scuba

No, it shouldn't.

If you go up in tire size, your RPM's will decrease at a given speed (on a linear bases) due to the decrease in miles per revolutions of the tire.
If you gear down (numerically higher) your RPM's will increase at a given speed (again, linear).

This is why we re-gear to a lower ratio. So we can get higher RPM's at a given speed vs. stock gearing.

With larger than stock tires, you'll be at a lower RPM at 70 and thus (theoretically) have less power. Gear it lower and your driveshaft (and your speedo gear) will turn A LOT faster thus, getting you back to -pre tire size change (or hopefully higher)- RPM's at a given speed.

It's really not rocket science, I promise.

You tach is not off. Grimm jeeper is not off. Your speedo is off because you have larger than stock tires.


/thread.

millball

Originally Posted by Batousaii:
...Regardless of the Diff-gears or tire sizes, the gauge speed to Engine RPM in a given gear should remain constant. Not sure how this is being missed.

I believe that this is absolutely true.

Yotard

Absolutely not true the rear gears and tire size absolutely change your rpm and speed in a given transmission gear. You need to research more

millball

Of course changes in gear ratios and tire sizes change the engine rpms at any given speed. (True speed over ground)
This is what causes speedo error.
What absolutely does'nt change is the relationship of engine revs to speedometer indication, as Batousaii points out.

Yotard

The rpm is always going to read correct on the tach because it is reading the sensor spinning around. For instance you will not always go 60 mph at 2900 rpm it depends on what tire and gear you run. You are right that they tied by the same ratio as the tire size to the rear gears.

You can calculate an rpm given a gear ratio and a tire size or a speed given an rpm and a gear ratio. So yes they are linked but it is a ratio not a fixed amount.

millball

The ratio between engine rpms and the speed that the speedometer cable turns in any given (tranny) gear are absolutely fixed. The only way this ratio can be changed is to change the speedometer drive gears. This is exactly how ordinary speedometer error caused by changing tire sizes or diff gear ratios is corrected.
When you add larger tires or go higher gears the speedo cable must be made to turn faster, in order to remain accurate.
When you go to smaller tires, or to lower gears, the speedo cable must be made to turn slower to remain accurate.

dark_fairytales

Well, I can say this much, the gears you are wanting in perfect world, would be 4.68:1; how ever that gear set is not made. If you chose 4.56:1 gears, you Yota would loose 3% torque. How ever with 4.88:1 gears, your torque would increas by 4%.

Now your Speedo with 4.56 gears, and 32” tires would show that you would be traveling 3% slower then you actually are (if your Speedo says 60 you would actually be going 62mph).

If running 4.88 gears with the Speedo at 60mph, you would actually be traveling at 57mph; which would mean your Speedo would be 5% too fast.

I would personally choose the 4.88's as I would hate too loose any torque on the highway, and which ever you choose, there will still be math too do inorder to know the actual speed.

Hope this all helps with the choice ahead of you.

Batousaii

Cool TY, - So 70 mph in 5th gear at 3000 rpms is via your gauge, since you dont have a GPS?


TY, Helpful. Is this reading via gauge or GPS? and in 5th gear i presume?


Exactly. Makes it hard to determine the correct gearing for 32" tires when all/some of the other figures are off.

I wondered about this too. 70 in 5th at 3k seemed pretty fast for that little truck.

Very Helpful! - This being correct, I think I'd preffer 4.88 gears. i rarely drive 70 for any length of time. I usually do around 60 - 65. but there are a fair ammount of hills along my commute. it's 90% daily driver / 10% Bugout-HAM radio light off road hobby radio truck.

Batousaii

double post

gillesdetrail

iTrader: (1)

Take into consideration that a 32'' tire isn't actually 32'' diameter. My 31x10.5x15 are roughly 29.5'' and my friends 36'' iroks are 34'' to the tape.

lledwod

iTrader: (2)

Just a note, I checked; my 1982 stock longbox 4x4 5spd runs about 62-63 mph at 2900rpm.

Batousaii

Awesome! - Thank You.
- I presume this was in 5th gear?

kawazx636

iTrader: (3)

I think that you are relying too much on the accuracy of your GPS to give you an accurate speed. A GPS needs constant and uninterrupted data to make accurate calculations. Things like a windshield, trees, buildings, overcast skies and the general accuracy of the device all attribute to inaccuracies in GPS readings. For your typical car GPS, it only needs to be as accurate as the road width since as it calculates your location it will adjust your on-screen position to the known road map so that you aren't going "off road." Have you ever noticed that when you are following your GPS and take an exit off the highway that it will sometimes "jump" from one road to the next? This is that correction that I am talking about. GPS also averages your speed based off of several linear calculations. Theses calculations may be taken while you are acceleration or slowing down and may cause the reading to be skewed. Also, these calculations may not account for change in elevation. For instance, you may travel one mile in one minute (60mph) but it may be up 22.5* hill (3D) which in a 2D representation may only appear to be 3/4 of a mile which would make it appear that you are only going 45mph over that distance. Make sense?

Then you have to add in the fact that you are trying to add in additional variables in your gearing, tire size and the truck's equipment functionality.

Bottom line, I would forget the GPS and RPM readings and just go with the numbers. Your speedometer is cable driven so the the likelihood of it malfunctioning is less likely. For me, I do conversion factors for my trucks. In my 83' I know that one revolution of my 31" tires takes me 1.096 times farther than the stock tires would. So to make it easy on myself, I round that number up to 1.1 and multiply it by the speed on my speedmeter. so If I'm going 55mph on the speedo then I know I am really going 60.5 and 60mph is 66.0. Luckily 1.1 is an easy conversion to do on the fly - take the speed, move the decimal one place to the left and add it to the speed.

If you are trying to figure out new gear ratios then you're going to have to trust the numbers. So far, I have found the numbers on the grimmjeeper site to be very accurate.

kawazx636

iTrader: (3)

And just to verify my knowledge about GPS, I work with GPS and mapping software everyday in my career as a GIS professional.

Batousaii

Good info. I am fairly aware of this, but most of my drive (after the hills) is stretches of straight 60mph road. It's fairly easy for me to keep a constant speed for a good length on flat straight road to determine GPS info. However, I have been scrutinizing everything here, including GPS.
- I am concluding that my Speedo gauge and RPM gauge are fairly accurate (mechanically speaking), so now I just have to decide on gear. A little over, or a little under stock Speed/power.

At this point it comes down to Gas-milage, as power in 4.56 and 4.88 would be an improvement either way, and either one brings me close to stock ratios on 32" tires. As dark_fairytales noted:
again, good info. It's all apreciated, so now i just have to consider how I want to balance the trucks perfomance on road vs. off: Gas milage vs. road speed, vs Off road ability. Having the gauge match isn't my priority (I can eye-ball my road vs. gauge speed), it's simply a tool of measurment that's been plexing me a bit since it doesn't seem to match the other tools.

Knowing what other trucks (like lledwod's bone stock truck) run is very helpful too. In all of this, i been trying to find the base-line to judge the modification on, with the numbers not matching between tools, it's been frustrating at best to determine the original (& acurate) road speed, to gauge speed, to gearing, to tire size formula.

TY Guys, the info is helpful indeed.