How do axle ratios affect engine longevity?

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I run 4.88s on a truck that originally came with 3.91. Love the extra gearing for trail riding and surpassingly there has been no MPG drop because of it. The thing just feels reved out at times but I have no problem with that. I was driving the other day and I got thinking what kind of long term affects this has on the engine compared to a stock setup.

Naturally I’d say high rpm situations create more wear but that’s also because I think most of us naturally think of racing/wide open throttle/ high load applications when it comes to high rpm. What about guys like me who are revving high because of different gear ratios but in all reality are driving around with a low load on the engine?

If you do the math you’re essentially doing an extra 25% RPM. So instead of cruising at 2200rpm you’re doing 2900rpm. And I know this isn’t exactly “high” in the world of engines but it is higher and I’m curious nonetheless

Is that extra 600rpm getting you better splash lubrication on the cylinders?

Do your piston rings stay cleaner due to more oil flow through the ring packs?

Do valves run cleaner because of it?

Or is it all just extra wear and tear?

i do want to say that I’m not worried about it as being an issue or anything as much as it’s a curiosity and wanted to see what other people thought.
 
I will exclude insane RPM in the comments; I'm sticking to "normal" rpms in the mid-range of the allowable engine speed and not bouncing it off the redline all day long.

That said, engine speed has far less effect than does load.

For example, many cars with smaller engines often will run at or close to 3200 rpm on the highway at 80mph. My 2019 2.0L MX-5, for example.
Other cars with more low-end torque can run lower engine speeds; my 2018 Taurus 3.5L runs 2000 rpm in the same conditions.
Yet the wear rates are both admirably low. So rpm has little effect. I do understand this is sort of "apples to oranges", but it's valid tho an indirect example.

When I tow my RV and really heavy loads, my truck has a higher wear-rate than when running the same speeds unloaded. Further, my buddy has a F250 with the 6.2L engine and 4.30 gear. I have a similar truck with 3.73 gears. He runs that 15% higher rpm all the time. Yet wear rates are very similar.

I won't say rpm doesn't matter; it may be a small contributor to wear. But loading is far more of a factor. The increase in rpm in your situation does bring more "contact" per unit of time. As the engine speed increases, the amount of time any one part sees "action" increases ... more times the rings pass any spot on the cylinder, for example. But, the engine is also under less load at that higher speed, because it has more torque available for any situation demand. So less throttle is required to climb an incline.
 
In addition to rear gear ratio, the transmission gear ratios need to be discussed. I have 3.73 gears in my Burb, but my 6L90 has two overdrive ratios. I cruise at 65 mph at under 2000 rpm. Generally speaking, low rpm at highway speeds will contribute to engine longevity, IMHO.
 
Here are the ratios in Ford’s 10 speed tranny. It has three “overdrive” ratios, the lowest being 0.63 to 1. This allows a high rear end ratio and will still keep the highway rpm low.

27EDD2B7-5C9E-47A1-BF51-3A81DC04E06A.jpeg
 
Older Toyota M/T equipped cars & small pickups (Hilux) tend to be geared like a farm tractor, but tend to last nearly forever with decent maintenance. I think lugging is worse for an engine than reasonably high RPM, lugging is hard on pistons, even rod & main bearings. High RPM only hurts the gas gauge!
 
Here are the ratios in Ford’s 10 speed tranny. It has three “overdrive” ratios, the lowest being 0.63 to 1. This allows a high rear end ratio and will still keep the highway rpm low.

View attachment 134547
I hope most people that have the 10 speed have better ones than I do, I’ve never seen a transmission slower to go into gear, or more erratic shifting/slipping-Ford needs to give the TCM a brain transplant! This transmission was ONLY designed to wring out a slightly better EPA MPG number, I wouldn’t recommend it for any real towing or heavy loads.
 
I think lower gears (higher numerically) actually help the engine last longer because it is less likely to be lugged at low rpm with a heavy load on it. With a manual shift this naturally depends on the drivers habits. Same thing with prop pitch on boats, the engine should be propped so that it reaches the specified max rpm at wide open throttle. Inboards usually list 4600-4800, I propped mine to be just short of 5,000 because the engine is on the small side for a boat that size & weight (20’, 4200 lbs, 4.3 V6 4-bbl).
 
I hope most people that have the 10 speed have better ones than I do, I’ve never seen a transmission slower to go into gear, or more erratic shifting/slipping-Ford needs to give the TCM a brain transplant! This transmission was ONLY designed to wring out a slightly better EPA MPG number, I wouldn’t recommend it for any real towing or heavy loads.
The truck I drive at work has the 10 speed. It's terrible if you leave it in " normal" mode. I always put it in "tow/ haul" mode and it's much more responsive.
 
Some of the best running and longest running engines are in old clapped out trucks with steep gears. I can't explain how that works

The standard gear in a jeep like mine is 3.08.... mine has the 3.73 and still runs great
Seems like every 80s/early 90s truck I've driven with a manual that runs high rpm on the highway always runs so much better than their automatic counterparts.
 
Seems like every 80s/early 90s truck I've driven with a manual that runs high rpm on the highway always runs so much better than their automatic counterparts.
Less loss from the automatic? Any heat going out the ATF cooler represents energy not being used to drive the wheels.
 
Less loss from the automatic? Any heat going out the ATF cooler represents energy not being used to drive the wheels.
Could be. A lot of those old automatics loved to lug along at low RPMs/part throttle when you'd have shifted a mile ago in a manual. Depending on what you're doing high load at low RPMs is pretty terrible for an engine that runs well, much less a farm truck with a vacuum leak you can hear and a rotor/cap/spark plugs that are old enough to buy cigarettes.
 
Any heat going out the ATF cooler represents energy not being used to drive the wheels.
I think the heat sloughed off by an ATF cooler (in-radiator or free standing in the air) is made by the friction within the transmission, thus it (the heat) was never in the power transmission chain.
 
Some of the best running and longest running engines are in old clapped out trucks with steep gears. I can't explain how that works

The standard gear in a jeep like mine is 3.08.... mine has the 3.73 and still runs great
Hi Chris, do you mean high numerical gears like a 4.11 or 4.56 rear gear ratio or something like a 2.76 or 3.08?

I remember coming across a lot of old late 60's Buicks and Cadillacs that had really tall gears for highway cruising. It seemed like those engines just held together pretty well?
 
Could be. A lot of those old automatics loved to lug along at low RPMs/part throttle when you'd have shifted a mile ago in a manual. Depending on what you're doing high load at low RPMs is pretty terrible for an engine that runs well, much less a farm truck with a vacuum leak you can hear and a rotor/cap/spark plugs that are old enough to buy cigarettes.
True. Worse, once that convertor unlocked, it was operating with some amount of slippage (making heat), and the lower the rpm, the more slippage it was undergoing.

I think the heat sloughed off by an ATF cooler (in-radiator or free standing in the air) is made by the friction within the transmission, thus it (the heat) was never in the power transmission chain.
Some of the heat is from friction, sure. But most (if not darn near all, outside of tractor-trailers) manual transmissions do not need coolers, their internal friction is rarely high enough to be a problem. Now automatics do have a pump which has to make pressure to run the transmission, and that does make heat—and some amount of heat is made each time the transmission shifts, and those frictions have to grab onto something.

But the most heat is being made by that torque convertor. A convertor outside of lockup but above stall speed has a few percentage of slip, making heat. That heat represents driveline loss—and all the energy, lost or put to pavement, comes from the engine.
 
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Did you change your speedometer gear to match? If not, I could see why your fuel mileage appears the same. Likely getting X% worse fuel mileage and your speedometer/odometer also reads X% too high, making the numbers appear the same but more fuel is being used.
 
Hi Chris, do you mean high numerical gears like a 4.11 or 4.56 rear gear ratio or something like a 2.76 or 3.08?

I remember coming across a lot of old late 60's Buicks and Cadillacs that had really tall gears for highway cruising. It seemed like those engines just held together pretty well?
A big engine doing highway duty will hold together well.

Back in the day, before the mid 70s, if you bought a Buick, Olds, or Cadillac you got a motor from the same marque, and it was unique to that maker. And the higher dollar brands were "better" with "more nickel in the casting" and other stuff you can't see.

There was a big lawsuit when someone bought an Olds with a Chevy motor. After that went down all the magazine ads for GM had a disclaimer that you might get an engine from another division.

Now it's just from "GM Powertrain."
 
I remember that so-called fiasco. The Cadillac had an Oldsmobile gasoline and diesel engine sometime during the mid 1970's. That is when I started tinkering on old cars during high school. I learned to look at distributer, water pumps and intake/exhaust manifolds to tell which division of GM made the engines. Good times..
 
I think the heat sloughed off by an ATF cooler (in-radiator or free standing in the air) is made by the friction within the transmission, thus it (the heat) was never in the power transmission chain.

The energy needed to make that heat comes from the power transmission chain though, and reduces what you end up with at the end
 
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