Odometer and MPG Conversion Tools

I recently switched from 17 to 18” wheels. At the same time the tire increased from 31.6” to 32.6”.

Are there any online calculators to demonstrate the impact to the odometer, and miles per gallon?

Most of the sites tell the difference in revolutions per mile, but I don’t see any that do anything beyond that.

I can run the math and correct things, but I’m looking for a calculator that takes all of the variables into account. Well, as many variables as possible. For instance, when I plug in the numbers, fuelly says my last tank average was 16.3. That is incorrect. It was at least 16.8.

Does this exist online for free? I’m not really interested in buying UltraGuage.

 

Here you go…

https://www.tacomaworld.com/tirecalc

Edit: re-read your post and I’m sure you already knew about that link so disregard lol my apologies.

 

No worries.

 

If you assume the odo was correct before, use the % change in tire diameter to correct your odo (manually, not in vehicle) then calculate your MPG.

Only the tire diameter matters, not the wheel size.

You can also GPS verify your odo error and use that % to adjust your mpg calc

 

Yes, I know. Just wondering if anyone automated it.

 

There are some tools on the market that aim to do that, but unsure if any are 4g tested yet.

Since I do all my tracking in Excel I just popped a formula in to do the correction for me.

However I also found, like many others have, that my tire up sizing actually corrected my speedo, meaning my MPG wasnt right to begin with.

 

Not to the extent that will account for tread wear, tire pressure (and atmospheric pressure) changes, sidewall bulge from load, things that alter the effective tire size as driven.

 

Thanks. Not to mention weight differences.

I guess I am going to assume the number I get by a simple miles/gallons is not accurate, but good enough.

 

Use one of the AI tools like chatgpt.

 

No vehicle does that now, nor any correction tool.

Using a GPS based mileage recording method would get you very close though, as fuel used for distance covered is all MPG is a result of.

All those other things would only impact fuel used.

 

Impact on odometer reading is straightforward and involves making sure you have an accurate distance measurement (trust GPS methods over odometer reading), but impact of different tires and tire size on MPG is much more complicated. You have to be able to account for details a tire company will never have or want to provide if they did. Moment of inertia by far is the most important. That will be affected by tire compound density, sidewall thickness, tread thickness and other factors each specific to each tire. Then there's rolling resistance, which is a factor of tire compound softness/hardness, total surface area of tire contacting the ground, type of ground you're driving on, and then others including engine, wind resistance and other characteristics specific to different vehicles. I too wish there was a tool able to integrate all this data, but unfortunately many of these data points will not be available so we are left to do imperfect back of the napkin type estimates and rely on extrapolation from others' examples with variable utility.

 

An exercise in futility. I've done the math on this multiple times and with a 1" tire difference it always shows about 1/2 mpg difference which is meaningless.

If you drive 1000 miles @ 16.8 mpg you'll burn 59.5 gallons of gas. At 16.3 mpg you'll burn 61.3 gallons. That's less than 2 gallons more to drive 1000 miles.

And that is only accurate when both tires are new. Your tires gradually get smaller as you drive them. Your new 32.6" tires will be 31.6" by the time you replace them.

 

Exactly. Trying to get precision results when precise distances aren’t tracked and as readily available as the odo is pointless. And the amount of fuel used is “only” the other bit of info used to calculate mileage. If the distance hasn’t changed then the amount of fuel used has. I keep track so I have a general idea that I’m maintaining good habits and there are no developing problems a drop would indicate.