# Mileage math?

Car has trip computer. I always get 26mpg on hwy. stock tires are 26.6" I put on a 2nd set of used tires that are 25.6" or 3.7% smaller. I did not and cannot recalibrate my trip computer for different tire size. But now it says I am getting 30mpg at same speed. Approx 60mph or so. My computer seems off? Or am I confused? A 4mpg jump is about a 15% increas in mileage?

Trip computers are not all that reliable at the best of times. Confusing them with different tires makes it worse. Measure your mileage the oldfashioned way. Fill the tank to the first click and drive until close to empty. Then fill up again at the same gas station, same PUMP until the first click. Then calculate miles driven divided by gallons to get mpg.That will give you a reasonable figure for that type of driving.

Then repeat for several tanks.

The mileage computers are NOT ACCURATE. The ONLY way to get an accurate mpg estimate is when you fill out the tank…take the number of miles driven from last fill-up and divide by the number of gallons you put in. I completely ignore my mpg estimator. So should you.

If you change tire diameter, then your odometer will be off as well. If you really want to be accurate, use GPS to get accurate distance and divide by measured gallons. How important is this?

The smaller tires will fool your fuel economy computer as well as the odometer. Run 60 MPH and use a stopwatch and the mile markers to estimate your speed. One minute equals a mile. If your mile takes 65 seconds, you’re doing 55 MPH. You would have to check it with the other tires too, but you will at least know how far off your speedometer is.

@twotone agree. Until miles are registered using gps is a standard in cars, everything is an estimate. So, use your gps direction finder to figure it out.
Mpg estimators are not absolutes but ARE valuable to denote “change”. This could be important or a variety of reasons but is suspect for finding absolute values with the estimated info thrown into the computer by tire rotation sensors.

GPS may not be the answer. It’s known to not always be reliable - for example when driving in cities between tall buildings or when there is heavy cloud cover.

I have owned car for yrs. always get 26 mpg on hwy. right or wrong, never varies. Of course the instant mileage goes up/down depending on gas pedal use. But, I changed tires and instantly, my hwy mileage jumps to 30. Right or wrong. Say it’s wrong with stock tires, it is also wrong with smaller tires? I think the car is really Susceptible to being lowered? Maybe it’s wind resistance?

@Stoveguy Everbody’s highway mileage goes up compared to mixed city and suburban driving. My city mileage is about 35 mph for our two cars while trip highway mileage goes up to 40 at least and 45 when driving below the speed limit. None of this has much to do with the tires, except the tire inflation; low tire pressure robs gas mileage.

Starts and stops absorb a lot of energy, and use more gas. If you drive above 60 mph, gas mileage drops again because the wind resistance goes up sharply.

What has yet to be mentioned is differences in tires.

Even tires of the same size can have HUGE differences in rolling resistance. That alone can account for what is being experienced.

Small tires are max performance summer tires, 50 series sidewall. Orig are all season Michelin passenger tires, 60 series. Car is a heavy caddy. Aren’t all caddys heavy?

Your odometer and I would imagine the trip computer operate on the number of revolutions made by the axle. These are computed for the specific tire the manfacturer specified for the car. The circumfrence of the tire is pi x diameter. You put on tires with a smaller diamteter. Therefore, the distance traveled for one revolution of the axle is less. However, the odometer and trip computer count the revolutions of the axle. When it has counted the number of revolutions to make a mile with the manufacturer’s specified tires, the car with the smaller tires has actually traveled a shorter distance and used less fuel. Therefore, it appears that you are getting better mileage. You have used less fuel, but the distance as measured by the odometer and trip computer are greater.
In actuality, with the smaller tires you are simulating a lower gear and probably getting lower mpg.
I noted the opposite effect when I put oversize tires on an AMC Javelin. The car originally came with 6.95 x 14 tires. I got a good deal on some used 7.75 x 14 tires (money was tight at the time). I didn’t realize how much it threw off the speedometer and odometer. On a trip we averaged 81 mph by the mile markers on the road. At a fuel stop, we had to add a quart of oil which we had never had to do before and the gasoline mileage was about 15 mpg for a 6 cylinder engine thagt had been giving us around 20. It then dawned on me that the tires were making the difference. I thought I was running between 65 and 70 by the speedometer. I was actually going between 85 and 90 mph. We used a watch and the mile markers on the road to figure what the speedometer should be for 60 mph and went from there.
Here is a money making idea–sell kits for automobiles that will “increase the mileage”. The kit will consist of undersized tires and a capacitor device that plugs into the accessory socket to “condition” the electrical voltage in the car. Advertise that both must be installed to recognize better mileage as the plug in device receives a signal from the pressure sensors in the tires; Motorists will be thrilled that their mileage has seemingly improved (when, in reality, it hasn’t changed a bit).

Because your smaller tires have spun more times over each mile thanyour OEM tires that the computer was calibrated for, the computer THINKS you’ve gone farther than you have. At the same time, it’s reading the fuel used, and is determining thta you’ve gone father on a gallon than you actually have.

Hey triedaq, I see it took u 344 words to say my tire diameter is now less than before. No kidding. I said that in my first sentance. So, my tires are 4% smaller but my mpg is 14% better? How come?

Your mileage isn;t really better. Your computer just THINKS it is because it thinks based on the number of times your tranny shaft has spun that you’ve gone farther than you have.

It is still measuring the amount of fuel used correctly, but no longer correctly measuring the distance you’ve traveled.

Arithmatically, it isn’t 1:1. You need to figure out the difference in the real number of rotations of the wheel for a mile’s distance, and adjust the fuel used for that new number of rotations (multiply the mpg indicated by the computer by the old number of rotations per mile divided BY the new number of rotations per mile). That’ll give you your true mpg.

Try the arithmetic. See how you make out.

I have to respectfully disagree that an MPG indicator on a car is not accurate. There may be cases of chronic or erratic figures but speaking solely for my past and previous Lincolns I have found them to be almost dead-on all the time. This has been verified repeatedly for many years by the fill/miles driven method and it’s always within .1 MPG.

The only exception was my prior Lincoln which would take a spell about once a year and go stupid for a day or maybe two. Once it hiccupped it was back to normal.

@ok4450 My experience with MPG indicators is thr same as your experience. The MPG indicator in my 1990 Ford Aerostar Eddie Bauer edition, my 2003 Toyota 4Runner, my 2006 Chevrolet Uplander and my 2011 Toyota Sienna were all accurate. Ironically, the MPG indicator in the Ford Aerostar behaved like the one in your Lincoln. About once a year it would take a spell and then return to normal.
Whether an MPG indicator is accurate or not is not really that important to me. Miles per gallon for me, once I establish a baseline with a vehicle. If the MPG drops below this baseline, I investigate the cause. The MPG indicator gives a good indication between calculations at fillup as to the health of the vehicle. If it really drops, I check things out (tire pressure, engine temperature, etc.)

The only one here that has gotten close to the problem is Pleasedodgevan2. The smaller tires will cause the odometer to record more miles than the old tires did. Therefore your MPG will seem lo figure to be more. Now different tires have different drag coefeciences and so does different tire pressure. I don’t believe you are using a MPG meter on the car, but they can tell you when you are getting your best mileage.

“…So, my tires are 4% smaller but my mpg is 14% better? How come? …”

Difference in rolling resistance.

I think the mistake being made here is that the OP is making two erroneous assumptions.

1. a 4% difference in diameter does NOT equal a 4% difference in mileage. It’s simply a variable in the equation. They are NOT related 1:1.
2. the OP is assuming that the mileage computer is still using an accurate measurement of the miles traveled. It isn’t. It’s now doing its calculations using an erroneous distance measurement, caused by the smaller tires.