Tire size calculator dependant on gear ratio?

By chance I found the awesome tire size calculator from this forum. Are the calculations assuming a 5 speed fifth gear ratio? Mine's a '09 2.7 5 speed

 

Doesn't actually matter how many gears you have this is purely based on a tire size difference. So if your new tire is say 5% taller then your speedo should be off by about 5% as the calculations show you.

Now since you have a 2.7 I am assuming you had the smaller stock tires like I did and my speedo was off by around 4-5% high so when I was showing 65mph on the dial my gps showed around 62 or 63mph (as to which one is off... not sure).

Now that I bumped up in size on the tires my speedo and GPS are pretty much synced.

 

I have gotten a lot of use out of the tire size calculator too!

It's my understanding that Tacoma's use a wheel speed sensor at the hubs to determine speed, so the calculations are independent of any transmission's 5th gear ratio.
But at the bottom of the page the gear ratio's are in reference to the gearing of the differential(s)

When I moved up in tire size (from 265/65R17 to 285/70R17, see ratio screen grab below) I also regeared from the factory 3.73 to 4.56

For an '09 with a 2.7 I think you diff gear ratio might be 4.10, but I'm not sure

 

I'm gonna have to dig a little deeper. At 65mph (according to my GPS, which is pretty accurate) with 235-70-16 tires, I get 2550 rpm in 5th gear (?)

 

The RPM's you are talking about is engine speed...

The RPM's that the Tire Size Calculator is talking about is what your Rear Differential and Wheels are seeing with the new tire size

 

Ahhhh, I see! I'll start again. Thanks man

 

Man, I am REALLY struggling with this math. Here's my calculation; 235-75-16 tire has a circumference of 93.86 inches or 7.82 feet. According to the calculator, with a rear end ratio of 4.11, that tire turns 3100 times in one minute. Each time it turns it covers 7.82 feet. So, if it turns 3100 times in a minute, it covers 24,242 feet. Roughly 4 miles a minute. I'm hoping someone can set me straight

 

I stand corrected, the calculator is not calculating how fast your ring gear (and tires) are spinning, I believe it is calculating how fast your pinion gear (and drive shaft) are spinning.

 

So if your drive shaft is spinning 3100 in one minute and your gear ratio is 4.11 then your tires are spinning 754.3 times in that same minute (3100/4.11=754.3)

Now you can multiply 754.3 revolutions by your tire circumference 7.82 feet and you get 5898.6 ft (754.3 x 7.82=5898.6) which is a little more than a mile in that same minute.

 

Here is a screen grab of a spreadsheet I put together a while back when I was looking to regear.
I created this calculator in excel to compare drive shaft speeds of the factory 3.73 differentials to my new 4.56 Nitro Gears

 

Awesome. If you know your tranny gear ratios, you can create a speed in gears spreadsheet. That was more of what I was looking for! Thanks

 

Any one happen to know what the 5th gear ratio is? TIA

 

Maybe you'd be better off with this calculator if you are wanting your specific rpms in whatever gear and speed: Gear Ratio Calculator (grimmjeeper.com)

Almost all of those generic RPM/diff gear charts are based on 1:1 final drive ratio, which really hasn't applied since the 80s, but it does allow you to compare different gear sets. Don't get too worked up over those generic gear ratio charts though.

 

Thanks man. If I knew the OD ratio of my fifth gear, I can make the tire one work. I want a 235-85-16 tire and an overall lower (higher numeric) gear ratio for trailer pulling etc.

 

The good thing about the calculator I posted is you don't have to know your transmission gear ratios. You just put your transmission in and it auto populates with the correct ratios and shows you your rpms in every gear at any speed you want. You can make changes just in tire size or gears or both.

 

I looked at the list of Toyota trannies and am not sure which is mine. Any help would be appreciated! Cheers, Dave

 

If you look at the sticker on your door jamb there will be a sticker with what transmission model you have.

I have an A750F Automatic Transmission (A/TM)

 

Once you find out what model Transmission you have, this website might come in handy:

https://www.marlincrawler.com/tech/transmission/transmission-gear-ratio-chart

 

My guess is you have the R155.

 

Good guess! Thanks all for the help. Cheers, Dave