Took my truck on its first hwy trip, about 300 miles. Looks like about 15 mpg. I noticed that, compared to my late SD, this truck seems to be geared lower somehow when cruising in OD. Going down long grades in the SD it seemed I could take my foot off the pedal and the truck would roll well on its own, maintaining hwy speeds for some distance without slowing down. In the OBS I felt like I needed another gear almost and had to keep power on to maintain speed.
On flat windless hwy I would cruise at 60 mph at about 1900 rpm. I'm betting that, like the SD, the 35's are causing my indicated speed to read about 9% low so I'm probably moving about 65 mph. Do these numbers seem normal for the 3.73 (?) rearend that I think I have?
11-26-2007, 04:02 AM
3.55s or 4.10s for the OBS. The "drag" you describe is part of the 4.10 gearing, they like to cost down & cruise around 30-35mph. When you get used to it you'll find you don't have to hit the brakes as much as you would with a truck geared higher. 15mpg sounds about right, that's about what my old IDI does & what other OBS Powerstrokes with 4.10s I've seen will do.
I guess that explains why the tag on the diff. says 10 10 xxx. I don't know how I missed that before buying the truck. Although I have to say that, as a daily driver, I kind of like the off-the-line quickness that this truck has over the SD and I am assuming it is the gearing that causes this.
Seems that 3.55 to 4.10 is quite a jump. I'm not sure I'd want the 3.55 with the larger tires. Tradeoffs I guess?
11-26-2007, 05:42 PM
How large of a tire do you want to go with? If you make a good amount of power & don't tow anything too large a set of 3.73 might be alright for you. It just really depends what your trying to do with your truck & how much $$$ you want to spend.
I doubt I'll ever go over 35s. I'd like to try some 37 x 12.5's sometime but the 35's already rub on the leaf at full crank.
And I don't foresee any serious towing; just small utility trailers.
The more I think about it the more I think the higher gearing would ONLY benefit on fairly flat hwy and higher speeds. In the city, towing, offroad, or even on grades or winding roads I think the higher gears will have benefit.?? Basically, any time I am operating under 60 mph.
I don't want to spend money on drivetrain, other than tranny. I am getting ready for exhaust, then tranny, then gauges, then chip.
11-26-2007, 07:35 PM
I think I'd stick with the 4.10s, that's what I'm running in my truck. I want to try a set of 35s on mine just to see how it acts. Generally if your doing alot of highway driving 65-75mph you'll want a set of 3.55 gears. For alot of 55mph country roads & town 4.10s arn't all that bad espically if your running larger rubber. Also considering how big of a truck your pushing down the road I'd stick with the 4.10s on your truck.
12-02-2007, 01:03 AM
yup 4.10-- the tag likely says 4L10 (for ltd slip) or I think just 4 10 for open. Your 4 is probably under the bolt that goes through the tag. 65 mph@ about 2400....