Axle Gearing: How to choose the ratio

[DaveInDenver]

Just got back to the computer and read the last couple of posts. Don't really have anything to add at the moment (be sure that I'll reread them and let it bounce around the noodle for a while, tho), but just wanted to say that this is pretty interesting and a neat thread. I like the idea that others around go to the same over analysis to figure out something like gear ratios!

 

[Hornito]

"Cruising at 80% (2,880) of peak torque (3,600 rpm) for my Tacoma 3.4L generates the greatest MPG, and leaves a range of about 1,000 rpm's for climbing and passing."

Hey Scott, how did you determine this? I can see how this would apply to inclines, but it would seem that 2000-2500 rpm would be more economical. I regeared to 4.56 with the AT, V6, 33's and moderate gear/armor, and on a flat piece of hwy I can do 75 at about 2300 rpm in OD. Are you suggesting that 488s would result in better fuel economy at the same speed?

Thanks!

Hornito

"Alcohol: the cause and solution to all of life's problems."

H. Simpson

 

[Scott Brady]

80% of peak torque is ideal for a DOHC motor in a heavily loaded truck. I worked from by ideal cruising speed back (70 mph or so), along with tire size to determine the appropriate gearing.

DOHC motors (most gas motors for that matter) are not efficient when they are being lugged down.

I gained about 2-3 mpg back by going to 5.29 gears, as the motor is back operating in the power band.

HTH.

 

[DaveInDenver]

Hey Scott, how did you determine this? I can see how this would apply to inclines, but it would seem that 2000-2500 rpm would be more economical. I regeared to 4.56 with the AT, V6, 33's and moderate gear/armor, and on a flat piece of hwy I can do 75 at about 2300 rpm in OD. Are you suggesting that 488s would result in better fuel economy at the same speed?

Don't confuse the engine speed where you're consuming the least amount of gas as the most efficient engine speed. Efficiency is a measure of fuel consumption against power produced. So while you are consuming less fuel at 2000RPM than 3500RPM, the engine is also producing less power. If your engine makes sufficient power through its power band that at a lower RPM it can use a lower RPM and still move the truck. The engine isn't running at it's most efficient RPM, but it's also consuming less fuel and so your economy will be fine.

It's hard to visualize, but look at it this way with a generic engine (apparently a diesel).

• At 1000RPM it makes 100ft-lb and burns 1 ounce/min, that's 19HP and 19HP per ounce per min
• At 2000RPM it makes 125ft-lb and burns 2 ounces/min, that's 48HP and 24HP per ounce per min
• At 3000RPM it makes 150ft-lb and burns 3 ounces/min, that's 86HP and 29HP per ounce per min
• At 4000RPM it makes 135ft-lb and burns 5 ounces/min, that's 103HP and 21HP per ounce per min

So the most efficient point is at 3000RPM, since you are making the most power per fuel burned. If the job required 50HP to produce the work, then 2000RPM would be fine and you'd consume less fuel. If the job required 75HP to produce the work, then the engine will consume more fuel but still be within its efficiency curve. If the job took 105HP, the engine might still do it but burn a ton of gas. Since the real work done is measured at the wheels, the idea of gearing is to put the engine at it's optimal speed to produce sufficient power to do the work. What that ideal gearing is depends on how fast you want to go, how much reserve power you want to keep, etc.

 

[TeriAnn]

DOHC motors (most gas motors for that matter) are not efficient when they are being lugged down.

Sometimes there's something good to be said about old school. Low tech V8s like to cruise in the mid 2000 RPM area and usually achieve their best fuel mileage there. From the factory they tend to have a wide power band and plenty of power to push most trail rigs. I personally am a fan of wide power bands & reserve power. My 302 Ford V8 does all that just fine. If I need more low end torque there is always a stroker kit. Being a law abiding slow poke who prefers to drive past filling stations, I tried to set my gearing for maximum economy at 65 MPH.

The charts & formulas you have been sharing don't really lend themselves well to Land Rovers which is why I thought I would inject my two cents worth into this thread. Also Land Rover engines are old school technology (designed in the mid 1950's) with fairly wide power bands.

The high range of a Land Rover transfer case is an under drive which varies by model and engine used. There are just a few R&P ratios available for Land Rover axles (3.54:1, 4.1:1, 4.7:1, 4.75:1 (much stronger than the 4.7:1) and I think someone has come up with another around 3.9:1). As modifiers there are a couple overdrives available that can split all gears and Ashcroft Transmissions offers a hi ratio transfercase conversion for the Series transfercase that is an overdrive in high range but does not affect low range ratios.

Here's some ratio charts for Land Rover gearboxes & transfer cases

Here are some ratio charts for gearboxes commonly used with Land Rover engine swaps

To figure a Land Rover's axle ratio you need to multiply your gearbox ratio times your transfercase ratio times your R&P ratio. I like to plug the axle number into the bottom calculator on this web page.

I found the stock Series Land Rover low range first gear ratio (40.7:1) to be too tall for most rock crawling and many climbing situations. Having 8 years experience with a 70:1 ratio I have found it to be too low for anything but the most technical rock crawling situations (with 33.3 inch dia tyre). At this time I'm leaning towards somewhere around 55:1 as being the best overall low range ratio for use in many different situations with 32 - 34 inch dia tyres. That low range ratio seems to fit Land Rovers well.

My advice to Land Rover folks in general is to pick a gearbox, transfercase and R&P that best anchors a low range first gear near 55:1 and high gear for best economy at normal highway cruise and fine tune with the tyre diameter. Most Land Rover folks are concerned about fuel economy as 15 - 16 MPG is the highway norm for Series rigs and aspired to by coiler owners.

 

[DaveInDenver]

Sometimes there's something good to be said about old school. Low tech V8s like to cruise in the mid 2000 RPM area and usually achieve their best fuel mileage there. From the factory they tend to have a wide power band and plenty of power to push most trail rigs.

I don't know that this is a uniquely V8 characteristic, but rather a displacement and size question. Old, big displacement inline engines share a lot of the same characteristics, low RPM torque peak, etc. As the displacement goes up, the bore and/or stroke increase and that has as much or more to do with the performance of the engine as does the piston configuration.

My advice to Land Rover folks in general is to pick a gearbox, transfercase and R&P that best anchors a low range first gear near 55:1 and high gear for best economy at normal highway cruise and fine tune with the tyre diameter.

This is good advice. You gear the axles so that you are comfortable on the highway and the tranny/xfer low-range are geared for off highway. Too often people try to get lower crawl ratios by putting very low gears in the axles and I think that is short sighted. BTW, I run a 45:1 low-low and think your 55:1 is not unique to Rovers. I think this turns out to be a pretty good ratio for a lot of trucks.

 

[ntsqd]

The way I figured out what ratio would be ideal for the Ballena Negra was to find what the stock tire specs were. Then I worked out what kind of RPM that resulted in at the various speeds I tend to drive.
I then worked things backwards using my new tire size.

The base for my approach was a reasonable assumption that GM's engineers knew the engine & trans combo better than I do & would have set up the axle ratio and tire size combos such that the engine would be at or near it's best BSFC at legal highway speeds.

 

[ntsqd]

Optimizing mpg at cruise speeds was my goal for the Ballena Negra.
One must keep in mind that if the engine is operating at it's best BSFC RPM that it is going to get the best mileage it possibly can at that vehicle speed, regardless of any other factors. Those other factors do have a subtractive effect, but changing the RPM in either direction will result in an MPG loss.

In re-reading this I realize that I should define BSFC. It stands for "Brake Specific Fuel Consumption." It's units are typically lbs-hr/HP. In an SI engine best BSFC occurs at the torque peak RPM. I suspect this to be true of diesel engines too, but I can't state it with certainty.

Were towing or hill climbing ability my goal I would still use the same process, but then after finding the stock equivalent gear ratio I would look at the available ratios one to two steps lower (higher numerically). If you're looking for tow capability then you would want to use the towing package ratio for comparison, if available. If this mod is made I would then still operate at the best BSFC RPM and accept that cruise speed would be reduced.

The reason behind my approach is that finding torque & HP curves for OE engines in light duty vehicles has not proven to be all that easy to come by. Finding the stock tire size & axle ratio combos is.

 

[DaveInDenver]

The reason behind my approach is that finding torque & HP curves for OE engines in light duty vehicles has not proven to be all that easy to come by.

An aside, ExPo-member Brian McCamish has aggregated them for several small Toyota engines.

http://www.brian894x4.com/Hiluxengines.html