5% larger tires (effect on odometer/MPG calculations?)

[litesong]

Originally Posted by California98Civic

I have used 80-110 mile routes to derive a multiplier.

Ah, you're looking for an accuracy of 1 in 10,000. You must be an aircraft designer. Me..... I'll take tires 8% to 9% greater in diameter. Don't care if they're 8.5%.

[California98Civic]

Originally Posted by litesong

Ah, you're looking for an accuracy of 1 in 10,000. You must be an aircraft designer. Me..... I'll take tires 8% to 9% greater in diameter. Don't care if they're 8.5%.

Yes, a little precision helps the knowledge. For example, what has been surprising is that it is not about a mere 0.5% variance as you're suggesting. The specifics of my car and the condition of the tires makes it a much more significant difference. If I just went by stated tire diameter and assumptions about how the odometer is actually measuring and reporting distance, I wouldn't be just 0.5% off. I'd be wildly off, maybe 5% off. I'm not an aircraft engineer, but I'll play one on Netflix if they pay me well enough.

[MeteorGray]

I use the mileage markers on long stretches of Interstate to check the accuracy of my odometer. Over a 25 or 50 mile stretch or more, it seems to work pretty well.

When I first got my Mazda3, I found the odometer to be about 2% off compared with the mile markers. The speedometer also was off the same proportion.

Then, when I got slightly larger tires (20565R16) to replace the factory size tires (20560R16), I found the odometer to be about about 4% off compared with the mile markers, which was predicted by the tire comparison charts I consulted.

I got the slightly larger size because I find that a larger sidewall is more comfortable to ride on when going over the pothole-filled roads too common today. I know this is against the stylish rubber-bands-on-wagon-wheels so popular today, but at my age, my lumbar is more important than my ego.

[litesong]

Originally Posted by California98Civic

I wouldn't be just 0.5% off. I'd be wildly off, maybe 5% off. I'm not an aircraft engineer....

Considering a 0.5% to be a 5% error, I can tell you ain't an aircraft engineer or any kind of engineer. My acceptances of 0.5% errors, are 0.5% calculations..... not "maybe 5% off". I wouldn't accept 5%. That is why my car, normally fitted with stock 175x70x14 inch tires now has 205x65x15 inch tires...... close clearances. Since changing two of my cars to much larger tires/wheels, they ride, handle, brake, & corner with superior results than new for the last 75,000 miles..... & no vehicle parts failures or problems with the bigger tires/wheels.

[litesong]

Originally Posted by MeteorGray

I got slightly larger tires (20565R16) to replace the factory size tires (20560R16)..... I know this is against the stylish rubber-bands-on-wagon-wheels so popular today, but at my age, my lumbar is more important than my ego.

Totally agree with you. I even went to percentage larger tires/wheels than you did. Love the comfortable ride of the big shoes.

[California98Civic]

Originally Posted by litesong

Considering a 0.5% to be a 5% error, I can tell you ain't an aircraft engineer or any kind of engineer. My acceptances of 0.5% errors, are 0.5% calculations..... not "maybe 5% off". I wouldn't accept 5%. That is why my car, normally fitted with stock 175x70x14 inch tires now has 205x65x15 inch tires...... close clearances. Since changing two of my cars to much larger tires/wheels, they ride, handle, brake, & corner with superior results than new for the last 75,000 miles..... & no vehicle parts failures or problems with the bigger tires/wheels.

I'm glad the tires are working great for you. I like them larger too. Next time you have a long trip to make, use google maps and measure an approximately 100 mile long section of it. Then drive it. See if your stock odometer shows what google reports. The difference between google and the odometer gives you a multiplier. In my experience, google maps is very close to the GPS readings I get, though google rounds to the nearest tenth, and the GPS measures hundredths. Calculations based on reported tire diameters or revs per mile is just not enough data to know the actual distance traveled. I want to know the actual distance. With my current tires, which are larger than my stock size by quite a lot, I measured an undercount of over 7% when comparing my car's odometer to the GPS and google maps over an 86 mile stretch. It's significant.

[hat_man]

Originally Posted by California98Civic

In my experience, google maps is very close to the GPS readings I get, though google rounds to the nearest tenth, and the GPS measures hundredths.

The way I'm seeing this is that Google maps rounds to the nearest tenth of a mile and GPS is to t he nearest hundredth?

So Google adds 528 feet to every mile but GPS only adds 50 feet. So for every 10 miles Google shows it's actually 11 miles? Sounds a little extreme to me. Unless I'm not understanding what you are getting at.

I can't imagine Google being 100 miles off for the 1000 mile trip I make to see my family in Myrtle Beach.

[CapriRacer]

Originally Posted by hat_man

The way I'm seeing this is that Google maps rounds to the nearest tenth of a mile and GPS is to t he nearest hundredth?

So Google adds 528 feet to every mile but GPS only adds 50 feet. So for every 10 miles Google shows it's actually 11 miles? Sounds a little extreme to me. Unless I'm not understanding what you are getting at.

I can't imagine Google being 100 miles off for the 1000 mile trip I make to see my family in Myrtle Beach.

I think you are confused about the concept of rounding.

If I measure a trip that is actually 10.06764 miles to the nearest 0.1 mile, then I display this as 10.1 miles. And if I measure a trip of 1000.06764 miles to the nearest 0.1 miles, then it is displayed as 1000.1 miles.

But there is a certain amount of PRECISION in each measurement, so if the first trip was MEASURED as 10.06764 miles ± 1%, means that the ACTUAL distance could be as high as 10.783164 and as low as 10.569636 miles - and each would be displayed to the nearest 0.1 miles as 10.8 and 10.6 miles - where the second trip was measured 1000.6764 ±1% could ACTUALLY be between1010.683164 and 990.669636 miles and displayed to the nearest 0.1 miles as 1010.7 and 990.7 miles.

So a lot depends on the precision, not how it is displayed. I assume both Goggle Maps and GPS units have a fairly high amount of precision, but their display would read differently, but only barely different.

[hat_man]

Not confused so much as trying to see how it makes much of a difference in real world MPG numbers. If this 10 mile trip is your daily commute, then the difference is at most 2 tenths of a mile. About 1040 feet per day. So for an average work week the difference is 1 mile. Either 53 or 54 miles total. Even if you did this on 1 gallon of gas, how much more gas did you burn? Is it even worth calculating? If you only averaged 30 MPG, the difference in what you used is only 3.3/100 th's of a gallon. Just a hair over 4 oz for the week. Less than a dime at $2.50/gallon for the week. Not much of a deal breaker.

For an average of 30 MPG, to drive the two longer distances, it would take either 33 gallons or 33.66 gallons. $1.65 extra to drive 1000 miles at $2.50 a gallon. Again, not a huge meaningful difference.

Precision is nice, but when converted to real world numbers, it seems like overkill.

[litesong]

Originally Posted by California98Civic

.....over 7% when comparing my car's odometer to the GPS and google maps over an 86 mile stretch.

On my two cars, they have an odometer off-set of 8% & 9%, over an 11 mile stretch.