# Gear ratios - which to choose

Discussion in 'The Garage' started by W7NB, Dec 14, 2006.

1. ### W7NB1/2 ton status

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As the build saga continues...

I have a chance to snag a set of axles out of an 88 'burb cheap - like 250 bucks. Got to drive the rig and nothing seems out of place - he's just doing a major (like 2 1/2 ton Rockwells) upgrade and wants to get rid of these axles. Only issue is the axles have 3.73 gear sets. I run 32's and am trying to dump my 3.08 gears. I don't plan to go any bigger tires on this rig as I tow with it and would like to keep it fairly civil.

Here are the calculations I ran comparing the 3.08's to 3.73 and 4.11. These are based on a 700R4 OD Ratio of .7:1

3.08 with 33's 2100 rpm= 66 mph
3.73 with 33's 2100 rpm= 54.5 mph
4.11 with 33's 2100 rpm= 49.5 mph

Or I can convert that to RPM@Speed

3.08 @ 65mph= 2070 RPM
3.73 @ 65mph= 2507 RPM
4.11 @ 65mph= 2762 RPM

Highway crusing and towing would be a bigger consideration for me then grunt. If I'm running a trail I'll put her in 4 low and have at it.

My only problem is that I don't know where the ideal crusing rpm is for a TBI 350. I seem to remember somthing about 2400 RPM but I'm not sure - input here would be very helpful!

2. ### AussieK51/2 ton status

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Check your calcs 32-33's on 3.73's with a lock up 700 will be approx 1800 @ 65.

You need to look at 4.56 or 5.13's

Jas

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The charts and calculators don't take into account many real world factors , like wind , elevation , and lock up etc .

In my own use , I have run 31 , 32 , 33, and 35's . And I have had 3.08 , 3.73 , and 4.56 gears . I also have a good working lock up 700r4 .

With 3.08's and 33's , I could cruise 65-70 on flat ground and never see 2100 rpm .

With 3.73's and 33's at the same speed I could tickle 2100 rpm .

Now I have 35's and 4.56 , and its close to stock . I do 65 mph and maybe 2100 rpm at 65 mph.

4. ### W7NB1/2 ton status

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Problem is, I can't find any dino charts to determine what ideal cruise RPM would be. Was 2100 fairly decent for fuel economy and power, or should it be closer to 2400? I guess thats the missing element in all of this.

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Well , I have a 305 with higher compression LE9 , and I make peak torque around 2000 rpm anyway .

Yours is what , a LO5 ( tbi 300 ft lbs ) ? I think those were rated for peak torque under 2500 too .

6. ### iwaxmyjimmyCollege web wheelerPremium Member

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what formula are yall using to come up w/ these rpms? b/c ive got 33s and i blew my pinion gear in my rear axle and ive got a 4.88 ring and pinion but i dont know how bad its gonna be rpm wise. i plan on putting 38s on it ASAP maybe around feb. and also ima swap in some one tones around then well hopefully.

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Speedometer , and tachometer . The charts are a rough guide .

8. ### iwaxmyjimmyCollege web wheelerPremium Member

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my jimmy didnt come w/ a factory tach and its not that high on my list of things to do ya know. i was just tryin to get a ball park figure for where my cruising RPMs will be

9. ### W7NB1/2 ton status

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10. ### blazinzukBuzzbox voodooPremium MemberGMOTM Winner

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From what I have heard most poeple say your cruising speed should be as close as possible to where your engine developes its peak torque. I am not sure what that is for your engine. Also remember in doing your calcs you need to measure your tire from the center of the hub to the ground this will change your heigth calculations somewhat. By doing that you will get a truer rpm vs speed chart.

11. ### iwaxmyjimmyCollege web wheelerPremium Member

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according to them caculators i should be fine running 4.88s. says ill only be turning 2000 rpm at 57 mph. dunno if its right but i guess that makes everyone i talk to a dumba\$\$ i had one guy tell me i was gonna be spinnin around 4K rpms at 65

12. ### AussieK51/2 ton status

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I use this site http://www.4lo.com/ , its simple and has some good info on trans gearing plus a rudimentry guide for choosing rpm range. Its got crawl ratios etc etc

I also use a neat excel sheet that I found last year that lets you model your rpm range Vs speed, I find it quite handy. PM if you want a copy.

Jas

13. ### AussieK51/2 ton status

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A couple of months ago I checked this out by doing an rolling diameter experiment with the 33's on my K20. At the end of the day I concluded that you should just measure across the tire, the effect is negligable.

The figures on the pavement represent psi in the tire at each test. After crunching the numbers I came to the conclusion there would be as much variance due to tread wear over the life of the tire.

The rolling diameter is only very midly affected by the ratio of vertical (to ground) and horizontal measurements.

Jas