I'm running 34's in place of stock 32's. I know my odometer will be off, but does anyone know by what percentage? I dont want to end up waiting 6k miles for an oil change thinking its only been 4k or 5k

 

Miata.net is a good % calculator.. but, I'm not sure if they do inches.. you'd have to figure out your P/LT size.

 

You will be traveling 6.25 % fasted than what the speedo reads.

 

32/34 = 0.941... (Old tire size divided by new tire size)

That's a 5.88 % change down in rpm.

So if you were going 75 MPH true speed, the speedo would read 5.88 % lower, or ~ 70.6 MPH.

If the speedo is reading 70.6 MPH, its off by 4.41 MPH (As the true speed is 75 MPH)

To ADD back to the true speed, that 4.41 MPH is a larger percentage of 70.6 than it is of 75 MPH....so, you need a larger percentage than 5.88% when going back.

34/32 (New tire size divided by old tire size) = 1.0625...so adding 6.25% adds back the missing mph.

As the odometer and speedometer are proportional to each other, if the speedo is off, the odo is off the same percentage.

So, if the odometer says you went 2,824 miles, you'd add 6.25% to that to get the true miles, showing you actually went 3,000 miles.



Of course, the factory 32's are more like 32.1", and most 34's are less than 34", as these are NOMINAL sizes, not exact sizes...and that's normal...but, of course, it makes these calculations an estimate, not a precise projection.

If you take the specs for each tire, etc...you can plug in the actual numbers, and, except for rolling differences, tire pressures, etc...you'll be closer to the TRUTH.



Hope that helps!

 

Teej loves the questions.

but i just read that and then my head exploded. maff is hard!

 

I am on 35s.

When I am actually going 75mph my speedo is showing 70mph exactly.

This only true because the dealer flashed my comp for as big as they can go which I think is 33s but am not sure about that. ASK THEM.

So for me I have a .0667% error on my speedo.

7% error?
PFFFT
Brand new stock rigs of any car, can have as much as 3% error.

What I really like is that I won't get a ticket if I obey the speedo per posted limits.
But I will be speeding.
YEH!
Just a bit tho, and I only do so with out zig zag diving in and out crap that scares others.

But I bet if you got a flash you would never notice any error in daily use.

Luck to u and enjoy!

 

The MPG calculation is also off

 

Um, taking everything you need (32/35) and converting it to circumference, is MORE complicated, not less complicated, you are adding a step for nothing, and getting the exact same answer.



And, yes - the makers tend to calibrate for the LARGEST optional tire, so THAT guy can't sue them....AND, so that you think new rig is FASTER and gets better MPG....and needs SERVICE sooner, and runs out of Warranty sooner, etc.

 

For X owners....keep in mind that you didn't start with 32s. i jumped from 29.3 inches to 34.8 which makes for a large difference


Here were my calculations:

Stock tire size - 225/75/16 - 29.3 diameter; 92.049 circumference
BFG tire size - 35/12.50/15 - 34.8 diameter; 109.327circumference

(109.327-92.049) / (92.049) = 18.77%


Then the easiest way to calculate your real mileage or real MPH is:

Mileage x 1.1877 = Actual Mileage
MPH x 1.1877 = Actual MPH

 

wait i have an x and now have 37s on it..what is the difference in speed and miles