gas Milage
Junior User
Joined: May 2004
Posts: 83
Likes: 0
From: Leonardtown, MD
Just 'cause I want to make sure I understand - wouldn't going from a 3.50 to a 3.00 actually decrease your gas mileage, indepedent of doing anything else with the transmission? The original stump puller gears were 2.5-something, right? Thanks for educating me.
Matt and Toulouse
Matt and Toulouse
Hotshot
Joined: Oct 2004
Posts: 15,882
Likes: 88
From: Durham NC
yes, a 1.0:1 ratio means that for every turn of the pinion gear (driveshaft) the axle (wheel) turns one complete revolution. A 3.0:1 ratio means the pinion has to turn 3 times to turn the axle one revolution, 4.0:1 the pinion needs to turn 4 times for every revolution of the axle etc. When discussing rear end ratios the (X):1 part of the ratio is understood so we just refer to it by (X). A 4.10 rear end means the gear ratio is actually 4.10:1 . Clear as mud? thought so... 
Elder User
Joined: Jan 2005
Posts: 908
Likes: 0
From: Lindsborg, KS
One way to increase gas mileage isn't stepping on it. Now for me, that's just asking too much, i mean, those 8 cylinders and the sound of the dual exhuast is just too much temptation.......lol
Lead Driver
Joined: Apr 2002
Posts: 8,813
Likes: 13
From: Littleton, New Hampshire
I'll be running a 2:26 rear end ratio and P225:75/R15 tires (not sure how tall the tires are...garage is closed for the winter). I also have some P235:75/R15s in the wings if the P225s aren't tall enough. What will my RPMs be at 60 or 70 MPH? I think I've seen a formula for this somewhere but I don't remember what it is. Maybe some of you math majors can tell me.... 
Vern
Vern
Elder User
Joined: Mar 2003
Posts: 558
Likes: 0
From: Limestone, TN
Vern
Your tires are about 27" in diameter (varies a little by brand). If you are using a C-4 or C-6 it is a 1 to 1 in 3rd gear. You will be turning 2000 at 70 mph. I have that rear under mine (for a couple more weeks) and an AOD. I turn 1470 at 70 and it wont change to OD until 77 mph. Just 30 more knots and it will fly!
Your tires are about 27" in diameter (varies a little by brand). If you are using a C-4 or C-6 it is a 1 to 1 in 3rd gear. You will be turning 2000 at 70 mph. I have that rear under mine (for a couple more weeks) and an AOD. I turn 1470 at 70 and it wont change to OD until 77 mph. Just 30 more knots and it will fly!
Hotshot
Joined: Oct 2004
Posts: 15,882
Likes: 88
From: Durham NC
Something in the 3.0-3.5:1 range will give good gas milege with an OD tranny (single overdrive transmission high gear ratio is typically ~0.62:1) for long distance expressway driving, something in the 3.5 - 4.1 area is good for around town or a little stoplight drag racing. 3.7-4.3 will beat up on the kids from stoplite to stoplite or for hauling or towing without over reving the engine on the expressway. If you don't have overdrive then move up (numerically smaller) a ratio group for the same uses. I have a 3.70 rear in Gracie for trailer towing, but 6th gear is 0.50:1 so on the crosscountry trip home with her we averaged 27+ MPG at 80+ MPH! I doubt we'll use 6th much with a 5K# trailer on the back tho.
Lead Driver
Joined: Apr 2002
Posts: 8,813
Likes: 13
From: Littleton, New Hampshire
Jock,
Thanks. I suppose 2000 RPM at 70MPH is about what I had hoped for. I knew without an OD tranny (TH350) I would want a fairly tall rear end ratio to effect decent gas mileage. Actually, I thought I had a ratio in the high 2s when I first decided to use this rear end so when I determined it to be a 2:26 it was somewhat of a surprise. I expect the truck to be pretty doggy for around town and I hadn't planned on doing burnouts anyway. It should cruise great on the highway. I am a little concerned about shift points on the TH350 though. I may end up driving around in second gear around town. I might have to look into a variable vacuum modulator...
Vern
Thanks. I suppose 2000 RPM at 70MPH is about what I had hoped for. I knew without an OD tranny (TH350) I would want a fairly tall rear end ratio to effect decent gas mileage. Actually, I thought I had a ratio in the high 2s when I first decided to use this rear end so when I determined it to be a 2:26 it was somewhat of a surprise. I expect the truck to be pretty doggy for around town and I hadn't planned on doing burnouts anyway. It should cruise great on the highway. I am a little concerned about shift points on the TH350 though. I may end up driving around in second gear around town. I might have to look into a variable vacuum modulator...
Vern
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Logistics Pro
Joined: Oct 2002
Posts: 4,489
Likes: 5
From: Valley Center, CA
Hey Vern, you are right about driving around in second. But, I actually keep mine in first a lot. For driving through neighborhoods, I usually only go about 20 mph. I have found that keeping my C-4 in low allows me to get up to about 25mph before the engine starts to winde. The exhaust note and the engine sound make the truck sound like a standard transmission. 
So, the engine revs are up in the power band and the Magna Flowmufflers sound awesome. So, give it a try in low and see how you like it. My 289 sure likes it. Jag
So, the engine revs are up in the power band and the Magna Flowmufflers sound awesome. So, give it a try in low and see how you like it. My 289 sure likes it. Jag
Last edited by Jag Red 54; Jan 25, 2006 at 12:29 PM.
Logistics Pro
Joined: Oct 2002
Posts: 4,489
Likes: 5
From: Valley Center, CA
AX, you are right, of course. I usually only run it like that for a few blocks at a time until I reach a road with a higher speed limit. Also, I do have a nice auxillary trans cooler. However, I think running in second would be harder on the trany trying to leave from a standing start at each stop sign. 
Jag
However, I think running in second would be harder on the trany trying to leave from a standing start at each stop sign. Jag
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PAUL_2
1980 - 1986 Bullnose F100, F150 & Larger F-Series Trucks
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Aug 6, 2001 10:49 AM