If U were spec'ng a truck for fuel mileage...

my c15 has almost 700hp and 3:55 rears, always running 78,000 - 81,000 and I'll tell you this, with all this power when my rpm drops to 13, truck ain't doing so good, it's like a big hangover after ruff night, soon as I get it to high 14's, it's all good, anything above 15's big power! Thank you Mr. Honey for all the great help!

 

If U were spec'ng a truck for fuel mileage... the main item that has to be spec'ed is the driver.

 

Amen! The wind resistance created by your truck when running high speeds voids all possibility of great fuel mileage. The two choices are slow down and save or go fast and pay the price. Spending large sums to aid in fuel mileage when running high speeds will mostly just empty your wallet because you cannot change the wind resistance factor of high speed running.

 

I'm with u on this one! If u go with the flow and keep it low, everything is gonna be alright!

 

Reviving a dying thread here, but...

I built a spreadsheet that calculates road speeds in whatever combination of wheels, tranny finals, and rear end ratios you plug in. It has columns for several different gears so you can side by side comparison.

That being said, the numbers looked the best to me by using an 18 speed, running it in direct drive (16th), and tying it to 2.79 rear ends. I just put a combination like that together with a 500 hp 12.7 DDEC IV on a glider kit order I put in this last Thursday.

 

I have a double over 13 with 3.55 and lo pro 24.5 and that seems to be a good set up, but I wonder what numbers I would need on a different truck to use a 13 direct for the few per cent advantage that gives.

 

I have an 04' Columbia that has a C-15 (tuned by Mr. Haney), 10 speed, and 3:73 gears. I am looking at putting in a 13 speed and 3:42 gears. Any ideas about what I can expect???

 

A big bill.....

 

Thanks Krooser.... I already knew that... The trans needs to be replaced anyway, and the gears is to make it be able to run a little faster to make the driver happy.

I was looking for input on how this set up would run...

 

Is the C15 and Acert or an MBN, being an '04 it could have been either. If it's an Acert I would go with 3:36's especially if it's on small wheels/tires. If it's an MBN (or was thanks to Kurt) 3:42's might be better unless it's on 24.5's then maybe 3:55's. The advantage of a 13 over a 10 is axle ratio doesn't hurt you as much. With mine if the speed limit is 55 it stays in 12th and if it's 65 or above you can use 13th.