Fuel economy calculation when odometer is off?

So I wanted to present a question on calculating fuel economy when your odometer is off.

There are a couple of ways this could happen but the most common scenario is you change tire size.

Let's assume the Prius models Two-Four come equipped with a 195/65/15 and the odometer reads 100% accurate but the speedo reads 1.5mph - 2mph fast. Then you slap on a set of 205/50/17s (25.1") and now the odometer reads 0.8 miles lower than GPS readings (consistently) and the speedo is dead accurate.

So over the course of a 48 mile trip the odometer reads 47.2 miles and the GPS reads 48.008 miles. How should I approach calculating fuel economy for the trip and more importantly over the course of a tank?

Now let's assume someone uses a 215/45/17 (24.7") tire which is quite a bit shorter than the 195/65/15 (25") and the odometer now reads 0.8 miles higher than with the OE 15" tires. How would you adjust your fuel economy calculations? I do not know if the above error is correct. I am making assumptions based on the difference in tire size and how it affects the speedometer. If someone with a non-Five and a Five could measure this I was be grateful.

 

I generally agree but in this case the trend is in the right directions and the last 3 trips I have measured were calculated at 0.715, 0.827 and 0.808 difference. We know that changing tire size will change the odometer reading. This is backed up by the fact that the speedo changes when using 215/45/17 tires (speedo error goes up) and 205/50/17 tires (speedo error goes down).

 

kgall,

I think your math is correct but here is where it gets dicey. The revs/mile for the OE 15" tire is 829. The revs/mile for my new tires is 829. Yet the speedometer works different with both tires. I think there is less circumferential "squashing" of the 15" tire or there is more growth in the 17" tire. Somehow the revs/mile is changing despite the revs/mile being rated the same. How else can I explain the roughly 1mph-1.5mph speedo difference?

 

I have a new Galaxy S3 phone and the mileage difference between the GPS and the odometer is still pretty constant at .8 miles off over 48 miles (computer) vs. 47.2 GPS. If I use a 48mile sample size and calculate the difference I see about 1.69% less miles being reported. Over the course of a full 500 mile tank that comes out to approx. 8.45miles and a 1mpg difference. Is my math correct?

When comparing computer vs. hand calculations I observed an 8% error instead of the usual 5-6% error. I will have to track this better on future tanks. If this error is sustained then by proxy it is telling me that my odometer is off like I suspect.

 

Your tires are close enough to mine. In fact, I think the revs/mile rating on your tire is more accurate than mine. That would explain the speedo and odo error better.

I made sure not to measure distance with GPS where tunnels are parking garages are present.

 

Aye, now I just need for a few people to check the numbers on the OE 15" tires and the OE 17" tires. Both with air pressure around 44psi.

I used an app called GPD Odometer on my GS3 android phone.

 

Could someone please verify the accuracy of the odometer on a 2010-2012 GenIII with 15" 195/65/15 tires using a GPS unit/app with good accuracy? I've ran the GPS and the difference is pretty consistent. I would like to update my 17" Fuelly records but I want to ensure I am using the right correction factor. If the OE tires make the odometer accurate then I know my numbers are correct. Please list your tire pressure. Thanks in advance.

 

My last 4 tanks have all been within the normal 5% over-estimate range or so. Some would be even lower percentage wise if I used the correction factor.