How to calculate MPG when tire size changes

you have Torque right? There is a setting under vehicle profile where you can trim wheel diameter size (OBD Speed Multiplier). Create a dash with GPS and OBD speeds side-by-side and trim until they are close. The trim should trickle down through all your other distance-based calculations in the app (MPG, distance traveled, etc).

premature response. let me re-read the OP and think

 

It's all based on tire circumference and the ratio between OEM and your new tires.

The outer diameter of the EnergySaver tires is 25.0" - Circumference is 78.54"
The outer diameter of the PureContact tires is 25.1" - Circumference is 78.85"


78.54/78.85 = 0.996

Think of it this way - the error is less than 0.4%

If you are traveling at 60mph (GPS speed), and the OBD says (hypothetical) 60mph with the 15" EnergySavers, then theoretically the OBD will display 59.76mph* with the 17" PureContact tires (60*0.996).

*I believe when tire circumference goes up, displayed distance traveled goes down. If I'm wrong, it will be 60.24mph (60*1.004).

The cirumference of the tires is so close that the change in distance traveled per tire revolution is negligible.

Another way to look at this is lifetime car distance traveled. Imagine traveling 100,000 miles (GPS) before your car craps out. With the OEM 15" tires the display will show 100% correct 100,000 miles, but the PureContact tires will only be off by +/- 400 miles. It's negligible.

 

And to answer your original question, distance traveled at the obd is 99.999% likely calculated by wheel clicks, and not by another means such as an on-board GPS unit somewhere in the ecu, which is pretty much the only other way to do it.

Wheel clicks meaning wheel revolutions. The Ecu tracks wheel revolutions and multiplies that by the known oem tire circumference. Or, it adds 1/10 of a mile for every x tire revolutions.

 

One thing to keep in mind is that speedos on all cars are going to be conservatively set so they do not underestimate speed. If you calculate based on "true" speed and distance, your numbers won't be comparable to all other cars where MPGs are based on their speedos (odometers).

I think most cars will "overreport" MPGs by about 2%. The Prius V (5) overreports by more than 5%, but I don't know that it can all be attributed to the smaller OEM tire. If you "correct" the tire so that you match the OEM tires on the 15" wheels, there may also be other "corrections" to make. The Toyo A20 Proxes (215/45R17) OEMs for my 2010 Prius V had 843 revolutions per mile (RPM). I can't find the RPMs for the new Conti PureContacts F8L put on his 17" wheels show, but the Conti ProContact tires in the 205/50R17 show 832 RPM. The 15" OEM Yokohama AVID S33D show 829 RPM. According to these numbers, the Toyo tires would "overreport" by 1.7% vs. the AVIDs.

Looking at a 500 mile tank (reality, as opposed to what is reported by the speedo/odometer), the AVID will report 500 miles after 414,500 revolutions. If you were to assume the speedo/odometer in that situation actually had reported true for 500 miles, and you then replaced the wheel/tire to use the Toyo A20 Proxes, at 843 revolutions per mile, what difference would you find? Once the speedo/odometer reported 500 miles, the wheels/tires would have revolved 414,500 times, but each revolution would have traveled 6.26 feet (5280 feet divided by 843 RPM), as opposed to the 6.34 feet for each revolution on the AVIDs. Assuming the car actually traveled 500 miles on the AVIDs, it would have traveled only 491 miles on the Toyos.

I'm hesistant to take the math too much further, because the tires have difference performance characteristics, and there is no way to assume the HSD would have used the same amount of power to get the same number of revolutions on each tire. All it shows is that you have an overreporting of distance traveled for the OEM 17s vs. the OEM 15s, and that the overreporting is about 1.7%.

 

Yes, but the speedometer and the odometer are two separate instruments. The speedometer indicates speed a bit high on purpose, to discourage speeding. The odometer should be reasonably accurate, with the usual tire size. There's a lot riding on that accuracy, warranty limits for one.

Our '06 Honda Civic Hybrid underestimated mileage, typically by about 1~2%.

To the OP:

If (as an example) you put on a tire with a circumference that is 1% greater than stock, on any trip you will travel 1% further than the car (and it's odometer) thinks you travelled. When calculating mileage you would need to multiply any odometer reading by 1.01.

 

I didn't realize this. It is hard to believe there is a reason to have two different distance measurements taken, but I could see the car companies making one read differently simply to make certain the car did not under-report its speed.

Clearly, I've simply made the assumption the speedometer and odometer are linked because of the distance reading.

 

The do use the same data source. The speedometer reads high because the computer makes it read high

 

That's what I was thinking. why add the cost of a second instrument when you could do a simple X*Y or X-Y calculation of the distance instrument value to reduce the displayed speed?

 

Remember, if the distance reads high, the MPGs will read high and the warranty will end sooner. Both good for the car company!

 

That has been done before and the gumnt has spanked the car companies' peepees for doing it. The car companies tend to be pretty honest on odometer readings these days, so not so good for the car companies.

 

But, I bet you they are at the very top of the allowable variance. There is a lot of engineering that goes into that!

 

Yeah, that's a more accurate description.

 

How about over-inflation? Are the different tires inflated the same?

 

did you test the GPS vs OBD speed difference on the AVIDS and EnergySavers at the end of their useful life? vs the Contis which you're using now that are brand new?

how much does tread loss reduce tire diameter over the life of the tire? maybe 1/4 inch? haven't done any math yet, but that's probably a measurable difference.

 

1/4 inch is a good number for radius. New tread depth on tires runs about 10/32 inch, common recommendation is to not run them below 2/32 which = 8/32 = 1/4 inch of allowable wear so diameter is reduced by 0.5 inches on aroughly 24 inch diameter, or a 2% reduction in diameter when worn out.

Unfortunately for us calculators, turns per mile isn't a direct function of tire diameter, it's closer to the diameter of the tire at the steel belts so wear doesn't have the expected effect on turns per mile.

That wouldn't be the first time that a tire company posted the wrong turns per mile number.

Tire growth for steel belted tires at legal speeds isn't significant.