Jump to content

Rear end gears (formula?)

Matthew4:19

Recommended Posts

Matthew4:19 Enthusiast

Matthew4:19

Posted
Posted
I want to find out what gears would keep me as close to stock as possible. I have 3.73s now with 245/16 tires. I plan on using either 305/16 or 315/16 Thanx Don
Kansas Kid Senior Enthusiast

Kansas Kid

Posted
Posted
With your current setup (assuming you have the 4L60e transmission) you should be turning around 1970 RPM at 70mph.  So with 315/75R16 tires and 4.10 gears that would put you at 1935 RPM and 2035 RPM with 305/70R16 tires at 70mph.
americanmuscleman Enthusiast

americanmuscleman

Posted
Posted

First off, this post is going to make you do this --> :smash:

 

An important piece of information you don't have listed is the aspect ratio of the tire.  I'll assume your 245's are 245/75/16's and the other two tires you listed 305/75/16 and 315/75/16.  The 245 for example is the width of the tire in millimeters.  The 75 is the aspect ratio, which would be 75% of the width (in this case .75 times 245).  

 

Using the "tire diameter" formula on www.smokemup.com It will tell you your overall diameter in inches.

245/75/16= 30.47" tall tire

305/75/16= 34.01"

315/75/16= 34.60"

 

Plugging those known tire heights into the "speed vs rear end for tire and gear ratio" formula, you can compare gear ratios for given tires.  You have two choices with gears if you want to stay near stock.  Either stay with the 3.73's, or put in 4.10's.  The next deeper size is 4.56 which is going to be too low.  So I'll use 4.10 below.

 

You can enter the tire diameter, gear ratio, and transmission.  If you have an automatic, select 700R4/4L60.  That tranny has the same gears as a 4L60E.  Under Torque converter slip, put in "0".  4L60E's use a lock-up converter, so the slip is "0".

 

With your 245 tires and 3.73 gears, your running:

 

Transmission Ratios:

  1st: 3.06 2nd: 1.63 3rd: 1 4th: 0.7

 

Tire Diameter 1: 30.47 inches

Rear Gear Ratio 1: 3.73:1

 

MPH

RPM 1st 2nd 3rd 4th

1000 8 15 24 35

1500 12 22 36 52

2000 16 30 49 69

2500 20 37 61 87  

3000 24 45 73 104  

3500 28 52 85 122  

4000 32 60 97 139

4500 36 67 109 156

5000 40 75 122 174

5500 44 82 134 191

6000 48 89 146 208  

6500 52 97 158 226  

7000 56 104 170 243  

7500 60 112 182 260  

8000 64 119 194 278

 

 

For 305 tires and 4.10 gears:

 

Transmission Ratios:

  1st: 3.06 2nd: 1.63 3rd: 1 4th: 0.7

 

Tire Diameter 1: 34.01 inches

Rear Gear Ratio 1: 4.10:1

 

MPH

RPM 1st 2nd 3rd 4th

1000 8 15 25 35  

1500 12 23 37 53  

2000 16 30 49 71  

2500 20 38 62 88  

3000 24 45 74 106  

3500 28 53 86 123

4000 32 61 99 141  

4500 36 68 111 159  

5000 40 76 123 176  

5500 44 83 136 194  

6000 48 91 148 212  

6500 52 98 160 229  

7000 56 106 173 247  

7500 60 114 185 264  

8000 65 121 197 282

 

 

 

For 315's and 4.10's:

 

Transmission Ratios:

  1st: 3.06 2nd: 1.63 3rd: 1 4th: 0.7

 

Tire Diameter 1: 34.75 inches

 

Rear Gear Ratio 1: 4.10:1

 

MPH

RPM 1st 2nd 3rd 4th  

1000 8 15 25 36  

1500 12 23 38 54  

2000 16 31 50 72  

2500 21 39 63 90  

3000 25 46 76 108  

3500 29 54 88 126  

4000 33 62 101 144  

4500 37 70 113 162  

5000 41 77 126 180  

5500 45 85 139 198  

6000 49 93 151 216  

6500 54 101 164 234  

7000 58 108 177 252  

7500 62 116 189 270  

8000 66 124 202 288

 

:smash::fume::fume:

Ok.  Thats a load of information to take in.  Simply put, you can compare the rpm in 4th gear at say 2000rpm.  

With 245's and 3.73's at 2000rpm, your turning 69mph.

With 305's and 4.10's at 2000rpm, your turning 71mph.

With 315's and 4.10's at 2000rpm, your turning 72mph.

 

Summed up, 4.10 gears with 305's or 315's will be the closest to stock 3.73's with 245's.  Keeping the 3.73's with 305's or 315's will be like have a numerically lower axle ratio, like 3.42's.

 

I realize that was about 20 times more info than you probably wanted, but you asked for formulas and got me on a roll. :P:D

 

Hope that helps.

Matthew4:19 Enthusiast

Matthew4:19

Posted
  • Author
Posted

AmericanMuscleMan, Thanx great info. Now I see what I was doing wrong, I was told that the 245 was the circumfrance in cm.. Now with the info you gave me I was able to comfirm my formula was correct. If anyone is interested here is what I assume to be easier, well at least it is once you find out the diameter of the tire.

 

Take the original tire dia. and devide by the new tire dia.

30.47/34.6=0.8806

then divide your original gear ratio by the quotiant above

373/0.8806=423.57 or 4.24:1

this is the ratio required to keep the speedo and tach accurate.

 

 

I found the tire Dia. by using this formula below

For you calculations there are 0.03937" per mm.

245/75/16 is 245*0.75=183.75mm, 183.75*2=367.5mm, 367.5*0.03937=14.47", 14.47+16=30.47"Dia.

Shorter formula is

315/75/16 is  (315*1.5*0.03937)+16=34.6"Dia.

 

Your links work great especialy if you want to predict your RMPs or MPH with a given set of gears and tires. Looks like the 4.10 gears will be closest to the stock 3.73, but since I plan on towing I will probably choose the 4.56 gears which will put it slightly lower than stock. This choice will put me at a gear ratio very close to me having the stock 4.10 gears with my existing tires. 4.66 would be the same as having 4.10s now.

 

Thanx for your help, I hope the way I worded this is undertsandable and helpful.

Archived

This topic is now archived and is closed to further replies.

Go to topic listing

  • Recently Browsing   0 members

    • No registered users viewing this page.

  • Forum Statistics

    250.3k
    Total Topics
    2.7m
    Total Posts

  • Member Statistics

    342,737
    Total Members
    8,960
    Most Online
    Christopher Miller
    Newest Member
    Christopher Miller
    Joined

  • Who's Online   4 Members, 1 Anonymous, 1,354 Guests (See full list)

  • Latest Articles

  • Posts

    • txab
      2001 GMC Sierra - Banging feeling in steering wheel

      By txab · Posted

      Without knowing how bad your banging is, one suggestion is the intermediate steering shaft on the steering wheel. These used to be a problem on 800 and earlier series. The feeling/sound was sometimes described as "clunking" and was felt in the wheel. Also there is a plastic piece in the lower column that is referred to as a bearing. Not your traditional looking bearing though. Your ball joints, bushings and tie rods or steering rack (if equipped) are the main point of issues. You never mention total unit miles, only that you've put 60k on it.   You can search the site for help on the ISS description. Plenty of these have been replaced   https://duckduckgo.com/?t=ffab&q=site%3Agm-trucks.com+intermediate+steering+shaft&ia=web
    • GAMark
      Road Master Active Suspension

      By GAMark · Posted

      Yes I have is set with the 1mm disk (Minumim Tension), for spring spacing and I raised my truck in the rear 1.5". You can go 2mm  and I think 3mm if you need.Minimum. We are on our way to TN from GA.. My wife is driving and stated she cannot believe  and   stated she can not belive the difference.
    • GAMark
      Road Master Active Suspension

      By GAMark · Posted

      Most 1 ton trucks come with the factory overload spring, which need to be removed for installation of RoadActive Suspension. Removing the overload allows proper fitment and ensures the system works as intended. The RAS® Mega-Duty kit will reduce squat and provide significant improvement vs the factory overload spring.
    • No1UKnow
      Transmission Woes :-(

      By No1UKnow · Posted

      I wonder as well, why they would send me a text message with links to these suggested procedures totalling over $10,000 without even the slightest hint of how they arrived at these being necessary.
    • Grumpy Bear
      OCI, not when but why?

      By Grumpy Bear · Posted

      https://precisionlubrication.com/articles/oil-filter-efficiency/     For them it is a problem of $$$$ and compliance. To cover both they have to lie from the heart.    Government wants low waste oil numbers so the bar needs to be very low on contamination to met that requirement.  Consumer wants cheap maintenance (perceived cost). Again requiring a low bar.  OEM wants high margins at a cost effective warranty rate and lots of repeat customers. More low bar.  Filter manufacture wants the same thing the OEM wants. See the pattern?   Consumers are fickle and also want perfection requiring a HIGH bar. Inconsistent with all of the above. Consumers can be, when the put their minds to it, inquisitive, love winning arguments which requires some useful information to be available to support them. And this is where it gets sticky and the lies begin.    Information needs a solid well grounded point of reference if we are to compare in a useful way. There is a standard for this.  ISO 4548-12. This standard sets the particle size at a specific Beta ratio and darn few are faithful to its intent.    If they don't state the test method they change the particle size or the Beta ratio to hit a perceived favorable standard. They obscure the information in a way as to prevent direct comparisons leaving the consumer with one method. UOA's with an ISO 4406 test result. Pricey and not allot of labs willing to do the work. Lobby work happens everywhere, not just at the seat of government.       

  • GM-Trucks.com Clubs

  • Popular Contributors

×
×
  • Create New...