35's regearing and speed correction

[Romer]

So next week I am going to strip both axles down and re-gear the diffs to 4.88's to go with my new 35" Toyo's.

So now it will take different rpms to get the same speed. Currently with the 35's, my speedometer is off by 12%, or I multiply the current speed times 1.12 to get how fast I am really going. It would seem to me the 4.88's would effect this relationship.

Does it, and if so how much?

 

[GXO]

speedo correction + gps + little screw driver = *shrug*

I just did what the computer told me to do!

 

[Arya Ebrahimi]

Yes, changing the diff gears will most definately affect this. By how much, I can't say w/o doing a lot of math. I believe I recall reading that 4.88s is slightly overgeared for 315s, so your speedometer will likely read a little fast when you're finished. I'm sure more will chime in w/ their experiences.

 

[hoser]

EDIT: Recalculated using precise tire measurements.

Here's one way to calculate it:

31.2" 4.1
---- X-----=
34.6" N

N= 4.55

4.55 would make your speedo stock

4.88/4.55=1.073

Your speedo will read 7.3% higher than actual. But, yes, use GPS for the most accurate reading.

 

[semlin]

i think you first need to remember that tires are not exactly 31 or 35

315/75/16 = 34.6"
275/70/16 = 31.2" stock

if I do it in stages then the tire size difference yields a 10% positive correction or multiply speedo reported speed by 1.1 to get actual speed

but then going from 4.11 to 4.88 yields a 16% negative correction or multiple by .84% to get actual speed

put it together and by my admittedly horrible math that means your speedo will be overstating speed and you need to multiply by .924 (or reduce by 7.6%) to get true speed.

so 60 on your speedo means 55

i'm sure i'm wrong but as usual when it comes to math i can't see why (at least until i post this when it will dawn on me)

also don't forget your stock speedo may not be accurate to begin with.

 

[Moxwell]

the math can be worked out here:

http://pitbulltires.com/calculate.php

 

[landtank]

I have that tire/gearing combo and it worked out to basically 8% as Semlin stated.

I used that to calculate the RPM error and then adjusted the truespeed accordingly. Years later I finally had access to a GPS and realised that at 75 mph I was reading 73.

I'd get the yellow box and use the 8% value when setting up the switches.

 

[ginericLC]

I thought 80 series speedos were set a little fast from the factory? Both of mine when stock read 60mph when the GPS said 56.

 

[Romer]

Thanks guys. I'll order the Yellow box.

That will be a switch showing faster than I am going.

 

[cruiserdan]

I thought 80 series speedos were set a little fast from the factory? Both of mine when stock read 60mph when the GPS said 56.

Mine is dead-on.

 

[alkaline747trio]

With 31's, mine was dead on...with 35's, I have no clue. Diff gears do definately change the speedo vs actual speed relationship.

 

[LongMonster]

Romer,
So you make it sound like you are going to tacke the re-gear on your own. I'll be very interested to hear how that goes, and I'm sure that you'll share your experience. I've heard it is a pretty tough job but maybe you'll be able to shed some light on the whole process.

Best of luck

 

[Tools R Us]

Romer,
So you make it sound like you are going to tacke the re-gear on your own. I'll be very interested to hear how that goes, and I'm sure that you'll share your experience. I've heard it is a pretty tough job but maybe you'll be able to shed some light on the whole process.

Best of luck

Some good pix on a front regear;

http://www.4x4wire.com/toyota/tech/landcruiser_elocker/

 

[Romer]

I'm just stripping the axles and giving the diffs and gears to someone with experiance.

 

[MaddBaggins]

I got 8.5% fast on the Speedo
https://forum.ih8mud.com/showthread.php?t=110368&highlight=4.88's

 

[OGBeno]

Some good pix on a front regear;

http://www.4x4wire.com/toyota/tech/landcruiser_elocker/

Great link Kevin. Thanks.

 

[medtro]

...... So now it will take different rpms to get the same speed. .....

You give me the reason to do this. I always wanted to see the RPM difference between running 4.56 and 4.88.

 

[reffug]

Mine is dead-on.

Is there anything on your rig that is not????????

 

[Romer]

This is a good candidate for the FAQ.

So how about a simple discussion applying the above table that it takes more RPM's with the re-gear to reach the same speed as with stock gears, yet it translates into a "peppier" setup.

 

[Hants]

Here are my takaways from the chart above and real-world experience with 33" at altitude and stock gearing:

At 60MPH,


31" 33" 35"
4.10 2056 1957 1846
4.56 2287 2177 2053
4.88 2447 2330 2197


- 4.56 with 35" is basically back to stock gearing
- 4.56 with 33" is basically the same gearing as 4.88 with 35"
- 100 RPM makes a noticeable difference in drivability (31" ==> 33", 4.10)
- 4.56 with 33" or 4.88 with with 35" looks like the sweet spot (backs up consensus here on MUD)