MPG revelation...not in a good way.

I've had my truck almost 12 years and I always used my built in computer to see what mpg I was getting...usually around 15.3.

For the first time I just calculated manually with a calculator.

I was floored, only 12.8 mpg. I couldn't believe it.

I do have over-sized tires, 275/75R 18's.

Now I'm wondering if this somehow throws off my calculations.

 

Is that based on your trucks odometer reading? If so, have your verified it’s correct? I’m willing to bet a six pack it’s not reading correctly. Go for a drive and check it. Mine is off by 5%.

 

It'll throw it off a little. But let me ask you...do you reset the MPG with each fill-up, and did reset it at the fill-up PRIOR TO the fill-up when you did your manual calculations?

 

Tires with a larger circumference will skew your dash readout down. Meaning, actual MPG is higher. And it's higher by the same % that your tire is larger because your truck is using stock size tires to calculate your speed, odometer, and MPG.

You can find what your discrepancy is easily using .

For example, if your factory tires are 275/65R18, and you're running 275/70R18, you can put them into the comparison specs (use stock size as Tire Size 1), and you'll see that the 275/70s are 3.4% larger.

Add 3.4% to your dash readout to correct for the larger tires. Or just multiply your dash readout by 1.034. 15.3 x 1.034 = 15.8

My hand calc and dash readout are always very close. If yours is way off, I would suspect something else is going on.

 

I'm calculating a difference of 6.4% .... 589 revs/mile now vs. 629 revs/mile on OEM tires, a difference of 40 revs/mile or 6.4%.

That said, if the OP doesn't reset the MPG on his display every time he refuels, his comparisons of the two calculations for one refill is meaningless. He's showing a 20% drop in mileage, and that wouldn't be due to tire size alone. The displayed MPG average is calculated since the last reset, not the last refill. That's why I asked what I did. He might have gotten just the 12.8 mpg on this refill, while his display shows the average for however long it's been since the last reset of the display to zero.

 

fuelly app is pretty good. Its free (or can be) and you just enter you odometer reading, gallons, cost per gallon, and if you want the octane and city / highway ratio. I enter the info while tank is filling (I have plenty of time). By doing this at every fill up I have more data points to compare. If its winter gas + extra idle time + only driving around town then I get about 12. If I'm driving 55-65 and not stopping much or changing speeds I'm getting 15+.

I am not sure about using the revs per mile and size vs stock in real world considerations. My 285 / 75 / 17 (claimed 33.9") tires are allegedly nearly 2" bigger than stock, but my speedometer is only off 2 mph at 70. That is the number I use. 72 / 70 = 1.0285714286. If you care to "correct" your odometer then multiply it by whatever your difference in gps vs speedometer speed. For me that correction doesn't really make a difference as a) it gets bad mileage and b) I only track it to ensure I don't see any drastic sudden swings.

 

I just go for a drive and clock the mileage via GPS. Double check by mile markers and you should be pretty exact.

 

Thanks, I'll double check everything and do it again.

I did think the larger tires might give me better mpg, not worse.

Maybe I just made a miscalculation somewhere.

 

Are you running highway tires or AT's? Load range E?

 

See @Terndrerrr post above. You need to add the % difference in tire size to the number of miles traveled, then divide by gallons used. If your average is 12.8 without the correction, you are actually getting around 13.3ish based on adding back the 3.4% difference from stock 32in tire to the new 33in tires.

 

You also need to consider the added weight of the larger tires. That extra torsion required to start turning them requires extra energy.

 

Just filled truck up for the first time and drove it 50 miles. Looking forward to calculating mpg manually to see how accurate the truck computer is. The drive included a lot hills and some stop and go traffic.

 

You hand calculated MPG will be 0.3-0.7 MPG lower than the computer reports after driving at least half a tank's worth.

 

still boggles my mind that anyone who buys a tundra has any complaints about mpg. you brought this upon yourself! 12.8 vs 13.3 oh my god wow an extra 9 miles outta your tank

 

Yep.

I'll follow my gas mileage just out of pure curiosity and scienticious tracking of various changes in driving, hauling, and the vehicle itself. I will never be running a 12" lift and 37's, but that's mostly to preserve the ease of entering the bed (cuz I use it a lot), and reliability of vehicle components.

If you don't need the use of a truck, get rid of it seek optimal fuel mileage. If you DO need (or want) a truck, gotta pay to play!

 

Agreed. I think of it almost like a game, or a puzzle, to try to eeek my MPGs up by a few tenths during my commute.

But… I don’t feel bad at all hitting tow/haul and appeasing my inner child on occasion.

 

It's about the smiles not the miles

 

Are you actually running a 275/75r18. Thats a rare size. Taller then people are thinking. Compared to the gauge you are most likely 5-7% low on the mileage. But depending if you are reseting the gauge when you fill will determine a lot.
I don't just blindly fill as I could pay for a newer vehicle with the difference in mileage. Great example is my father's camping truck. He was using a beat up paid for 1992 Chevy 454 on propane. He felt like he was getting decent mileage. Turned out he was getting 6mpg and drove about 15000 miles a year with that mileage. We found him a newer used version of his truck and he now gets 17mpg and uses the truck more because of it. I get 14mpg in town and 18+ on the highway. Goes down slightly above 70mph as I'm on 35" duratracs