I know, this is all over the internet...

Seems the most accurate way, without pulling the gears and counting teeth, is to spin one tire and count driveshaft rotations.

So, right, 2 full wheel revolutions, count driveshaft revolutions. Number of driveshaft revolutions in 2 wheel turns = your gear ratio. Except I know my ratio, on my 98 Tacoma TRD, is going to be either 4:10 or 4:56. So telling the difference between 4.1 rotations and 4.5 rotations is kind of splitting hairs, and pretty subjective (I think).

So I went with more wheel spinning. Now forgive me, my brain doesn't always work so well and my math skills are seriously rusty.

I followed 4Crawlers advice to spin the wheel more rotations, to get a more accurate number. But on his site he doesn't give the actual equation. I'm assuming that's because it should be totally obvious, but...you know.

So I spun my tire 20 times. Driveshaft spun 41 times. So if 2 tire rotations equals my ratio, 20 rotations equals my ratio x10, right?

So if 2=x(ratio), then 20=10x. So if I get 41 driveshaft rotations with 20 tire rotations, then my ratio is 20=41, so 2=41/10 which equals 4.1

Right? So my gears are 4:10? So my question is, is my math correct, or did I just mess around with numbers until I hit a number that made sense, but isn't good math? Cause I did this a lot of different ways first and got numbers that didn't make sense...

 

Correct.

 

Thanks!

So that confirms that I could drop from the 31s on there right now, down to 30x9.50s without being geared too low.

 

So...I wonder if I counted wrong. I put on some 30x9.5s, and now my speedometer is off by almost 5mph, reading higher than my actual speed. And definitely high rpms at highway speeds. I like how it feels and rides a lot better, but...I'm going to see what kind of gas mileage I'm getting and I guess if it's gone down, I'll switch back to 31s. If it's the same or better, I'll stick with the 30s.

What I don't get is that my door plate says the stock size is 225 75 r15. Even smaller...

But clearly 31s are what it's set up for.

 

Sounds like it has been regeared. 4crawler has a write up on his site on how to check gear ratio. You should verify front and back ratios are the same.

Oops, seems first post covered it.

 

If it was modified, the dealer did it. My friend bought the truck brand new in 98 and never modified anything.

4wd works fine, so I'm sure the front and rear gears match.

 

Well, all axle codes and my apparently faulty reading of driveshaft turns say 4.10 gears. BO3A. But it seems pretty clear I've got 4.56. I was at about 2,700rpm at just under 70mph with my 30x9.50s..

 

Not so fast....

You have a 5 speed manual which has a .838:1 5th gear. In 5th gear with 30.5s @ 2700 rpm with 4.10s = 71 MPH

With 4.56s it would be 64 MPH @ 2700 rpm in 5th.

http://www.apexgarage.com/tech/gear_ratios.shtml


Oh wait...you're adding 5 MPH...right? So that's possible maybe? Even though it wouldn't have been stock. And you do know the faster you go the more off the speedo get's right? It's not linear. 2 mph off at 25 might be 12+ mph off at 70mph. I do know with 4.10s and 35s in 5th gear(.838) I'm doing close to 90 when the speedo says 75. And only 26 when it says 25.

 

Yeah. I'm not totally sure.

I'm only going off the police speed check radars they have set up around town. It was closer to my speedometer with the 31s. Now it reads lower than my speedo. Not quite 5mph off at around 35mph.

I had cruise set at about 73mph, which I'm guessing equals around 67ish, and was definitely over 2500rpm.

The gear charts I've been looking at still show a mostly "stock feel" with 4.56 and 30" tires, though..

I don't know. Just annoying. I felt it lugged a bit in first and was a little sluggish with the 31s. It feels great with the 30s but the rpms seem high at interstate speeds.

I guess I'll stick with using gas mileage as my deciding factor.

 

I have used a GPS app on my phone as a speedo.
You can also find decent used GPS'S on ebay
I think I paid $40 for this little Garmin on ebay.
Shows speed and total miles on one screen option.

 

Yeah I can probably use my phone too. I'll try that.

 

According to my phone, I'm going 5-6 mph slower than my speedometer says, at highway speeds. At 65mph (actual speed), I'm at 2500rpm. 70mph, 2800rpm.

Pretty sure I've got 4.56 gears. I'll figure out my gas mileage next, I guess.

 

The trucks ID plate and your count both indicate 4:10.

Why would you think that it is anything else??

Auto speedos and tachs are very unreliable instruments.

 

Well... if that's correct then you probably have 4.10s. With 4.10s at 2500 RPM it should be 66 MPH. That's 1 MPH difference
That doesn't makes sense either way, because if you had 4.10s at 2800 RPM it should be 74 MPH. But if you had 4.56s it should be 66 MPH at 2800 RPM. You're off +/- 4 MPH there. Meaning, your either wrong or way wrong. So the evidence is inconclusive...at best.


I trust the math, that calculates. And the axle code. At this point.


4.10s

 

Why would I think anything else? Have you been reading?

Well, whether my reasoning or math are right or wrong, I'm not convinced. For every person who points to the axle code thread over on ttora or wherever it is, there's a person who will say it's been disproven and you can't trust it. So I don't 100% trust the ID plate.

My count...well...I'd like to do that again too. Counting two numbers at once (I did it by myself) is pretty tough. Pretty easy to mess up.

What I know is that since dropping down a tire size, my speedometer is now off. It was on with 31s.

Either way, there doesn't seem to be enough of a difference in rpms between the two to worry about it. My mileage, at about 90% highway driving for one tank, was right at 19mpg. Which is pretty much exactly what it was with the 31s. So the slightly higher rpms are probably not a big deal, and offset a bit by the lighter, smaller tires. Only annoying thing is that my speedo is off. I'm used to that with my older trucks, though, so whatever.

Thanks.