Revs Per Mile

[ctrlraven]

What's the best way to find out for new tires?

I've normally use Tire Size Calculator per www.miata.net/garage/tirecalc.html which has normally seemed to be just about right. I just mounted my Nitto 265/50-18s which per Nitto's website says 731 RPMS and the Tire Size Calculator says 709. Since there is such a big difference between the two number is makes me question which to use. My old BFG KDW2 255/55-18s were 694 RPMS per Tire Size Calculator and my speed has been spot on with going through stand alone speed displays and usually go 5 mph over when a LEO is running radar or pacing.

[Zack]

Compare it to a GPS and adjust til its correct.


FYI the revs/mile in the stock tuning file is not that close to BFG's advertised number on the website.

[ctrlraven]

Compare it to a GPS and adjust til its correct.


FYI the revs/mile in the stock tuning file is not that close to BFG's advertised number on the website.
Yeah I remember you telling me that before. I'll just go by the Tire Size Calculator for now until I can borrow someone's GPS or have a friend pace me.

[Krytin]

When I put the Nitto 255/45 18's on I used their rev/mile # and it turned out to be dead on w/GPS.

Trial and fine tuning would still be best.

[ctrlraven]

When I put the Nitto 255/45 18's on I used their rev/mile # and it turned out to be dead on w/GPS.

Trial and fine tuning would still be best.
The new 265/50s are .6" smaller than my 255/55s. Comparing the Nittos 265/50-18 diameter to the stock 245/55-18 diameter and I think the RPM was 718 or 724 for that tire. The Nitto is slighly smaller than the stock rear so maybe 731 is about right. Also yay for 4.15 effective rear gear now yay! lol

[Krytin]

I hear that on the effective grear ratio improvement!
I was VERY happy when I put the 255/45's on.
That was the the best bang for the buck change I have made short of the dyno tune!

[ctrlraven]

Yeah my 255/55s gave me a 4.04, every little improvement helps I guess. lol

[Blk Mamba]

Real quick with my calculator, I get 709.48, (rounding up, and down), do you have the formula to figure the new rear end ratio with different size tires, I'd like to know what mine is with stock gears, and 285/50/18's.

[ctrlraven]

I went with 731 (730 by Xcal) and it seems to be right with the rpms.

To find out your new effective rear gear ratio use this formula
http://www.4lo.com/images/geartire-formula.gif
If you want to be lazy they have a java app on the page, just input the info.
http://www.4lo.com/calc/geartire.htm

[Krytin]

Very good - thanks for the calculator!

[Blk Mamba]

Thanks, so as I figure it, I have 3.63's.

[boatmangc]

You guys got too much time on your hands Jeez!

[Dennis Reinhart]

What's the best way to find out for new tires?

I've normally use Tire Size Calculator per www.miata.net/garage/tirecalc.html which has normally seemed to be just about right. I just mounted my Nitto 265/50-18s which per Nitto's website says 731 RPMS and the Tire Size Calculator says 709. Since there is such a big difference between the two number is makes me question which to use. My old BFG KDW2 255/55-18s were 694 RPMS per Tire Size Calculator and my speed has been spot on with going through stand alone speed displays and usually go 5 mph over when a LEO is running radar or pacing.

I have a program for this send me a mail I will send to you.

[breeze]

Thanks, so as I figure it, I have 3.63's.

285/50s are taller then stock so your gears will be less then 3.55s

[ctrlraven]

285/50s are taller then stock so your gears will be less then 3.55s
Yep, stock rear 245/55-18 is about 28.6" tall. The average 285/50-18 tire is about 29.2", taller tire which means slower rotation and lower gear ratio. If you went with a 265/50-18 you'd be around the stock diameter and be around 3.57-3.59 for effective rear gear ratio.

[Doohicky]

Is this like a figure of speach?
It just sounds like smaller tire diameter means more rear wheel horse power/torque, compared to larger diameter less rear wheel horse power/torque.

[Blk Mamba]

Then the formula listed above is incorrect.

[Doohicky]

All that's changing is turning torque. Gear ratio is still the same. If you have a 3.55 gear ratio, no matter how big or small your tire is, the drive shaft still spins 3.55 times to one rotation of the tire.

[ctrlraven]

That is correct, hence the term "effective gear ratio" but with using smaller tires that one rotation will be completed faster than with a larger size tire.

[Dragcity]

To fine tune my rotations and get my Speedometer to read the exact miles per hour I used to do this:

While driving on the highway, set the trip odemeter to zero when you get to a mile marker on the road side. Every mile passed on the mile markers, note where your trip OD is. After 10 or 20 miles, you should have a good idea where you are at.

Once you have that data, it is a percent change game. Adjust your revs up or down to accomodate where you need to go. Just use what lies inside your cranium to figure it out. You may have to do this a few times, but once you are there, you will be exact.

The GPS thing is quicker and easier.... That's just in case you don't have one.....