True speed after regear formula.

for first gens with stock 4.10 going to 4.88, and 31's going to 33's

@ 55 real speed = 48
@ 60 real speed = 52
@ 65 real speed = 56
@ 70 real speed = 61
@ 75 real speed = 65

Gear ratio change:
(4.88-4.10)/4.10 = 0.190243902
(New gears - stock gears)
Tire ratio change:
(33-31)/31=.06451629
(current tire size- stock tire size)

Then 0.06451629-0.1902= -.012568371
Tire-gear ratio.


Then add 1 and use the number to multply by your speed.
-.012568371+1 = .87431629

Take .87431620 and multiply by speed on the speedo. The answer will be your actual speed.

33 with 4.88 is off by about 12.55 percent

 

for second gens with a 3.73 rear going to 4.56. With 285/75/16.
(4.56) -(3.73) then / by 3.73= .222520107

(32.83) - (30.61) / 30.61 = .072525319
(285/75) - (265/70) / 285/75 = ^^


(.072525319)-(.222520107) = -.149994788

-.149994788 + 1=.850005212


.8500 x 55 mph = 46.75

speedo reading = actual speed
55 = 46.75
60 = 51
65 = 55.25
70 = 59.5
75 = 63.75

 

for second gens with a 3.73 rear going to 4.88. With 285/75/16.
(4.88) -(3.73) then / by 3.73= .350753617

(32.83) - (30.61) / 30.61 = .072525319
(285/75) - (265/70) / 285/75 = ^^


(.072525319)-(.350753617) = -.278228478

-.278228478 + 1= .721771522


.7218 x 55 mph = 39.699

speedo reading = actual speed
55 = 39.69
60 = 43.31
65 = 46.92
70 = 50.53
75 = 54.14

 

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I don't understand this. Why would you be going slower than what your speedometer read? And what are the effects of re-gearing?

 

Because your driveshaft is spinning faster than usual. It gives you more low end power.

Stoking for me is 4.10 the drive shaft spins 4.10 times per wheel revelation. I went to 4.88, which means the driveshafts spins 4.88 times per wheel revelation. The speedo gear to determine speed is reading off the driveshaft.

 

So how exactly do you do this, and are there any downsides at all to re-gearing?

 

It's all in your differential. You swap the gears in it out. Or swap the actual third member.

Downside is your truck will be in a higher rpm. At 80 mph I'm at 3000. Before the regear I was at 2200. But I rarely drive 80 I mainly do 70 I saw a bit in mpg increase.

Upside you have more lowend power. Great for 33 inch tires and bigger

 

I'll probably never do it. Thanks for the info, man!

 

hey andre,

just came across your thread, seems like you're the guy to answer my question.

i'm running 305/55 20's. the online tire converter tells me these are 33.2" tires. they're more like 33" when i measured them (tread wear).

i have a 2nd gen and run stock gearing. can you tell me how much faster i am going relative to my speedo?

 

For that the tire size calculator tells you your new speed and all.



from the tire calculator

 

^ thanks for the link!

 

I just used this, thanks.

With my 33's and my new 4:10's I am only off by 0.08%

When my speedo says I am going 65mph, I am actually going 64.525mph...
Or, if I go 300 miles on a tank, I actually traveled 297.8 miles.

(I came up with a correction factor of .9926979659.)

Sweet, time to do a MPG check!

 

I dont think this is accurate. On second gens the speed sensor is at each wheel so it measures the speed at each hub. Gearing does not affect the speedometer. Tire size does how ever. When going from a stock 30.6" tire to a 315/70/17 (34.5) the speedometer will be off between 12-13% giving you a slower reading on speedometer than actual speed going. When I am showing 65 on my speedometer I am actually going 72-73 mph. Same is true for mileage every 10 miles I drive I show 8.6 on the trip meter due to larger rolling diameter of the tires.

 

Then disregard the second gen part.

 

I just found this out also. I was driving down the interstate and using the milepost markers to double check my odometer.

20 miles traveled registered 18.4 miles on my odometer.
30 miles traveled registered 27.6 miles on my odometer.

I found out I am actually off by 8.6682%

I am running 255/85/16's (and I re-geared to 4:10's.)

The good news is that at 65mph my rpm's are right around 2100rpm's.

 

so will 4.10 gears help me get better mpg? with 285/70/17

 

Everyone that has 4.10's and 33's said they saw an increase in fuel efficiency. Lets get some more responses from those guys.

 

Yes. On the second gen. Going to a 4.10 from a 3.73 is like the first gen going to 4.56 from 4.10.