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Tool Talk Discussion Forum

O/T Gas mileage/gear ratios

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Angle Iron

10-07-2005 08:45:43




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I have an 87 Suburban and am wondering what the good and bad would be going from a 3.73 to a 3.08 ratio on mileage and power. Its a silverado 10 350 overdrive with throtle body. It used to pull a travel trailer and car trailer. Now just ocasional use to pull 19 ft boat. Main use is just to get around. 15 mpg is the best it will do for now. Wondering if a 3.08 would help mpg without too much lost towing ability. Only tow the boat around home, pretty flat here on the coast and the boat landings are prettygood here.
Simple fix would be to get a different truck but this one is pd for and only has 118000 miles on it. Thanks Angle Iron

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thejdman01

10-07-2005 17:28:28




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 Re: O/T Gas mileage/gear ratios in reply to Angle Iron, 10-07-2005 08:45:43  
if you currently have say 235 65 r16 try goign to a 265 80 r16 or so i woudlnt go out and buy different tires but when it is time to get new ones change to the biggest size available for your rim. you got to buy tires anyways the price for a little bigger size isnt very big



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jdemaris

10-08-2005 05:31:20




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 Re: O/T Gas mileage/gear ratios in reply to thejdman01, 10-07-2005 17:28:28  
Bigger tires (larger diameter and increased width) cause more rolling friction and thusly, less MPG, not more. I've got two 87 3/4 ton 4WD Suburbans, both diesel - one has the T-400 trans and the other the 700R4 overdrive trans with lockup converter. Both have 3.70 axle ratios. I live in a hilly area in Central New York and here, the overdrive makes no difference in fuel mileage. I average 14 with either Suburban, and both can get about 20 on a flat highway (a two-wheel drive 1/2 ton diesel Suburban can get 24 MPG and 350 gas about 17 MPG). I've also got several diesel K-5 Blazers, and again, there is no difference in fuel mileage between the overdrive and non-overdrive. I DID put oversize tires on my 86 K-5 and the fuel mileage dropped by almost 2 MPG. On a long trip on a flat highway, not pulling anything, the overdrive does make a slight difference, but otherwise, not much. With a two-wheel drive, 1/2 Suburban, about a 2 MPG gain can be expected on flat highway, and otherwise, no gain at all.

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Angle Iron

10-07-2005 14:46:57




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 Re: O/T Gas mileage/gear ratios in reply to Angle Iron, 10-07-2005 08:45:43  
Thanks for the response. Advice was pretty much in line with what I figured. Loss of towing capacity and long if any payback. I bought it about 11 years ago and it's kinda like an old friend, hard to part with. Angle Iron



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DigDugNC

10-07-2005 11:55:07




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 Re: O/T Gas mileage/gear ratios in reply to Angle Iron, 10-07-2005 08:45:43  
I would look more at your tires AND leave the rear axle gear alone. If you swap the gears it will take a long time to recover the money you spent doing the swap and the 3.73 ratio is a good mix of power vs economy. Truck will still pull good but maintain good highway speeds. Make sure your tires are the correct recommended size and are properly balanced and properly inflated this makes a big difference as it effects the resistance placed on the motor to freely turn the tires(similar to the effect of pulling a trailer). Also ask your tire dealer about the roll resistance of the tire. I read about that once in a truck magazine. I don't remember how the concept works but obviously it has something to do with the tread design and hardness of the rubber, which determines how much friction is required to rotate the tires. A lower number is less roll resistance. Also a proper tune-up and good air breather such as a K&N would help, with good driving habits.

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surveyor

10-07-2005 10:06:10




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 Re: O/T Gas mileage/gear ratios in reply to Angle Iron, 10-07-2005 08:45:43  
No easy answer to this. The speed you drive factors in as well. If you go to too low a final drive ratio the engine will be lugged down while towing. Then you may not have enough line pressure in the tranny to keep the clutches from slipping. I've got the 700r4 tranny in several vehicles and have had mixed results. 3/4 ton sub w/ 4.10, poor mileage but tows anything, good reliability when I drive it, when others drive at 35 for ever and let it shift back and forth from 2 - 3 new tranny. Blazer w/ 6.2 diesel, 3.42 good milage, but short on power for trailers and big hills. Tranny built for performance at 1800 rpm.
Put one in an old firebird, worked great once I replaced the 2.56 ratio with a 3.55. Original one lugged down too much. Poor engine performance and slipping clutches with hard acceleration.
Short answer - I think the 3.42 was the lowest offered and it had a reduced tow rating. If it was mine I wouldn't change it unless the current one was shot and then probably still wouldn't go to the 3.42

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Bob

10-07-2005 09:03:56




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 Re: O/T Gas mileage/gear ratios in reply to Angle Iron, 10-07-2005 08:45:43  
I had a 1992 Sub with 3.42 gears, and it was just fine, as I did not tow with it. However, I don't think you'd want to go to a numerically lower gear ratio than that, with the OD transimission.



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