Which gear ratio for best MPG

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Collector

Apprentice
Aug 28, 2011
61
17
8
Hoofddorp,Netherlands,Europe
Hoping to get some help in sorting out my gear ratio, have a stock monte carlo from 1979, 305 stock engine running on LPG, th200 and i dont know what ratio the diff has. (GM 7.5" 10 bolt)
Present diff has a nasty "boink" noise know and then, and was reason for me to buy a spare diff, the spare diff is getting an overhaul from me with fresh bearings and seals, so when i get my MC on the 4 post i can swap the rear axle.

I have also bought an TH2004R which i am going to swap with the present TH200.

The spare diff says on the ring gear 15-41 if i am correct thats a 2.73 ratio

If i want to change the ratio is have a choice in these ratio's (according to rockauto)
2.73
3.08
3.23
3.42
3.73
3.9
4.11
4.56
At 9 dollars the gallon over here, a little fuel efficiency would be very nice, can someone explain what happens when i change my ratio to (example) 3.42.
I am dutch so its a PITA to convert from inch to metric and vice versa so please keep the answers as simple as possible
 

tlonz88442

Not-quite-so-new-guy
Jul 8, 2014
18
0
0
2.73s- 3.23s is keeping mpg in mind but if u go up past 3.23s to 3.42s that should be easing up on performance and decreases mpg , if ur not hotrodding but only crusin with a small block then ur org. Gears is what u need
 

Collector

Apprentice
Aug 28, 2011
61
17
8
Hoofddorp,Netherlands,Europe
tlonz88442 said:
2.73s- 3.23s is keeping mpg in mind but if u go up past 3.23s to 3.42s that should be easing up on performance and decreases mpg , if ur not hotrodding but only crusin with a small block then ur org. Gears is what u need
Plan is to use a TH200R4 transmission, and i dont want to smother the engine while in 4th gear and in lockup.
 

pontiacgp

blank
Mar 31, 2006
29,270
20,391
113
Kitchener, Ontario
the spare carrier you have is a 2 series so that limits you to a 3.08. Any higher you need a series 3 carrier or a spacer or thick gears to make them work on a 2 series carrier. I chose the 3.08 cause I think it's a nice gear for city and highway, especially with a stick so I'm not shifting every 20 feet
 

FE3X CLONE

Comic Book Super Hero
Dec 2, 2009
2,714
47
48
Ohio
Your stock gears are likely something in the 2.14 to 2.41 range.
Since you'll be using a TH-2004R you could use the below ratios that would keep you in the same ball park as your current 3 speed:

3.73x.67 = 2.49 final drive
3.42x.67 = 2.29
3.23x.67 = 2.16
 

Collector

Apprentice
Aug 28, 2011
61
17
8
Hoofddorp,Netherlands,Europe
FE3X CLONE said:
Your stock gears are likely something in the 2.14 to 2.41 range.
Since you'll be using a TH-2004R you could use the below ratios that would keep you in the same ball park as your current 3 speed:

3.73x.67 = 2.49 final drive
3.42x.67 = 2.29
3.23x.67 = 2.16

This is how i look at this
3.73 is diff gear ratio x 0.67 is gear ratio from the TH200R4 in LU that makes the final gear ratio 2.49 am i correct ?

If this is the case then i can do the math from here, Thanks for you're patience and the help

[edit] Ordered 3.42 "thick"gears from rock auto found this site who is very good at explaining everything http://www.crawlpedia.com/thick_gears.htm
 

BOWTIEKID

Not-quite-so-new-guy
Apr 21, 2013
7
2
3
Collector said:
FE3X CLONE said:
Your stock gears are likely something in the 2.14 to 2.41 range.
Since you'll be using a TH-2004R you could use the below ratios that would keep you in the same ball park as your current 3 speed:

3.73x.67 = 2.49 final drive
3.42x.67 = 2.29
3.23x.67 = 2.16

This is how i look at this
3.73 is diff gear ratio x 0.67 is gear ratio from the TH200R4 in LU that makes the final gear ratio 2.49 am i correct ?

If this is the case then i can do the math from here, Thanks for you're patience and the help

[edit] Ordered 3.42 "thick"gears from rock auto found this site who is very good at explaining everything http://www.crawlpedia.com/thick_gears.htm

That's what I would have picked, at least it won't fall on it's face in city traffic. How expensive is the LPG over there?
 
Oct 14, 2008
8,806
7,746
113
Melville,Saskatchewan
I run at 1850 rpm at 100 km/hr with a short 25.6" tire (stock height). I also have 3.42 gears and a 2004r trans.
 

jacobdavis1979

G-Body Guru
Feb 18, 2011
700
14
18
If your going with an overdrive transmission then 3.42 will give you nice power and you should cruise at 60 mph about 1900 with i think a 26 inch tall tire. My math may not be correct but that should be about right

Olds307 im sorry i just saw your reply i promise i wasnt trying to rip that from you lol
 

cyrus_the_great

Greasemonkey
Sep 8, 2009
237
0
16
IRAN
just wanna join this good topic .. i have 2.41:1 and 2004R , on highway with 110km/h speed i can not hear the engine even :mrgreen: tachometer dosent work so i don't know exact RPM
 
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