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73-87 Chevy _ GMC Trucks => 73-87 Chevy & GMC Trucks => Topic started by: spacetransient on July 05, 2011, 08:17:25 am

Title: Optimizing MPG's: Gear ratio/RPM's/for mph
Post by: spacetransient on July 05, 2011, 08:17:25 am

I want to change out my rear to something more practical and got good responses here but since it is a project and the recommendations were such a jump (3.73/3.42) over my current 2.73 ratio that I thought of a better way to ask the question: What rear end ratio will get me the closest to 1950 rmp's in a '89 military series 6.2 diesel with dual exhaust with a 700r4 transmission at 74 mph? My tires are a little oversized at 235/75/15's but still 15's. (I usually run at 70-74 mph for a 65 mph speed limit) I have an '86 Sierra 1/2 ton.
Thanks!!

Title: Re: Optimizing MPG's: Gear ratio/RPM's/for mph
Post by: Irish_Alley on July 05, 2011, 11:08:03 am

3.23 will get you there

Title: Re: Optimizing MPG's: Gear ratio/RPM's/for mph
Post by: spacetransient on July 05, 2011, 06:24:42 pm

Thanks. They do not seem to have a gear set for that at drivetrain.com but they calculated my target 1950 rmp's with 27.5" Diameter tires (235/75/15) with the 6.2 diesel and the 700r4 tranny comes out to 3.08 almost exactly (1948 rpms). 3.23 would work if I went with taller tires.
Appreciate the feedback.

Title: Re: Optimizing MPG's: Gear ratio/RPM's/for mph
Post by: Irish_Alley on July 05, 2011, 08:13:45 pm

for some reason i thought those tires where 28.9 idk lol

Title: Re: Optimizing MPG's: Gear ratio/RPM's/for mph
Post by: Stewart G Griffin on July 10, 2011, 09:05:49 am

What mpg are you getting with the 2.73?

Title: Re: Optimizing MPG's: Gear ratio/RPM's/for mph
Post by: Edahall on July 10, 2011, 09:04:32 pm

At 75 mph, 3.08's will give you the best gas mileage with the tires you're using.

Title: Re: Optimizing MPG's: Gear ratio/RPM's/for mph
Post by: Irish_Alley on July 10, 2011, 11:07:57 pm

at 75 yes but its getting up to the 75 that will probably burn what you save

Title: Re: Optimizing MPG's: Gear ratio/RPM's/for mph
Post by: Edahall on July 11, 2011, 08:48:56 am

Quote from: Irish_Alley on July 10, 2011, 11:07:57 pm

at 75 yes but its getting up to the 75 that will probably burn what you save

Hmmm, that's interesting but that's not what I'm experiencing with my 1982 Suburban with the 6.2L.  I've got 3.08's on the rig and I get excellent fuel economy, around 30 mpg on the freeway.  It does very well around town as well.  The 700r4 has a very low 1st gear so it'll even burn a little rubber if I really stomp on it.

On the other hand, my dad has 3.73's and I can't get any more than 21 mpg driving his rig.

Title: Re: Optimizing MPG's: Gear ratio/RPM's/for mph
Post by: Irish_Alley on July 12, 2011, 12:14:33 am

if thats what your getting then i just might be doing a swap lol

Title: Re: Optimizing MPG's: Gear ratio/RPM's/for mph
Post by: Edahall on July 12, 2011, 07:39:01 am

Quote from: Irish_Alley on July 12, 2011, 12:14:33 am

if thats what your getting then i just might be doing a swap lol

Regarding gas to diesel swaps, right now, I've got a 1999 4 door Tahoe with the 350 Vortec and I'm getting ready to yank that engine out and put in a 6.2L diesel with 3.08 gears.  The Vortec gets no better than 19 mpg while a 6.2L diesel set up properly will get 30 mpg.