Trying to understand gearing
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Fleet Mechanic
Joined: Apr 2008
Posts: 1,724
Likes: 2
From: Vancouver BC Canada
Trying to understand gearing
Forgive me for asking a common question but I'm not exactly clear on how this works. 94 5.8 4x4 3.55 31's. I have a line on a good deal for some 3.73 gears for the rear for $50. Thinking of buying a matching set for the front and installing them on the truck. I know that ordinarily, the shorter gears (bigger number) will increase power but hurt fuel economy. With my taller tires though, would these gears actually give me better power and FE?
I have a very light foot and my truck runs great. I seem to average 11-12mpg in mixed driving unloaded exept for a canopy.
Truck currently spins about 1900rpm at 60mph. I'm wondering if the 3.73's would put me more in the sweet spot of the power curve also helping fuel economy?
I have a very light foot and my truck runs great. I seem to average 11-12mpg in mixed driving unloaded exept for a canopy.
Truck currently spins about 1900rpm at 60mph. I'm wondering if the 3.73's would put me more in the sweet spot of the power curve also helping fuel economy?
Posting Guru
Joined: Nov 2009
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ok with 3.73 u will be turning about the same rpms u are now maybe a few more. thats being said u will probably get about the same mpgs but u will notice more torque and lot better mpgs if u tow a trailer. and if u ever go off-road u will crawl a little better in low....
Fleet Mechanic
Joined: Jan 2009
Posts: 1,964
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From: Fort Worth
Here is a site with fantastic animations to help you understand differentials and gearing:
HowStuffWorks "Open Differentials"
Also, when you increase tire size, you are effectively gearing your truck higher. To have relatively the same gearing as stock, you must regear the diff with numerically larger gears to offset the larger tire. You aren't going to notice much, if any, difference in fuel economy going from a 3.55 to a 3.73.
HowStuffWorks "Open Differentials"
Also, when you increase tire size, you are effectively gearing your truck higher. To have relatively the same gearing as stock, you must regear the diff with numerically larger gears to offset the larger tire. You aren't going to notice much, if any, difference in fuel economy going from a 3.55 to a 3.73.
Thread Starter
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Fleet Mechanic
Joined: Apr 2008
Posts: 1,724
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From: Vancouver BC Canada
Ok, so is there any real value in the switch to 3.73's then? I do tow a small 5th wheel (19 1/2' , 4000lbs) for summer vacations.
The stock tires were 235/75/15 which I think were 28" and my 31's are 30.5" I believe.
The stock tires were 235/75/15 which I think were 28" and my 31's are 30.5" I believe.
Posting Guru
Joined: Nov 2009
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Posting Guru
Joined: Jan 2008
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From: Trenton, MI
Wow that worst.
I get 16 mpg out 96 E350 15 passenger. 351W with 3.55 plus E40D. It weight 4 1/2 tons.
How I got 16 mpg. You need add vacuum gauge it will tell how bad your foot is.
Don't tell me how vacuum gauge can help if I have light foot. Just go get it $15 dollars.
You better keep 3.55 and you need raise to 65 mph since 60 mph is little lug for 351W.
I get 16 mpg out 96 E350 15 passenger. 351W with 3.55 plus E40D. It weight 4 1/2 tons.
How I got 16 mpg. You need add vacuum gauge it will tell how bad your foot is.
Don't tell me how vacuum gauge can help if I have light foot. Just go get it $15 dollars.
You better keep 3.55 and you need raise to 65 mph since 60 mph is little lug for 351W.
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Posting Guru
Joined: Jan 2008
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From: Trenton, MI
Just find one you could get at store.
Sunpro or anything.
that truck with 302 that get 14-15 mpg is that in city or highway? That sound little normal for city.
I am not sure if it ok to put link here to other forum but don't want get in trouble
info about vacuum gauge. Econometer/Vacuum Gauge Installation Guide - Fuel Economy, Hypermiling, EcoModding News and Forum - EcoModder.com
FTE Legend
Joined: Jun 2006
Posts: 31,930
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From: Ottawa, Ontario
If you're considering a gear change jumping to 4.10 makes more sense, it won't cost any more and will put more TQ on the ground which will help when towing. Overall milage won't be negatively impacted much if any as long as the speeds are kept conservative. 4.10's will give you a 16% increase in TQ across the board over what you have now while only increasing cruising rpm by 200-300, and while that is a negative it will be offset by the fact the transmission won't downshift as much on grades and the lighter engine load will reduce fuel demands.
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Fleet Mechanic
Joined: Apr 2008
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From: Vancouver BC Canada
Late back to this thread - sorry.
Paul - I did recalibrate the speedo for the tires.
I paid attention to the rpms the last time I did some highway driving and the truck runs about 1750 rpms at 100kms. Does that seem too low? I'm wondering if the motor is lugging, not really sure what that sounds like.
The choice of 3.73 was mostly convenience. The guy basically gave me the rear gears ($30cdn).
Paul - I did recalibrate the speedo for the tires.
I paid attention to the rpms the last time I did some highway driving and the truck runs about 1750 rpms at 100kms. Does that seem too low? I'm wondering if the motor is lugging, not really sure what that sounds like.
The choice of 3.73 was mostly convenience. The guy basically gave me the rear gears ($30cdn).
Post Fiend
Joined: Mar 2002
Posts: 19,402
Likes: 38
From: Bonita Springs FL
I don't think you realize what you are getting into with a cheap set of used gears. One, you don't know that they're good. They could have small stress fractures in them from people towing too much with the truck. Not a huge worry but something to think about. I would never put used gears in my truck if I was going through the effort of changing gears. Another thing you have to think about is that it's not just the cost of the gears. You have to buy an install kit, which includes shims and bearings. Then you either have to pay to have the gears set up, get a buddy to help you who knows how to set gears up, or try it yourself. Then multiply all that by two because you have to change the front as well. Last, going from 3.55 to 3.73 isn't worth the effort and cost no matter how you look at it. The gains are very minimal and you're going to end up spending $500 on the swap.
FTE Legend
Joined: Jun 2006
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From: Ottawa, Ontario
That would be good for cruising with an empty truck on flat ground but it works against you when you add weight and start climbing hills, more throttle needed to maintain speed and the transmission downshifts more.