Bigger Tire Odometer Math - Am I wrong??

I'v been keeping track of my MPG after installing Toyo ATIII pizza cutters and doing my own math but I took to the internet to double check and now I'm questioning everything lol

According to the internet the idea is that the % difference in tire diameter directly correlates to % difference in distance traveled (odometer vs actual).

My math says otherwise...if I'm wrong, why??

If I'm right y'all been getting worse milage than you think haha

In the end it's all about the smiles per gallon anyway!

 

That’s a lot of work. All I can tell you is that I’m off by just about 5% and all I used was the speed on my gps vs Speedo which matches up to mileage.

 

I'm sure someone who actually knows how an odometer works could answer this question.

It's complicated and nuanced because not all wheels travel the same number of revs per mile unless you're always traveling in a straight line, so there must be some kind of mechanical averaging or otherwise computation to determine distance traveled.

 

I have a headache for some reason.

 

You did a lot of math, but there is an easier way. Plug the numbers in here Basically the new tires are 8.2% larger. When compensating for the speedometer and odometer simply multiply the numbers your truck displays by 1.082. For example if your odometer says you drove 250 miles then 250 X 1.082 = 270.5 miles actually driven.

You can also recalculate to find your true axle ratio by clicking on that link. Assuming you have 3.73's from the factory you now have 3.45's.

Speedometer calculations are now given. If your speedometer says 70, you're actually doing 75.

 

interested to see how this works out

 

I tried doing this back in 2009 when I went to larger tires as well, and my head hurt so I stopped.

What I do know is:
1. My speedo is now spot on based on multiple traffic radar staions that show your speed. Before upsizing my actual speed was 3 MPH less than what the speedo was showing.
2. I like my tires and the way they look on my my truck, so any difference it "might" make in my MPG doesn't matter to me.

 

With stock tires, my speedometer reads 72 while my gps says 70.

 

Based on a recent highway trip where I measured against the mile markers. I noticed that my odo/speedo was reading ~5% high, so I did the math. If my tires were 0.75" greater in radius, my odo would read correctly. Some of my overage is due to tread wear - time for some new sneakers on the ol' taco.

All the odo does is basically count revs, and it is engineered to a certain tire size. The smaller the tire, the more revs it takes to cover a tenth of a mile, but the odo thinks its the 'correct' size, and thinks it's gone further.

For more than you want to know about it, just search on 'how do automobile speedometers work?' There is a wealth of info, including videos.

I found out that it is illegal for speedos to read low, and legal for them to read up to 10% +2.45mph high. I guess the tendency is to design them to be a bit over in case people put bigger tires on? Kinda pisses me off that my mileage on my odo is 5% high, though. And it screws up mpg calculations.

 

Me too. I first became interested in looking into this when they set up a radar speedo display on a route that I drive every day - I notice it always said 39 or so when my speedo said 42 or so. I'm usually slowing down at this point, so it's hard to be sure - I tried going through at a steady speed, and that's pretty much the story.

 

You made the mistake of assuming your Speedo was correct from the get go.

 

Wow.....you took the long way around the calculations.

Its really a simple ratio between the old tires and the new tires based on circumference. This can be simplified to use the overall tire diameters, since 3.1416 is a constant in both.

33.06299/30.60629 = 1.080.......

This is the multiplier to use to correct your speed and miles driven.

Done. EZPZ.

 

i get what you're trying to figure out OP. you want to know why the math doesnt work out like it should. i suspect something like what @gotoman1969 said, maybe 250 miles isnt actually 250 miles on your stock tires, its a little less. but that still doesnt make the math any more wrong.... idk

 

Yes, that's the point. One expects one's speedo to be accurate. More to the point, one hopes one's odo is accurate. With stock tires, why shouldn't it be?

It turns out there is a lot of room for error in the system. The way the wheel revs are turned into distance traveled is relatively simple, based on gearing. But the way wheel revs are displayed as speed is somewhat funky, involving magnets and eddy currents and springs and such.

Of course, newer vehicles have electronic sensors to measure wheel revs, but the same wheel size issue holds true.

 

8% difference from stock plain and simple. Your ODO from the factory with factory tires reads spot on, your speedo will read high. This is a "trick" that manufactures will do to make cars safer so people dont travel too fast (its even moreso on japanese motorcycles). Needless to say, take ODO reading with new tires and x by 1.08 to get actual miles traveled.

Your problem in math arrives from the fact you compared the difference in size to the new tire size (vs stock) and the assumed miles per tank.

 

Commenting just to follow, this is actually interesting

 

OP i think it is in your "additional distance traveled per mile" step that something is hosed up. the 377 additional inches per mile doesnt make sense to me. still working on it. thats only 31ish feet additional distance.

 

i think the way you subtracted the two different factors and then multiplied them wasnt the right way to find the answer to your problem.

if you take the 658.95 revs/mile of the stock tire and multiply that by the inches/rev of the new tire, that will give you the inches/mile driven for that tire on the same number of revs that the stock tire turned. then divide that by inches/mile (63360 in/mi) and that will give you miles drive for 658.95 revolutions on the new tire. which, unsurprisingly, equals 1.08.

 

To ask a basic question, why?

That is a lot of work to go thru, unless you are really bored or trying to escape talking to someone (like my wife). Then after all done, would you end up changing tires back if the fuel economy is really bad?

I am lazy, so I just don’t understand it.

 

Why was it necessary to do about 50 different unit conversions when all you have to do is say to yourself, my actual travel is increased by my diameter increase over stock (assuming proper odometer calibration to stock). So, apply the same ratio to your odometer since the new tires and you magically have your actual miles traveled.

I think somewhere in your minefield of unit changes you probably made a math error. I thought about trying to find it, and then I decided I’d rather watch a movie.

Also, why bring your gas mileage into it?