I notice my truck appears to get 20mpg. But on closer inspection, I find that the speedometer reads about 5 mph faster than I’m actually going (at 60)…and the odometer reads that I’ve gone 10.4 miles when I’ve really gone 10.
If I get taller tires will the accuracy of my speedometer and my odometer get better?
If I get taller tires, will my measured mpg (as measured by the odometer) get better, worse, or stay the same? (Assume rolling resistance, friction and inertia at starts are negligible, and any differences are only due to the increase in distance traveled by one revolution of the taller tire)
Right now your speedometer/odometer are about 4% off. Getting tires 4% larger in diameter will fix that.
If you do that, you’ll find the speedometer & odometer are now accurate. And, given your assumptions about no other performance changes, your computed gas mileage will improve by the same percentage. I think those assumptions will not hold true however.
Instead of different tires you could take the car to a shop who can adjust the calibration of the speedo/odo to match your current tires. It’s a simple adjustment.
Taller tires will make your speedometer read slower, so I guess if it’s reading high it’d make it more accurate. Your measured MPGs will probably go down, but your actual mileage may or may not go up. Taller tires increase the overall gear ratio which can help with highway mileage, but it also makes the truck taller and less aerodynamic.
On modern vehicles, the speedometer can be recalibrated electronically but it usually requires a special (possibly dealer-only) tool. Most speedometer/odometers read a little high from the factory, which is of course to keep you safe and has nothing to do with inflating MPG figures or making the warranty run out sooner.
Diameter or circumference? A tire that’s 1 inch taller will travel 3.14 inches further for each revolution. A tire who’s tread is 1 inch longer will travel 1 inch further per revolution.
Oddly enough, some quick math shows that a 4% increase in diameter might actually work out to a 4% increase in circumference as well.
It’s not mathematically odd. The circumference is only dependent on diameter, pi is only a constant, so a percentage wise increase in one will give the same increase in the other.
Try thinking of it this way:
I get an annual salary of $ 100 000 (not my actual salary)
But I get paid in Norwegian kroner, so I get paid 600 000 NOK
A raise of a hundred dollars would be a raise of 600 NOK, but a raise of 10% in my “dollar salary” would of course be a raise of 10 % in my “NOK salary”.
You probably find that to be obvious, so all you then have to do is realise that pi and the exchange rate between dollars and NOK function exactly the same. Except one is a universal constant of flat space and the other fluctuates with the world economy.
I can’t help wondering – Your actual gas mileage is going to vary, from one trip to another, by at least 4%, because of wind, air temperature, eccentric driving speed, quality of gas, etc. So why do you care if the speedometer and your economy calculations are not within a 4% margin of error? You already know that it is off by 4%, just account for that in your calculations, instead of going through expensive and maybe fruitless mechanical remedies. Is it important for you to know whether your MPG is 20.0 or 20.8?
I have owned dozens of cars over the years, and the vast majority of them have very similar properties to that describe by the OP. When the speedometer reads 65 mph, you’re actually going about 60-62. When the odometer says you’ve gone 10 miles, you’ve actually gone about 9.6-9.7. In my experience it is very rare to find a speedometer/odometer combination which read accurately. I have NEVER seen one that reads lower than what the car is actually doing.
Buying taller tires won’t fix the problem. Or, rather, it might fix one of the problems but can’t fix them both. If your odometer is reading 4% high and your speedometer is reading 8% high, then you could fix the former by getting tires which are 4% taller but then your speedometer would still be off.
I can think of three reasons why car manufacturers would do this. #1 If the speedometer reads lower than your actual speed, you could easily end up getting speeding tickets when you thought you weren’t speeding, so it’s better to be slightly over than slightly under. #2 If the odometer reads low, it would make it seem like you’re getting worse fuel economy than you actually are, which would make people mad, but we’re okay with the illusion that we’re getting better fuel economy than we actually are. #3 We’re used to it and no company wants to be the first to change the status quo and face the backlash when people complain that this car brand is different from the others.
Bottom line is, if you drive an average car 280 miles in exactly 4 hours and it burns exactly 10 gallons of gasoline (28 miles per gallon), your odometer will say you drove 291 miles, so you’ll calculate your fuel economy as 29.1 miles per gallon. And even though you were going 70 mph during the trip, your speedometer said 75 mph. You thought you were going 10 mph over the speed limit but really you were only going 5 over. So that’s why you didn’t get pulled over.
Oh, by the way, if your car has a fancy display with instant MPG calculation, it probably told you that you were getting 30MPG during the trip.
None of this affects the EPA estimate, which is based on actual performance rather than what the car’s dashboard display tells you.
Cars lie, a little bit. Nearly all of them do it. Most of us are used to it.
Aren’t the estimates on new cars what the manufacturer reports to the EPA? I see Hyundai/Kia are in trouble again for advertising inflated mpg figures.
The odometer and speedometer are always going to be off by the same amount on a digital cluster, and generally also with a mechanical cluster too unless the odometer is outright broken.
The EPA has a standardized test that the car makers administer themselves, although the agency audits a certain number of them randomly or if there’s been complaints.
The way the test works is the car is run on a dynometer while a device meausures the amount of carbon in the exhaust. This measures speed and fuel consumption very precisely, so there’s not much fudge room there, but because it is on a stationary device the car makers have to come up with a conversion factor to account for aerodynamic drag and rolling resistance. In the car maker’s defense, coming up with that number isn’t the easiest thing in the world, although on the other hand I’ve never heard of an EPA audit finding higher than the reported numbers!
I don’t know very much about how mechanical speedometers work, but isn’t the swing of the needle based on an electrical impulse overcoming spring resistance? On the other hand, the speedometer is counting revolutions of the axle, driving gears which have a fixed number of teeth. So I would not find it surprising if one could be accurate while the other isn’t.
Anyway, all I can say is, on every single car I’ve owned, whenever I calculated what % off the speedometer is and what % off the odometer is, I always got different answers. Typically, the speedometer would be 5-10% off but the odometer would be only 2-4% off. Granted, all of these were analog. I’ve never tried it with a digital cluster.
Whether the car manufacturer does the test or the EPA does the test, the fact remains that they are measuring with the dynamometer itself. Whatever the instrument cluster on the dashboard says is irrelevant to the outcome of the test.
I’ve found out that mileage can vary all over the place, dependent on driving conditions, operator technique, outside temperature, and maybe even, it seems, the phase of the moon.
I kept records of every gallon of gas I put into my Ford Contour (1997 model) over a six year span, and found the average mileage to be 34.2 mpg, with a standard deviation of 1.86 mpg. Over that span of time, the minimum was 28.2mpg, and the maximum was 36.2mpg. And several times I had checked the calibration of the speedometer by means of mileage signs on a highway plus a stopwatch so I knew it was accurate
Basically this seems to indicate that a single calculation of miles per gallon, taken by itself, is of not much use.
The aero/rolling drag of a vehicle (“road load”) is assessed through a coast-down test: the car is run up to 70MPH on a relatively flat surface (e.g. test track or runway) and the time required to decel to some predetermined lower speed is measured. From this, the appropriate coefficients are calculated to be used in dynamometer testing.
There’s an industry standard procedure for doing this coast-down test. I don’t recall the details, but it involves multiple passes in both directions, along with an anemometer to measure ambient winds (along with limits on max wind speed allowed during testing). The procedure exists so that repeatable test results can be had, i.e. if two different people test the same vehicle and adhere to that standard procedure, they should get pretty comparable results. When EPA did their own coast-down testing and came up with very different values from Hyundai, a detailed review of Hyundai’s test records showed that they had totally disregarded this procedure during their coast-down testing.
