Calculating mileage with new tires (MATH!)

[tiderfish]

Greetings, I have changed the rims of my ZX2. I know the new tires are slightly less in diameter. They are from a 2004 Miata. I want to know how to adjust my mileage to account for the faster tires. Here are the stats:


Stock Tire - 185/60R15
Section Width: 7.28 in 185 mm
Rim Diameter: 15 in 381 mm
Rim Width Range: 5 - 6.5 in
Overall Diameter: 23.74 in 602.99 mm
Sidewall Height: 4.37 in 110.99 mm
Radius: 11.87 in 301.49 mm
Circumference: 74.58 in 1894.3 mm
Revs per Mile: 876.1
Actual Speed: 60 mph 100 km/h

NEW Tire - 205/50R16
Section Width: 8.07 in 205 mm
Rim Diameter: 16 in 406.4 mm
Rim Width Range: 5.5 - 7.5 in
Overall Diameter: 24.07 in 611.37 mm
Sidewall Height: 4.03 in 102.36 mm
Radius: 12.03 in 305.56 mm
Circumference: 75.61 in 1920.4 mm
Revs per Mile: 864.1
Speedometer1: 59.1 mph 98.6 km/h

Speedometer Difference: - Speedometer Difference: 1.388% too slow
Diameter Difference: - Diameter Difference: 1.38%

So do I just divide miles traveled by 1.388%

My last tank was 294.7 miles. When I divide that by 1.388, I get 212miles. That seems way too low.

Any thoughts?

[ProDarwin]

Multiply miles traveled by (24.07/23.74)

[tortoise]

Originally Posted by tiderfish

I know the new tires are slightly less in diameter.

Stock Tire - 185/60R15
Radius: 11.87 in 301.49 mm


NEW Tire - 205/50R16
Radius: 12.03 in 305.56 mm

To start with you show the new tire as larger in diameter. When we straighten that out, we can go on from there.

[tiderfish]

Thanks for the quick reply.

So it goes:
Odometer Miles: * (24.07/23.74) = Adjusted Miles / Gallons used = Adjusted MPG
294.7 Miles on ODO * (1.0139) = 298.8 Miles / 9.87 Gallons = 30.27 Adjusted MPG


Is this correct? If so it's great. I can't believe I got that good of mileage, since I have been driving a lot of stop and go.

[tortoise]

Originally Posted by ProDarwin

Multiply miles traveled by (24.07/23.74)

You mean multiply miles indicated on odometer?

[NeilBlanchard]

You must measure actual miles driven somehow -- the easiest way is to drive on a highway that has markers every 10th of a mile, and reset the Trip (B?) odometer at an even mile marker. Drive for at least 20 miles (as far as you can) and compare the Trip odometer to the measured distance. Add (or subtract) a percentage to the Trip odometer to get you close to the measured distance; erring on the conservative side.

For example, if you drove 25 measured miles, and the trip odometer only read 23.8 miles, you need to add 5% to get to 24.99 miles.

Another way to measure the actual distance you drive is use a GPS, though this sometimes doesn't account for elevation changes and might be off a bit. A third way to measure the actual distance is to use Google maps for directions to go between 2 points, and then drive that specific route, and measure it on the trip odometer.

You cannot depend on specs of the tires as the tire may be worn, or you may be running them at a higher pressure, etc.

[gone-ot]

Originally Posted by NeilBlanchard

...You cannot depend on specs of the tires as the tire may be worn, or you may be running them at a higher pressure, etc.

...another caveat to be aware of is that *if/when* you attempt to mathematically calculate the distance traveled during each tire rotation -- because of tire deflection (roughly about 0.97) under load -- you'll aways end-up getting a number that's slightly too high. In fact, even the 0.97 number is only a rough approximation, which varies somewhat with manufacturer, tire construction, tire diameter, tire loading, tire inflation pressure and tire temperature.

...you can visualize this tire deflection as the slight "flat spot" you see where the tire is in contact with the pavement.

[tiderfish]

All good points. I thank you for bringing them up. I will try to get some real-world numbers using Google maps, and my gps. I have several Android apps that will track distances, but I never considered it may not account for elevation changes. I know the one called My Tracks by Google will tell you the elevation change, but I am not sure if it relates to distance traveled.

[tiderfish]

Originally Posted by tiderfish

Thanks for the quick reply.
Odometer Miles: * (24.07/23.74) = Adjusted Miles / Gallons used = Adjusted MPG
294.7 Miles on ODO * (1.0139) = 298.8 Miles / 9.87 Gallons = 30.27 Adjusted MPG

Just to tide me over, are my above calculations correct? At least in a idealized world?

[gone-ot]

tire vs. MPH GUIDELINE:

...a LARGER diameter tire will UNDER report odometer distance and MPH, so the "correction" is to multiply by number greater than 1.0:1.

...a SMALLER diameter tire will OVER report odometer distance and MPH, so the "correction" is to multiply by a number less than 1.0:1.