Batousaii

OK, So… I’m NOT a math wiz.. but I’m not a dumbass either (At least I don’t think I am….) I am however hella confused here, and maybe you guys can help?

I plan to put 32” tires on the truck (Has a 4” lift) and considering either 4.56 or 4.88 gears. I know everyone says you’ll eventually go bigger… Honestly, I probably wont... 32’s seem a respectable compromise between price and functionality for the trucks intended purpose (Daily driven, open field long distance capable bug out zombie rig + Rolling HAM radio) 32” seems just right to me.

Here is my problem: I currently have 265/70 R16 tires, and I assume the original 4.11 Diff gearing (L52 Transmission). At 2900 rpm in 5th gear my Speedo says 60 mph, and my GPS says 68 mph..

Correct me If I’m wrong, but the transmission does not know what gears the diff has or what the tire size is, and will simply post all speeds based on info from the T-case output regardless of diff & wheels dimensions or Engine RPM, Transmission Gears Etc. I can coast in neutral at idle, and still know my road speed.

Now, according to the grimmjeeper(.com) calculator, a bone stock 1982 Toyota (L52 gearing, 4.11 Diff and 225/75 R15 tires) should go 70 mph in 5th gear at 2900 rpm. So my Speedo should say 70... not 60 .. right? <sigh>

So, then, could my speedometer be off or malfunctioning? Or… am I missing/overlooking something here? I’d like to have my truck geared as close to bone stock while wearing the 32” tires…. And I’d like my speedometer to act accordingly so I know how fast I am really going at a glance.


Any thoughts, or things I should check would be appreciated, this is really throwing me off.

Thanks Guys,

Bat

P.S. - I'll double check my speed to RPM when i drive home tonight, but pretty sure I'm correct here.

JasonYota

iTrader: (1)

You have larger than stock tires on your rig. Meaning your going faster than your speedometer is showing. Your way over complicating it.

Batousaii

The speedometer gets it's info from the T-case and doesnt know what tires or gears the truck has.

- A stock truck should read 70 mph in 5th gear at 2900 rpm. Mine reads 60.
- Can speedometers be re-geared, or an adjuster someplace?

JasonYota

iTrader: (1)

Yes it's matched from the factory using the stock gear ratio and tire size.
You increase the tire diameter, your distance traveled is increased. Your going further per tire revolution than stock.

dropzone

iTrader: (6)

i have not found a speedo gear to correct for different gears.
That being said with 5.29's & 35's my speedo is pretty darn close.

I have a cheap GPS i use as a back up speedo

lledwod

iTrader: (2)

I don't think an 82 4x4 will be going 70mph in 5th at 2900. At least not here in Canada! I could be wrong, and I will check tomorrow. I've owned several and I think its closer to 60mph.

dropzone

iTrader: (6)

punch numbers in here
http://www.grimmjeeper.com/gears.html

toyota transmission ratios can be found here
https://www.marlincrawler.com/tech/t...ar-ratio-chart

Batousaii

According to grimjeeper(.com) it is, unless the grimjeeper is wrong ?

ya, i got my gear ratios there, and punching in numbers here is what got me thinking my speedo is wrong (or Something is wrong/off).

The factory sets it speedo to the t-case final out (Drive shaft) based off tire/gears ratios- correct. But the READING (at guage) will stay the same even after changing tires and gears because the t-case final out doesnt know ant different.. You will simply be going faster or slower depending on the changes you made, but the speedo will still tell you the same reading based on RPM of the drive shaft rotations.

Either the grimmjeeper is off, or my spedo is off it seems.

JasonYota

iTrader: (1)

Yes I am aware how the speedometer works. You are correct the reading will stay the same no matter what tires or gears you put on it. That's why it's off, you installed larger tires than stock. Simple as that

Batousaii

I already understand the my speedo is not currently telling my my actual road speed. I noted that it's 8-mph slow in my original post (due to tires)
- BUT, in 5th gear .... at 3000 Engine RPM, my speedo guage says 60. Grimmjeeper says it should read 70.

which one is incorrect? grimmjeeper or my speedo?

---> How fast does a stock 1982 Toyota 4x4 go in 5th gear at 3000 RMP? (60 or 70 ??)

JasonYota

iTrader: (1)

70mph for STOCK gears and tire sizes

83

Your GPS is right. It basically says 70mph, just like grimjeeper. Your speedo is off. It's off because you have non-stock sized tires (which it looks like you've finally agreed do throw off the speedo).

Why are you plugging numbers for stock tires into grimjeeper if you don't have stock tires?

It seems like you're confusing actual speed with what speed your speedometer is reading.

According to your GPS, you're going 70. Which is correct. You're speedometer is telling you you're going 60, because you have larger tires. All your t-case knows is how fast the gears are spinning. Not how fast the vehicle is moving. When you put on bigger tires, you caused the gears to spin at a different rate, at the same vehicle speed.

redbayredneck

I understand what you're trying to say... you sure it has stock gears in it?

edit: Any chance your tac is off?

dark_fairytales

I am trying to get my head around the fact that the OP is so concerned that the speedo does not indicate the actual speed. The rest of use who have changed tire size and gear ratio do the math to understand the difference of what the speedo indicates, and then make the adjustment to our skinny peddle.

When I had 35's and 5.29's, my speedo said I was traveling 5mph faster then I really was. So, 70mph subtract 5mph told me I was doing 65mph. My brain calculates faster then a speedo gear change.

P.s. I am not trying to be rude.

redbayredneck

The OP isn't trying to understand why the speedo isn't displaying the correct speed... he's trying to figure out why the speedo and tach don't show the same relation they should.


Example: My truck show me doing 60 at 2800 rpm on stock tires... the speedo will still read 60mph at 2800 rpm no matter what size tires I am running... even if I'm running 35's and am really going 75... the speedo will still read 60 at 2800.


What he is saying, is that his speedo is supposed to be showing 70 at 2900 rpm, but instead its showing 60 at 2900 rpm.

redbayredneck

I think the Grimjeeper site calculations are off... its showing that in 5th gear I should be showing 74 mph at 3000 rpm and actually doing 79mph with my W56 and stock gears... My truck is only showing around 60 at that rpm range in 5th gear

Batousaii

As stated in my OP.. I already knew that gear/tire changes throws off the speedo. My GPS being correct, my truck may have re-geared at the Diff. I will do the "Tire Spin vs drive shaft" trick on my day off to see if that gives me any clues to the ratio.

- I was plugging stock numbers in to grimmjeeper because I am wanting to gear a set of 32" tires to be close to stock, and I wanted to know the original speed to RPM per gear numbers to base the re-gearing off of.
- I am not confusing actual speed with gauge readings. Simply, the numbers are not adding up, so something is innacurate (either g-jeeper or my speedo.. or tach?).

It wasn't about making the vehicle match the speedo. I been running 8mph over for a couple years. am concerned with my gauge reading vs road speed, and compaired to g-jeeper because i 1) want to run gears that bring 32"s close to stock gearing, and 2) knowing the original numbers is helpful to acheive that goal. The gauge reading was not adding up to what g-jeeper was telling me it should be (speed vs eng-RPM per gear), so it was causing confusion when trying to figure out the proper gear set to use.
...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.




Thank You.. Finally.
I am not sure about the gears, If i go off my GPS-road speed vs RMP in a given gear, it may actually be re-geared already. I will try to check by spinning a drive wheel and counting the drive shaft rotations. I'm starting to think the speedo may be off, assuming g-jeeper is correct in saying that a 1982 actually goes 70mph in 5th gear around 3k RMP when stock.
Hmmm, the Tach... It's a brand new aftermarket tach, and i'd like to think it's not off, but i could test it against my multi meter (it does RMPs). If it is, that would explain alot, however it seems like i'd be noticably lugging the engine if the tach was off that much. I'd suspect a 32 year old speedo over a new tach, but who knows? Some tests will tell, and TY for the idea.

83

Ok, cool, gotcha.

This is what threw me off. Because if you're coasting in neutral with stock gears and bigger than stock tires, you will not know your road speed. At least not without doing a little math.

Batousaii

My bad - i could have worded it better there. I should have said that the guage will give you a speed based off the drive shaft even when coasting in neutral.
- You could remove your driveshaft all together, and the guage would still tell you your going X-speed in X-gear @ X-enging RPM, even though your standing still.
- So my truck thinks I'm going 60mph at 3k RPM in 5th... g-jeeper tells a whole different story based on stock setup, and my GPS tells yet another story all togther, and stories dont add up... so... someone is lying -lol.

1st Gen Crawler

Seems to be pretty spot on for me. i Usually can cruise at 70 mph in 5th gear at 3000 rpms. Mine is lifted, locked, with a W56, 5.29 gears, and 35" tires. i do not have a GPS though, and when i go one of those things that reads your speed it seems to be about 5 mph off.

Cool calculator though, i like seeing the dual case ratios and speeds. Super slow.