What gears should I choose??

DCSpecial

DCSpecial

Mr. Sinister
The question gets brought up often with lifted trucks regarding gearing choices with bigger than stock tires.

I've complied a little chart of sorts that shows how different gear ratios efffect RPMs with different sized tires.

Here's some info I posted awhile back in another post on another forum:

Here's a quick example of rpms with given gear ratios at 70 mph. All rpms listed are for a AOD trans with a .70 OD ratio....so they aren't dead on, but they are close (the 4R100 has a .71 or .72 OD ratio I think??). So, while the rpms aren't 100% on the money it will give you an idea of how each different gear ratio effects engine rpm at 70mph.

tire..............gear...................rpm
31.6"...........3.73....................1943 (stock truck)
31.6............4.10....................2136
31.6............4.30....................2240
34.7............3.73....................1770
34.7............4.10....................1945
34.7............4.30....................2040
34.7............4.56....................2164
36.7............3.73....................1673
36.7............4.10....................1839
36.7............4.30....................1929
36.7............4.56....................2046
36.7............4.88....................2189
38...............3.73....................1616
38...............4.30....................1863
38...............4.56....................1976
38...............4.88....................2114
38...............5.13....................2223
40...............4.10....................1688
40...............4.30....................1770
40...............4.56....................1877
40...............4.88....................2009
40...............5.13....................2112
42...............4.56....................1788
42...............4.88....................1913
42...............5.13....................2011
42...............5.38....................2109
44...............4.56....................1706
44...............4.88....................1826
44...............5.13....................1920
44...............5.38....................2013
46...............4.56....................1632
46...............4.88....................1747
46...............5.13....................1836
46...............5.38....................1926


I have most common choices pretty well covered....if you want info on a gear/tire combo that I don't have listed let me know and I'll add it to the list.
 
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Thanks.

For 35s, I think 3.73s are fine....as long as you are happy with the performance.

I ran 35s and 3.73s for 24,000 miles......got better mpg on the highway due to the lower rpms. I was happy with it.

If you didn't like it's performance with the 35s, I'd go with 4.10s.

Once you get up into 36s and 37s is when I'd start thinking to regear. Due to the size and weight of the tires.

38s+ I feel it's definately a must to do. I hated running 38s with 3.73s. I went with 4.56s and like the combo. Could have gone to 4.30s as well....little better mpg on the highway.
 
hey, im thinking of going to a 37 inch tire. would you recomend 4.10s. i do alot of highway driving. some of my friends that have gas trucks say that when putting on a larger tire like a 35 they lost about 4 to 5 miles per gallon. does this not happen to diesels because the engine makes so much more torque it doesnt notice the tire or would it be the gear?
 
Here's the simple version of the math.

Take your new tire dia (NDia) and divide your stock tire dia (SDia):

NDia / SDia = Correction (C)

To keep the same highway RPM, you now multiply the correction by the existing gears (SGear), to get new target ratio (NGear).

Example:

NDia/SDia = C

40" new tires / 30" old tires = 1.33

SGear x C = Target gear ratio.

3.73 x 1.33 = 4.97:1 gearing.

Now you have a decision to make. If you want a little more RPM, go to the 5.13? size, if you want a little less RPM, go to the 4.88 size. Seldom will the math be perfect.

The reason you do not use PI (3.14) in the calculation is because it divides out anyhow.
 
smoken02 said:
hey, im thinking of going to a 37 inch tire. would you recomend 4.10s. i do alot of highway driving. some of my friends that have gas trucks say that when putting on a larger tire like a 35 they lost about 4 to 5 miles per gallon. does this not happen to diesels because the engine makes so much more torque it doesnt notice the tire or would it be the gear?
Click to expand...


No, your mpg will go down with a 37.

Mine were good with 35s and 3.73s.....not as good since the 38s and 4.56s though LOL

What gear do you have stock?

4.10s and 37s would help keep your highway rpms low for better mpg.
 
hmm..... very good chart, has got me thinking..... with my 35's and 3.73 gears, the best mileage I've EVER gotten chip or not was 13mpg in the city, and about 17 on the highway (at 70mph). Now, IIRC, a stock 6.0 with 3.73s gets around 16-17city, and ?? highway (from what I've been told, never had mine stock height), so accoreding to that chart, if I were to go to 4.10s, or even 4.30s (I'm leanin 4.30s), I'll loose highway mileage (possibly gain highway towing mileage), but will I be gaining in town mileage?? Just a quick question to help me clear up my own mind. thanks.

--Ryan
 
The 4.10s or 4.30 will make it easier on the the motor to get the meats moving. But, higher speeds the mpg won't be too hot.

I'm surprised that's your mileage.....I get 13-15mpg around town with 38s and 4.56s :D
 
A little side note, I used to have 4.56's in my truck. For daily driving they were awesome, but I busted 2 or 3 sets of them at the track. I was told because there really wasn't enough material on the ring to hold the power. I've got 4.30's now which are great and haven't had any problems with **knocks on wood**. I'm sure it has a lot to do with weight and power output, but that was my experience.

Also, for the record, I saw virtually no change in ET when I went from the stock 3.73's to the 4.56's; they just made it a lot nicer to drive around town.
 
my tires are 325/65r18. what does that work out to be? 35? 35.5? 36? i have been thinking of changing gears and diffs.... just not sure of the gear range, what axles i have and what diff to buy?
 
txastuff79 said:
my tires are 325/65r18. what does that work out to be? 35? 35.5? 36? i have been thinking of changing gears and diffs.... just not sure of the gear range, what axles i have and what diff to buy?
Click to expand...


That works out to be a 34.6" tall tire....12.8" wide

Honestly, I'd stick with the stock 3.73s unless you tow heavy often.

I really liked my 35s and 3.73s.
 
I also really really liked the 35" / 3.73 combo. the 35's are still light enough to get moving easy and with the lower rpms of the 3.73's, the truck really purred on the highway. i got 21mpg on the highway with that combo and programming.

I felt the 4.56's were a tad low with the 38" (37.5 overall) Toyos but now that i've stepped up another size on the tires with these Pro Comps, i like the 4.56's more and actually think they may be too tall if I go any higher but 4.56's are a good overal big tire setup.
 
I got 38s on my truck, thinking about a gear swap up to 4.10s or 4.30s...will this help with my EGTs at all? with an innovative extreme street tune i hit close to 1600*....what to do, what to do?
 
BigPSD said:
I got 38s on my truck, thinking about a gear swap up to 4.10s or 4.30s...will this help with my EGTs at all? with an innovative extreme street tune i hit close to 1600*....what to do, what to do?
Click to expand...


Yes EGTs will drop.
I'd do 4.30s or 4.56s....most likely 4.30s since you have a SCT and some other go fast goodies.


On my 7.3 I ran 3.73s and 38s....then went to the 4.56s with 38s and my EGTs (cruising, WOT, etc..) dropped 200-300* across the board.

Now I'm running 40s and the 4.56s, EGTs are <125* more than they were with the 38s. But, I'll be dropping back down to 38s soon.
 
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