# Bigger Tires - ReGear or ADD HP???

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by Greg72, Jan 3, 2002.

1. ### Greg72"Might As Well..."Staff MemberSuper Moderator

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I was going through another one of my "super-calculator-problem-solving" phases today, and came up with an interesting question for myself:

After adding bigger tires, people usually re-gear the axles to reclaim the power that was lost with the larger diameter tire (and slightly increased weight). It's really pretty simple to understand why this is needed.....a larger diameter tire reduces the "effective gear ratio"....which in turn reduces the power available to accelerate the vehicle.

The overall gearing of the vehicle is a combination of tire diameter, rear end ratio, and the individual ratio in the transmission (1st, 2nd, etc) .....and these are mulitplied by the available torque from the engine to give a measure of acceleration (g-force, Wheel thrust, ft/sec)

Here are two combinations that appear to have nearly IDENTICAL abilities to "accelerate", that is, to apply force to the ground and move the vehicle forward at the same rate:

Vehicle 1:

33" Tires
5500 Lbs
3.73 Gears
Engine producing 300 Lb/Ft of Torque

Upgrade that vehicle with 37" tires (which also adds weight) and you might have something like:

Vehicle 2:

37" Tires
5800 Pounds
3.73 Gears
Engine producing 300 Lb/Ft of Torque

Unfortunately, vehicle #2 would need an engine with about 360 Lb/Ft of torque to give the same acceleration as vehicle #1. Most people in this situation would have changed gears.....(probably to 4.56s) and achieved the same result.....identical acceleration as vehicle #1.

So my question is this:

Is there any reason not to try for the extra engine power instead of always swapping gearsets? Changing out F&amp;R gears is not cheap.....especially if you have to pay a shop to do it. It seems that you could get the extra engine power more cheaply.

The reasons this might be GOOD are:

1. No gear swaps (saves money)
2. Higher gears (lower numerically) are stronger than higher gears
3. Highway RPMs actually go DOWN....so you can still drive at reasonable speeds without adding an O.D. transmission
4. Isn't it cool to have BIG HP &amp; Torque numbers anyway?

The reasons is might be BAD are:

1. Heat in the transmission....it's doing more work, when the axle gears do LESS.
2. Crawl Ratio does not improve. Re-Gearing always gets you better "crawl numbers"
3. HP &amp; Torque may be cheaper than gears initially, but costs for BIG power may surpass a re-gearing.

Anyway.....I was curious if I've missed the "CRITICAL REASON" that explains why you can't just "ADD POWER" instead of "RE-GEARING" when doing a larger tire swap???

-Greg72

'72 K5 Blazer - 427BB/TH350/NP205/6" Lift/35x12.50's
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2. ### zakk1/2 ton status

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i agree with both the good and bad, but there are trades off (as in everything) Also the cost of yor tranny buring out after running 3.08's and 36's is not added.

My tranny was dying after doing exactly that. But i snapped it in half before it totally burned out.

I am running 4.10's now with the 36's and will keep them wit the 38.5/39.5 TSL's i'll get once i win the lotto. regear is too much and a doubler would help more than a set of 4.88's on top of the 4.10's being stronger to hold up to the fabled doubler.

So i am riding the fence on this one...torquey 400 with 4.10's
but my 400 doesn't hold a candle to your 427

-zakk

'77 1-Ton K5

CK5's MOAB 2002 OR BUST!!

3. ### Burt4x43/4 ton status

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So the first thing I thought of is the #1 question everyone asks!!
What type of wheeling do you want to do?
If you want to try and takel the Con or some of the knarly stuff at Moab then I think lower gears would work way better than high HP. Also I think a higher HP rig without low gears would brake more Ujoints and other stuff.
I belive you need both to find the "What works best for you &amp; your rig".
Too much HP and too high of a ratio is good for hammering down the throttle thru a mud bog but thats about it.
.02c

72K5Led ZeppelinRock ON!

4. ### Greg72"Might As Well..."Staff MemberSuper Moderator

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Burt,

Using the vehicles from the original example:

Vehicle #1:

Low Range Crawl (uncorrected) = 18.42:1
Crawl Ratio before TH350 "Stall" = 36.85:1
Adding an ORD Doubler (uncorrected) = 37.03:1
ORD Doubler before TH350 "Stall" = 74.07:1

Vehicle #1 "Re-Geared" to 4.56's:

Low Range Crawl (uncorrected) = 22.52:1
Crawl Ratio before TH350 "Stall" = 45.05:1
Adding an ORD Doubler (uncorrected) = 45.28:1
ORD Doubler before TH350 "Stall" = 90.55:1

OK, I've thrown in a "Doubler" that wasn't in the original consideration.....because I don't want this thread to just die after a "depends what you want to do"-type answer . Without the Doubler, the crawl ratios are pathetic.....and that is why most people instantly tell you to "re-gear".....you'll never do any slow climbing with an 18.42:1 Crawl Ratio!!!! Might as well go hang out in the sand dunes and mud pits!!!

So try to stick with me on this.....let's say you are willing to add the "Doubler" to keep a decent crawl ratio, AND add a HUGE tranny cooler, to help keep the transmission cool and happy.... Let's face it, having a decent highway RPM without adding an expensive O.D. tranny would be a VERY desireable option for me, and probably others too!

-Greg72

'72 K5 Blazer - 427BB/TH350/NP205/6" Lift/35x12.50's
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5. ### Burt4x43/4 ton status

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Ok so it sounds like you have a goal here.
You want to have great Hwy and great crawl ratios.
I ain't all that smart when it comes to numbers so I can't 'feel' what the differnt #s are doing
But it does seam like you already have it all figured out!
So whats the question? Are you looking for someone who has a 'real world' experance. You sure have beat up that ratio calculator!!
I know, once again I am no help to you!! But hey at least I put this thread back to the top for ya

72K5Led ZeppelinRock ON!

6. ### mudhogTHEGAMEStaff MemberSuper Moderator

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"So try to stick with me on this.....let's say you are willing to add the "Doubler" to keep a decent crawl ratio, AND add a HUGE tranny cooler, to help keep the transmission cool and happy.... "

or just get a sm465

wheeling videoshttp://community.webshots.com/user/ssmith6333

7. ### txbartman1/2 ton status

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Couple of things you should look at is where the torque is being applied. If you increase the torque from engine, you are increasing the torque on your axle shaft. However, whether you increase the torque on the engine of re-gear, you are still "delivering" the same torque to the axleshaft. In simple terms, it takes the same amount of torque at the axleshaft to turn a big tire regardless of where the torque comes from. Now think about the other components: the u-joints in the driveshaft, the engine itself, the transmission. While yes, I know that technically there is the same torque applied everywhere because they are all connected, think of this. When you pedal a 10-speed in 5th gear from a stop, you have to work a lot harder than you would in 1st gear. So, if you re-gear, it saves the work that needs to be generated by the other components.

If you do not have an OD transmission, smaller gears could benefit you. Because, as you stated, your highway RPMs would go down. But, you need to keep them in the power band of the transmission. Going down too far can be detrimental to even a non-OD automatic. If you have OD, you will quickly hurt yourself without regearing.

But you also have to ask where do you need the power. If you need it at idle to 1500 RPM, I don't know many engines that put out high torque (enough to counter the effect of not regearing) at that low of RPM. Gonna use it for towing then you are really going to be in a bind (no pun intended). Gearing helps across all RPMs, whereas increased torque may only help within a limited RPM range.

That last fact is the biggest one that made me regear. Bigger engine torque will only really help and counter the effect of the bigger tires with a ceratin RPM range. Gears make an impact throught the RPM scale. And, yes, regearing can be expensive. Around here it runs \$250-350 and axle (less for rear, more for front). So, not including parts, regearing would run you around \$600. Inlcude parts and you are around \$1000. Can you get the same result from modifying the engine alone for \$1000 total parts and labor?

That is all just my .02.

Brian
Check out <a target="_blank" href=http://tx85gmcguy.alloffroad.com>My Jimmy</a> with all her projects!

8. ### muddin4fun3/4 ton status

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<blockquote><font class="small">In reply to:</font><hr>

Gearing helps across all RPMs, whereas increased torque may only help within a limited RPM range.

<hr></blockquote>

Very good point.

I added more hp and torque and regeared
Most engines put the most out inbetween 3,000 and 4500 rpm. Regearing will help more in idle to 1500 rpm which is what you sound like you're looking for. Plus, no worries about your tranny.

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9. ### 3car1/2 ton status

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the point of regearing is to keep the engine in its proper rpm range or powerband where it makes the most power and runs efficiently.gotta do the gear thing...

eatin'fords and shittin'dodges down in the sunshine state.

10. ### Greg72"Might As Well..."Staff MemberSuper Moderator

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Brain,

I think I just heard the REASON that I was wondering about!

Your description makes a lot of sense to me. All of my calculations only accounted for "peak torque" of the engine. While this is fine for comparing "apples to apples", it doesn't fully capture the 'real-world' experience of leaving the stoplight at 800RPMs and accelerating up into the "meaty" part of the torque curve.

The experience at low RPMs may be quite unsatisfactory, unless the motor was ABSOLUTELY built for gobs of torque down low...... I think I'll re-run my numbers using a variety of engine RPMs &amp; their corresponding Torque values to see if there's still a way to make this work.

Thanks for the insight.

-Greg72

'72 K5 Blazer - 427BB/TH350/NP205/6" Lift/35x12.50's
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11. ### Greg72"Might As Well..."Staff MemberSuper Moderator

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Burt,

HAHAHAHAHAHA..... Well, I'm not sure that I'd call 74.07:1 a "great" crawl ratio.....but compared to my current 36.85:1 Crawl (best case) it's certainly better.

Both you and Brian pointed out that U-Joints may become and issue. That's another potentially BIG problem with my idea. I'll have to look up the torque ratings of the various U-Joints to see if these combinations are going to simply "grenade" those joints all the time.

-Greg72

'72 K5 Blazer - 427BB/TH350/NP205/6" Lift/35x12.50's
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