Ok did a little checking since it been said to not use overdrive unless your on flat roads and not straining the T/Q<br>I have a 47RH and 3:54 gears along with 235/85-16 tires 230 HP 605 Torque<br>Without towing anything at WOT with O/D off Iam at 2750 RPM's and 61 MPH now this has to be off the HP/Torque sweet spot and fuel economy has to suffer<br>Same speed with O/D on Iam at 1950 RPM's which puts me in the Hp/Torque band which to me would be better as long as the engines not in a lug condition<br>My question is with a load like I haul total 18500-25000 lbs Ive never taken O/D off on hills unless I get into a lug condition and havent ever seen my tranny temp go over 240* so why would I want to take O/D off until I reach 1700-1800 RPM's fuel economy has to suffer badly

 

I agree with you, allthough @ 2000 rpm in OD my rig is running 75 mph. So for the guys with the newer stock trucks with 3.54 gears its pretty easy to lug them going uphill if you not carefull. I use OD even up steep hills towing my TT about 7K + lbs, but only if I can keep the rpm's up around 1800, and its not so steep as to lug it.

 

Iam wondering if I have 4:10 gears because seeing all the posts everyone with 3:54's run faster then me...but my tag says 3:54 now Iam thinking I should pull my cover and count the gear teeth

 

pappyman:

I haul my 11K fiver all the time in OD, and only shift out if my rpm's drop below 1800 or so. With your first gen engine, I think your hp/tq (therefore mileage) rpm sweetspot is lower than my 24v, and mine is around 2000-2200. Running at 2750 is deffinately out of yours. I have the 4:10 gears, so at 2000 rpm's I'm at about 61 mph in OD. I'm sure that some of the undervalvers here will chime in here to give you the info that you need. I wouldn't worry about hauling in OD, though, unless you are lugging, or the tranny is hunting between OD and direct.

 

Pappy, <br>I'm sure the tranny gearing is probably slightly different, but I have the same size tires. I have a Jasper version of the 727(no overdrive), 4:10 in the rear. I can get to 80 at WOT in the flats, empty. It's comfy at 70. 14-15 mpg in the summer. Yours seems a little slow for 3.54's. I don't have a tach yet to help you there.

 

Pappy, I'm running 4:10s with stock sized tires and I see 2100 rpm @ 64 mph.
Hope this gives you a point of reference.
Jack the rear wheels off the ground, set the jack stands, then put a mark on the driveshaft and one on a rear tire.
Spin the driveshaft and count the number of turns it takes to spin the tire one complete revolution.
3 1/2 times would be a 3:54, a little more than 4 times = 4:10s.

 

Found this chart on the web about gear ratio and tire size vs mph and in 3rd gear at 2700 rpm Iam running correct speed (my speedo is a little slow) 64.8 mph...Still cant under stand how other members get higher speed vs rpm then me if everything is the same<br><br> http://www.phantomcycle.com/speed.html?1040036470185

 

BoldtsWagon's spreadsheet that he posted here says you should be turning 2528 rpm in 3rd locked at 65 mph with 3.54s. Maybe your TCC was unlocked?

 

[quote author=pappyman link=board=8;threadid=8352;start=0#80853 date=1040036508]<br>Found this chart on the web about gear ratio and tire size vs mph and in 3rd gear at 2700 rpm Iam running correct speed (my speedo is a little slow) 64.8 mph...Still cant under stand how other members get higher speed vs rpm then me if everything is the same<br><br> http://www.phantomcycle.com/speed.html?1040036470185<br>[/quote] <br><br><br><br> Pappy I,m running 265 tires on mine vs your 235, the 265 are a lot taller.

 

Thanks I measured my tires they are 31.5&quot; so by the chart I found with my gearing in 3rd with O/D off I am close to the charts RPM maybe just a little high on RPM's...but next week if I get time I'll put in a T/C lockup switch to be sure my T/C is locking up

 

pappy: I checked today, and at 55mph with my 4:10s, in 3rd gear lock-up, I am turning 2600 rpm's. Just thought I'd let you know. I just put 215/85/16's on my 3500.

 

Chuck3 you are a little confused about the tire size, if this is a stock 265/75/16 your tire is shorter and fatter than pappymans 235/85/16. 1st#=width, 2nd#=wall height and 3rd# is rim size, try and fit a 235/85/16 into the space under the box and you will notice a lot less room for the spare. I replaced my 245/75/16 tires with 235/85/16 and picked up 5 mph at 2000rpm in 6th gear, dont forget overdrive amplifies the tire change X 27% as well. PK

 

Pappy you check to see if your tranny is locking up while you are cruising down the road in top gear lightly touch the brake pedal enough so your brake lights should light up and you should feel somewhat of a down shift. Matt

 

Checked it out I put in a lockup switch and tested speed with OD off using my GPS (speedo was low 3 MPH) at 2700 RPM speed was 64 MPH and T/C was locked up...so I guess everything checks out with the chart I found<br><br>http://www.phantomcycle.com/speed.html?1040036470185<br><br>.

 

I agree that the 235 is taller. I have both sets, one for winter and one for summer. If you can afford it put on a US GEAR splitter. The underdrive will drop you 100rpms for every 1,000 you are spinning. With 4.10's at 70 mph in overdrive you should be around 2200rpms, with the splitter you should be around 1980 rpms. Same thing with no overdrive. If at 2700 you can drop down to 24 something. Maybe if you want lower r's, change your gearing.