Question about rear axle ratio and transmissions

[Ford850]

Getting conflicting information about the dodge. Think maybe the payload is 1500 lbs.

Don't trust any sites, marketing material, etc on payload. The door or jamb sticker of the truck you are looking at will be the only source that matters. That sticker takes into account how the truck is configured including every option added or deleted. The more options added to the truck you are looking at will equal more weight, will mean lower payload. You could look at a row of trucks, all the same model, and the payload will be different on each one.

 

[Copperhead]

I'm not sure I understand the phyiscs of it, but the higher number rear gear ratio takes a lot of shock out of the drive line, taking off from a stand still being the most important. I'm not sure even with the extra gears in the transmission equals a higher numbered gear in the rear end or not.

You are correct, higher numbered rear ratios do take a lot of shock out. Commercial trucking has had to work around lower numbered ratios for a while since OEM's are wanting to dramatically down speed engines and going to direct drive final transmission ratios. That has meant diff ratios of around 2.47 to 2.64. Even down to 2.21. Really takes a toll on drive lines.

For what we are talking about with pickups, more gears in the trans doesn't really change things. The final gear is surprisingly close on say an 8 speed as it is a 6 speed. They just have tighter spreads between the gears that keep the RPM band tighter on the engine. For these taller (lower number) rear ratios, it is a smart move to use manual mode on these autos and hold gears so that the trans doesn't get into the top too quickly and have engine RPM's below optimum for a balance of economy and power. Down speeding can be a good thing with an engine, it all depends on terrain and load. I tend to use manual mode quite often to avoid the gear hopping nonsense that goes on running rural hilly two lane roads. Has multiple benefits in doing so.

 

[TMGT]

The numerically higher ratios are more efficient in a hypoid gearset, think emissions/mileage.

 

[handirifle]

The mileage will depend GREATLY on terrain. From my personal experience, the more gears a transmission has, the more it "hunts" for the right gear in the hills, and that is far harder on a transmission, that a lower rear axle ratio ever will be. About 90% of transmission wear is caused during shifting, the more it shifts, either up or down, the more it wears.

Taller (smaller numerical number) axle gearing is ONLY beneficial for mileage on fairly level terrain and steady speeds. That is the reason most are going for this, and that is due to ever tightening and ludicrous (IMHO) EPA standards for CAFE. I have changed rear axle gears from 3.42 to 4.10 for towing and I could ABSOLUTELY tell the difference. I seriously lost only 1 MPG and the difference when climbing grades was night and day. This is especially true with smaller vehicles and gas engines. Diesels can get away with taller gearing because of their torque, but gas engines have much less torque and usually less RPM range where it's available.

Towing is more then getting a trailer rolling, it's having the power available to keep it rolling on steep grades too.

 

[ClaySlayer]

I have owned both the 3.21 and the 3.92. I regret getting the 3.92's in the second truck. There is no difference in the towing performance other than the published numbers. It does however translate into about 1 mpg loss on average.

The first two gears in the 8 speed are very deep. Like a granny low in an old 4 speed. It has NO issues moving a loaded trailer, quickly.

Yeah, he can always lockout the upper gears if needed. Unless he's towing near max all the time and gonna put bigger tires on the truck he should be fine.

ZF 8HP70 Ratios:
5, 3.2, 2.14, 1.72, 1.31, 1, 0.82, 0.64, Reverse 3.48.

 

[94BULLITT]

With the 8 speed trans you can get buy with a higher rear axle ratio.

 

[TheGoose]

I have 3.55's on my current ride. I had 3.73's on my chevy before. the 4x4 extended cab chevy with 5.3 got about 16-17 mpg on average. The Ford SCrew 4x4 with the 3.5L EB gets about 13-14 on average. I have always felt that the "higher" gear ratio (numerically higher but lower gear) was better suited to my style of driving which is a mix of city/rural driving. By using higher (3.73) gears the engine can stay in it's power band when NOT highway cruising at a steady speed.

I would prefer to go back with 3.73's but it's almost impossible to find any truck that has them. Most Fords either have the 3.31 or 3.55. Most Ram's have the 3.21, very few have the 3.92. I haven't really looked much at chevy's. Ram doesn't even offer the 3.73's, they only have 3.21, 3.55 (I think it's EcoDiesel only), and then 3.92. Toyota has 4.30's but I think they have a really tall gear in the tranny to make up for it at hwy speeds.

 

[Steve1300]

My Titan XD offers only the 3.73 rear end.

The truck runs very close to max torque RPM all the time because of the 6-speed transmission. Really nice on hills.

 

[Copperhead]

The mileage will depend GREATLY on terrain. From my personal experience, the more gears a transmission has, the more it "hunts" for the right gear in the hills, and that is far harder on a transmission, that a lower rear axle ratio ever will be. About 90% of transmission wear is caused during shifting, the more it shifts, either up or down, the more it wears.

Taller (smaller numerical number) axle gearing is ONLY beneficial for mileage on fairly level terrain and steady speeds. That is the reason most are going for this, and that is due to ever tightening and ludicrous (IMHO) EPA standards for CAFE. I have changed rear axle gears from 3.42 to 4.10 for towing and I could ABSOLUTELY tell the difference. I seriously lost only 1 MPG and the difference when climbing grades was night and day. This is especially true with smaller vehicles and gas engines. Diesels can get away with taller gearing because of their torque, but gas engines have much less torque and usually less RPM range where it's available.

Towing is more then getting a trailer rolling, it's having the power available to keep it rolling on steep grades too.

Well said. And lest we forget, the numerically higher gears in modern transmissions really doesn't mean a lot in speed. I can have an 18 speed or a 9 speed and the top gear ratio is the same. The only difference is the 18 speed has a 200 RPM range on the engine whereas a 9 speed has a 400 RPM range on the engine on shifts. These transmissions coming out in the pickups with higher number of gears still have virtually the same top gear ratio. Only the engine RPM spreads have gotten tighter.

So when we then lower the rear gear ratio, having tighter spreads on the engine between gear shifts is more critical. But as was stated, only on flat or gently rolling terrain does it mean anything. CAFE standards are in play here. The chase for better fuel economy is the prime motivator for the lower rear end ratios and higher number of gears. It works ok depending on the setup and conditions.

My Semi, I have an 18 speed tied up with 2.64 rear ratios, pulling up to 80,000 lb. I have no problem, but I keep it out of the top two gears which are overdrive. I generally run in direct drive 16th which is more efficient and keeps the engine at optimum balance of power and economy. Even on hill and hard pulls, not a problem. And I get about 20% better fuel economy than the nation OTR trucking average. It is all in how things are set up. And the pickup folks could greatly benefit by giving the heavy truck builders a call and discussing such things with them.

I would rather, if they are wanting to do the lower rear ratios, is that they would forget the Over drives in the transmissions. My 2015 2500 6L90 has 2 overdrives. Ok with the 4.10 ratio I have, but criminal with something like 3.08 that they are using in the 1500 series pickups. On those they should eliminate one of the OD ratios in the trans and have more ratios below 1:1. So an 8 speed in something like a 1500 should have 6 ratios below 1:1, one gear at 1:1, and one OD. It would be far more efficient and user satisfaction would be better.

 

[rankrank1]

I almost bought a 2016 Ram 1500 CC 4x4 Outdoorsman 5.7 Hemi, 3.92 axle ratio , and 8 speed tranny. Outdoorsman has the big 20 or 22 rims so the 3.92 was perfect for it. Personally I would not go with a number less than 3.55 in the Ram myself.

Note: Some of the Ram 1500's have pathetically puny payload numbers. Make sure you have enough payload to cover whatever you weigh over 150 lbs, any passengers you haul, and tongue weight for the trailer. Even with empty bed payload can go fast as some of the Ram's are like only 900 lbs payload which is puny.