Gear Ratios

[RostyC]

What do think the best all around gear ratio is? 3:70 maybe? or lower like a 3:58 or 36?

[GMAN]

I have 354's in one truck. It does a good job of pulling and I consistently get around 6.29 mpg.

[Rawlco]

It depends on what tire size you want to run, what your transmission final ratio is, where your engine RPM runs best, and your top cruising speed desired.

Somewhere on Roadranger.com there is a road speed calculator that lets you play with those variables.

[Longsnowsm]

If you drive in the mountains don't you also want a higher numeric gear?

Longsnowsm

[brian]

horsepower/torque and at what rpm you make it also matters, I run 3.36`s with tall rubber and an 18 speed with a modified c15, you take the same gear, tires and tranny and try running the low rpm`s I do with a 375 series 60 and you`d be in a world of hurt

[ChrisSai]

I think much of it depends on the engine that you are running. You may have noticed that many 200/ 2004 trucks have a ratio of 3.55. With the 2006 models that typical ratio is 3.36. These are probrably the most popular ratio's and just depend on the year the engine was built.

[PhuzzyGnu]

One of my company's O/Os has a W9 with a 625 hp C15 cat, super 18, tall rubber and 3.08s!

I'ts only loping along at 1250 or so rpm at 70mph. Roadranger's website tells me it should be good for 115mph flat out at the governor. I think he says he gets low- to mid- 6s mileage wise, which is excellent for a non-aero truck.

On the other end of the spectrum, my 2007 Freightliner Columbia company truck has a 515 Detroit, straight 10, 11R-22.5 rubber, and 3.70 gears.

It's at about 1550 rpm at 70mph at the governer- right in the middle of the power vs. economy graph on Detroit's website. The drawback is that it is governed at 1800 rpm which doesn't leave much room for downshifting at highway speeds- 9th gear tops out at a little more than 60. I averaged 6.4 mpg in my identically spec'd '06 model, but the '07 was down in the mid 4 mpg (!) range when I got it at +/-8,000 miles. It has since freed up a bit ad in mid-to high 5s. I don't *****foot around -I certainly would pay more attention to fuel mielage if it wa my fuel nickel.

-p.

[allan5oh]

The newer trucks seem to run lower rpms to help out with the loss of fuel mileage.

[Truckdobe]

Rear end ratio depends on the motor, transmission and tire size. Cats tend to do better (on fuel) at low rpms while still maintaining power. I changed from 3.55 to 3.36 last year and that still wasn't quite enough. I just traded that truck and changed the new one to 3.08s. It wouldn't work without the 18 spd for me, the only time I'll use the top 2 gears is 65 and above; but I still have plenty of gears for startability and slower roads.

Different motors are efficient at different rpms... mine does great under 1400 and drinks fuel over that. 2nd quarter fuel average was 6.6+ with W900 (550 Cat).

[PackRatTDI]

A 10 speed direct with a 2.64 rear end will give you roughly the same cruise RPM's as a 3.55 rear end with an O/D transmission. I heard figures that you can gain 4% better fuel economy with a direct top gear since you're not spinning through a set of gears. Either method will work if you're planning on running around 65-70 mph on low profile (518 rev/mile) tires and want to stay under 1600rpm. Any faster and you'd have to consider strictly O/D setups since 2.64 is the lowest rear end currently on the market.

Back in the dark ages when Swift was still going 55-57 mph, they spec'd 10 speed direct trannies with 3.36 rear ends. That had the engine running at 1600 rpm at 55 mph! :P That didn't help resale value much (big suprise there) so often when the trucks were resold they were often equipped with 13 speed transmissions. Later they dropped the rear end ratio to 3.08 and today they sport 2.93 rear ends with 10 speed direct transmissions.

[number9]

We have a '96 FLD120 flattop, 60Series Detroit 430/470hp, 10-sp Eaton & 4.11's pulling flatbed. Driver says he likes to run around 70mph and it turns 1,700rpm at that speed. I was worried about the mpg and had planned to trade out the gears for 3.55 or 3.70. A month ago we had it in the shop getting an A/C dryer and charged up and got an ECM printout and it showed Avg fuel useage of 6.9mpg :shock: After seeing that, I think I will leave well enough alone. Someday when one of the gears fail, I may swap out to a 3.70....maybe.

[RostyC]

Man, funny you should post that, I looked at a truck yesterday at the Peterbilt dealer in Baltimore. It was a 96, 379 model with a Detroit engine. 430 hp at 2100 rpm with 4:11 ratio. I asked the salesman if the 4:11 would eat up the fuel and he said it wouldn't with the overdrive transmission. I think it was a 13 speed. I believe he said it should be around what you are getting.....6 to 7 mpg. I wasn't sure whether to believe him or not but I guess he wasn't BSing me.

[PackRatTDI]

We have a '96 FLD120 flattop, 60Series Detroit 430/470hp, 10-sp Eaton & 4.11's pulling flatbed. Driver says he likes to run around 70mph and it turns 1,700rpm at that speed. I was worried about the mpg and had planned to trade out the gears for 3.55 or 3.70. A month ago we had it in the shop getting an A/C dryer and charged up and got an ECM printout and it showed Avg fuel useage of 6.9mpg :shock: After seeing that, I think I will leave well enough alone. Someday when one of the gears fail, I may swap out to a 3.70....maybe.

When I worked for Stevens, I drove a T2000 with a 435hp Cummins N14 direct 10 speed and 2.85 rear end and 22.5 lopro's. It was governed at 68 mph but would have turned at roughly the same RPM as your truck at 70 mph. With grossing between 70k and 80k on the average, I always averaged over the company minimum of 6.2 mpg, usually 6.5 or better.

[Bandit102]

I have a 99 F/L Classic with the 500 Detroit. It had a 10, direct, I think and did well...about 6mpg. The trans suffered a hemmorage of some sort so I threw in an old 9 over that laid out back of the shop for a decade or so. Killed this Detroit. It won't pull a hill. It takes a lot of work to start off on a grade without spitting out the drive shaft and worse of all, the mpg is about 4.6 now. On the bright side, if it were still the 70's, I could run 100mph plus!!!