The truck in the Sig has a deep reduction 15 single over, 3406b, 3.90 rear, 24.5 LP. I usually cruise ~60-65 @ ~1600-1700 rpm hauling 80,000 in the Appalachians. Getting 5 usmpg on winter blend and 6 summer.
I'm not too fond of the big gaps between gears and the higher cruise rpm compared to the 13 double over. Will a 13 save enough on fuel to pay for the swap?
The trans would cost $5000 exchange + labor. The clutch doesn't need doing.
swap in a 13?
Discussion in 'Trucks [ Eighteen Wheelers ]' started by rank, Mar 26, 2015.
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If I'm understanding correctly, you'd be going from a .79 top gear to a .73 top gear. That would lower your rpm range from 1600-1700 to 1478-1571.
Now, let's assume you have a Detroit 12.7 (because I haven't seen a BSFC chart for the CAT). And let's assume that you're averaging 200 horsepower.
View attachment 81257
At 1650rpm and 200hp, the BSFC is ~0.324. At 1525rpm and 200hp, the BSFC is ~0.320 -- a thermodynamic efficiency improvement of 1.2%
So instead of 5.5mpg you'd get 5.57mpg. At 100,000 miles/year and $3/gal in fuel, you'd save ~$650/year in fuel meaning you'd be looking at a ~10 year payoff.
Zero real-world experience, by the way, that's just what the physics tells me the difference is in top gear. You may gain additional mpg from not having as big of a drop between gears too.rank Thanks this. -
If you don't need a clutch/ trans or the rear main seal done I'd save your money . And enjoy those low gears when needed.....
rank Thanks this. -
I have a rtlo16713a with 3.90s and a 3406e and run the same area as you and cruise right around 62-65 most of the time, 1450-1500 rpm on tall 24.5s. low gear is plenty low for what I do and I see a lot of jobsites...it works well for me. When this transmission goes, I will swap an 18 speed in to have the best of both worlds, I get 6.5 winter and 6.8 summer with my setup. Dropping your cruising RPM to 1450 will make a world of difference. That is a cool truck BTW!
rank Thanks this. -
Thanks.
Interesting comparison. I got 6.0 to 6.3 last summer. I wonder how much of that is the gearing and how much is the E model. Are you usually 80,000? -
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rank Thanks this.
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rank Thanks this.
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Yeah, I had it wrong in my head, the 10's are the shorter ones.rank Thanks this. -
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