayed MPGs and Trip Meter have the same built in speedo error.

 

Take the actual circumference of the new tire/divided by the circumference of the old tire, multiply this by the measured miles and then divide by fill up gallons.

for me:
old 265/70r16 GY Wrangler = 30.6*pi = 96.133 in.
new 265/75r16 GY Wildpeak= 31.65*pi = 99.431 in.

[(99.431/96.133)-1]*100%=+3.43%

Hypothetically,
Say my odometer said I went 300 miles on 15.5 gallons after fill-up (19.35 mpg), actual mileage is (1.0343*300)=310.29 (20.01 mpg), in reality the new mpg will decrease because to go the measured 300 miles, you will burn more fuel than the 15.5

 

Gallons used = Gallons pumped.

Not trying to get exact, but rather conservatively guess how much fuel I may use to drive 3000 miles.
May be cheaper to fly and rent a car.

 

What did you get before the lift? Also have you had that topper for awhile? You have added weight, unsprung I might add, in addition to the increased drag caused by lift. Take what mpg you had before and use .8 as a multiplier. That will get you conservatively close. 10-20% efficiency penalty.

 

Here's how I do mine.......when it gets towards empty, I fill it back up

 

Ah! You're talking about your available range, not computing MPGs. From that, you need a basis of more than one trip or a current tank of fuel. I log fill ups and have a separate chart that overlays each year to see comparisons and trends, which also includes the speedo correction for the new tires. I got the correction by comparing a GPS measured distance going down a more or less straight freeway for several miles compared to a Trip B distance. I wish I had done this with the Stock tires.

From that, I know that I will get at least 21mpg for conservative trip planning.


For me these days, I'd rather drive than fly if the time is available. More enjoyable and less screwy stuff going on.

 

Exactly lol. I left for Vegas on March 2nd, returned on March 16th. The whole world changed in that time period. Took four days to get there and three coming back, although I did set a personal record of 964 miles in one day. Most bizarre thing was, we had a tough time finding a room at hotels on the way out. Coming back, we were one of the few in the whole hotel.

 

Also to throw a wrench in your work, tire wear, esp for deep tread ATs / MTs, will have a somewhat significant effect over time. Approximately 3%.

 

fuel cost calculator for road trips, play with the numbers

https://www.gasbuddy.com/tripcostcalculator

 

Oof! That had to be surreal.


Given our sparse office situation, coming back in on the earlier side of things, it was killer commutes for about a month. Just the road and me.


Back in the day, my wife and I did some sprints from PA to here mainly due to no vaca time available. About 2,500mi in 2.5days. We enjoyed as much of it as we could, esp the kitch tourist trap stuff, road signs, and sights. One of those trips was in a Metro 5MT with no CC and tires that were out of balance over 70mph due to the shittastic hub caps. The thought of taking them off only occurred to me once we got here.

That $50.00 trip was fun though.

 

Go sign up on Fuelly, log your fuel ups, it gives you all sorts of neato stats;

 

I wish they would have a few more. Esp the year to year overlay that I mentioned and sorting the Taco categories better. They have the engines all confused with no transmission choice. Still a great site.

Here's my own set up using the CSV output from fuelly.com.

 

This right here. Don't use what the truck says, use the same pump several times to fill up at, and don't go past where it clicks off. Fill it on the "slow" setting. Calculate mileage each time using the diameter difference of the tires.

 

I could dump the csv out of my Scangauge as well..

 

It depends on the actual size of your tires before and after, but if you went from 265/65-17 (30.56") to 285/70/17 (32.71") and did not recalibrate your speedo, then adjusted fuel economy should be about 7% (32.71/30.56) higher than reported by the truck.

 

I will take that MPG all day long, truck is reading 16.4 average per tank, over last 5 tanks. Using the formula above 7% higher than reported.. 16.4*1.07=17.44
Heck yes, should of went bigger... haha

 

Hello All,

thanks for the response. Flew down this time because of all of the fires along the west coast. Next trip will be driving, and hopefully with the calibration done before.