3:70 or 4:11 ?

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sl33py

sl33py

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Hello all. My 87 60 is new to me. I am wondering if a PO swapped in 4:11's

I do 2900-3000rpm w/ 31 in tires. and my speedo reads 70-75. There is NO WAY i am going 70. More like 60-65ish, since i am in the right lane and getting passed.

it's running great, no worries there. I am just wondering if someone with 4sp and 4:11 can let me know if this sounds right? and what rpm = what speed would be perfect! My 85 always showed 60 and i would be doing 65 w/ 31's.

thanks for any help!

rob
 
With 31's and 3.73s your speedo "should" read slower than you are going.

I wouldn't use that as confirmation that your gears are not stock.
 
Find out for sure. Jack up the back and rotate the rear drive shaft and when you turn it just more than 4 turns the tire should complete one rotation.

That means you have 4.11's
 
Corbet said:
Someone correct me if I'm wrong but the speedo reads at the t-case. So changing diff gears or tires will not effect the RPM to speed relationship.
Click to expand...
Changing gears will not alter the speedo/rpm corerlation.

But it will alter the true speed/RPM correlation.

So will bigger tire.


My reccomendation..

Get some 35's..
 
Last time I checked, my Tcase was still ahead of my differential and wheels...

M
 
Mark Mathews Creek said:
my 84 also runs the same size tires and the speedo is approx. 10% off. So when the speedo reads 65, I am actually going about 60.
Click to expand...

Think you got that backwards... bigger tires are, well, bigger. One turn of the bigger tire moves you further than one turn of a smaller tire.

M
 
Mike S has it right. On my truck, with a 4-speed, 3.73 gears, and 33s, 40mph indicated is actually 47mph road speed. 20 is 24, 30 is 35, 40 is 47, 50 is 59, 60 is 72, etc.

That's all from memory, so it might be a bit off. But it's more or less right...
 
polarweasel said:
20 is 24, 30 is 35, 40 is 47, 50 is 59, 60 is 72, etc.

That's all from memory, so it might be a bit off. But it's more or less right...
Click to expand...


nice!!!! im not sure what 4, 5 , 7, 9, 12 have in common, but im dumb! ive always wondered how to figure the correct speed/tires size. guess ill just remember that. thanks!
 
rockin' the boat..

OK, I got one for you...

Before my recent mods my FJ60 had 3.70s, 31's and the H55f and when checked against my GPS and those roadside radar signs my speedo consisently read 10% higher than actual speed. yup..higher. And get this! my odometer read just about 10% lower than acutal when tested against the GPS.

Now, with 4.11s, and 33's it's about the same 10% variance on the speedo, but the odometer is almost right on, maybe 2% lower than acutal.

Weird.

I think of it as "speeding ticket insurance"

CardinalFJ60
 
Is the stock size on a 60 not 31's? If it is or smaller, then your speed-0 will read slower than you are going. If you have 4.11s then it will read faster than you are going. If you have the stock 3.70s and have 32's+ then your speed-0 will be slower than actual. If you have 4.11s and 34's or smaller than your speed-0 will be faster than actual (might be wrong on that one).
 
alkaline747trio said:
Is the stock size on a 60 not 31's? If it is or smaller, then your speed-0 will read slower than you are going. If you have 4.11s then it will read faster than you are going. If you have the stock 3.70s and have 32's+ then your speed-0 will be slower than actual. If you have 4.11s and 34's or smaller than your speed-0 will be faster than actual (might be wrong on that one).
Click to expand...


The stock size on the 60 was like a LT235 or something similiar. I have a stock tire floating around and it is small. My speedo with 31s and 3.70s was a little slow and with 33s and 3.70s it was slower.
 
You want an easy way to figure out how far off your speedo is?

Just pace yourself on the highway and do some simple math :doh:

Drive 10 miles on the highway using the mile markers on the side of the road. Reset your trip odometer right when you start.

After 10 mile markers note what your odomoeter reads.

I have 35's and 4.11's. After 10 mile markers, my trip odometer only registered 9. So you can take 10 divided by 9 and get 1.111111111111

Now I have my multiplier. If I need to figure out my speed, or correct odometer reading just multiply what you are reading by 1.111111111

example. Driving on the highway my speedo shows 65 mph. Multiply 65 x 1.11111111 I get 72.222222222
 
or option 2 (thanks for the math majanrk).

GPS! I will see how fast it says i am going. I am thinking that the speedo is buggered or i have 4.11's. I am showing 70ish and i know i am likely going 60ish.

thanks for all the ideas and help guys. new speedo cable is ordered, and possibly new guage - i'll figure it out eventually.

rob
 
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