Gear ratio

[EastCoastCB]

Originally Posted by MIKE N STEPH

10r22.5 tire so thats 40 inch
2300rpm at 60
2500rmp held to the floor 65

Ok, with that data and using Spicer's online gear/speed calculator your gearing is definitely not 5-something.
4.44 seems to be what you have. At least something close to that.
Try it out-
https://spicerparts.com/calculators/...rpm-calculator

For transmission ratio put "1" as your fourth gear is 1:1.

[Brad_SwiftFur]

My guess - the rear-gears were swapped at some point. The manufacturer simply relays what was originally installed.

[EastCoastCB]

Originally Posted by Brad_SwiftFur

My guess - the rear-gears were swapped at some point. The manufacturer simply relays what was originally installed.

Yeah it would only do around 50 or so if the gearing was 5.50.

[WARGEAR]

That makes much more sense to me. I’ve got the same size tires and a 4.78 rear ratio with a direct drive top gear. I max out on paper at about 60mph but I’m convinced my bus still has a road speed limiter based on how the engine reacts once the vehicle hits 55.

[MIKE N STEPH]

Originally Posted by eastcoastcb

ok, with that data and using spicer's online gear/speed calculator your gearing is definitely not 5-something.
4.44 seems to be what you have. At least something close to that.
Try it out-
https://spicerparts.com/calculators/...rpm-calculator

for transmission ratio put "1" as your fourth gear is 1:1.

hmm using that gear calculator you linked i see your point and i guess it deff can not have 5.57 and after putting some different gear ratios in it i think i should prob be either 3.73 or 3.58 (those ar the 2 sizes meritor offers for my axle near that number ) 3.73 gives me 58mph at 2000 rpm and 3.58 gives me 60

[TheHubbardBus]

There are (at least) 2 potential sources of error regarding your calculations...


1) Tire Height - can be different from that calculated from tire-size specifications. Best to measure.


2) The AT545 doesn't lock up, so you can't count on a precise 1.0 gear ratio in 4th.


I couldn't speak to the magnitude of these inaccuracies, or whether they'd be relevant in the context of the level of precision you hope to achieve.

[CHEESE_WAGON]

Not nitpicking, but you may want to update your profile info -- it's bit confusing to see 2007 3500 with a 6.6L (suggesting a shorty with a Duramax) when you're discussing a 3126 with an AT545.

That being said, I would look at re-gearing if and when the time comes to upgrade the AT545, given that you mention an impending MT643 upgrade.

Let's face it, folks, most of these buses were never intended for highway use. Indeed, many of them were never intended to exceed 45 mph, and never have. 5.58s certainly would be a low-speed city bus that was never intended to exceed 35-45. But as previously mentioned, it's not likely yours has 5.58s. The lowest I've seen in a bus were 5.29s, but I think most will likely have something in the 3.73 - 4.88 range.

Something that folks may understand already about gearing, but I'll explain it anyway. Higher numerically ratios are lower gearing, as most already know. The trade-off with higher gearing (numerically lower) is that while you gain top-end, you give up acceleration and put more strain on the transmission, as others have mentioned here.

You also are going to give up fuel economy, as the harder the engine works, the more fuel it will burn. Especially if it is turbocharged, higher gearing puts more load on the engine, making the turbo spool more, thereby requiring more fuel.

So your major consideration is this... Are you going to be driving this thing mostly on the interstate, or on back roads? Mostly interstate will warrant changing to slightly higher, but I wouldn't get too drastic.

However, if you're going to be using more back roads than interstate, or perhaps a bit of mountain driving, I would leave it alone... If you re-gear, you'll likely give up 10-15 mph on hills, torque or no torque, and you will REALLY miss your old lower gearing then. Just my $0.02.

[TheHubbardBus]

Originally Posted by CHEESE_WAGON

Something that folks may understand already about gearing, but I'll explain it anyway. Higher numerically ratios are lower gearing, as most already know. The trade-off with higher gearing (numerically lower) is that while you gain top-end, you give up acceleration and put more strain on the transmission, as others have mentioned here.

And to further complicate the subject, what you're gaining on the top-end is theoretical... it's the speed you could go at a given rpm in a certain gear if your engine has the oomph to make it happen. But if your gearing is not well matched to your weight, speed, torque, and increased air resistance at speed, then you very well may not be able to reach it at all, or if you do, you accelerate so slowly that you'll never see it outside looooong stretches of flat or descending roadway. So you shouldn't just pick out a speed you want to go and pick the gearing that - on paper - will make it happen, without some idea of whether your power-plant is up the the task.

Originally Posted by CHEESE_WAGON

You also are going to give up fuel economy, as the harder the engine works, the more fuel it will burn. Especially if it is turbocharged, higher gearing puts more load on the engine, making the turbo spool more, thereby requiring more fuel.

Either you mispoke, or I misread (most likely the latter), but unless you're lugging the engine way outside its efficiency range, higher gearing (lower engine rpm) should result in increased fuel economy. Or maybe you're referring to lugging?

[CHEESE_WAGON]

Originally Posted by TheHubbardBus

I'm sure you mispoke, but unless you're lugging the engine way outside its efficiency range, higher gearing (lower engine rpm) should result in increased fuel economy. Or maybe you're referring to lugging?

What I'm referring to is higher gearing making the engine work harder and spool the turbo for increased power under higher load, in situations such as acceleration from a dead stop or to keep the mass moving at a consistent speed (such as it were in this case) uphill or under other increased-load situations as load demand changes.

Which is why the primary use of the bus (back roads / interstate) is so important. Back roads will mean a lot more acceleration changes and thereby could cause the turbo to spool more frequently. Turbo boost = more air and thereby demands more fuel.

Yes, lugging is not good for any engine, but that's not what I'm referring to, and I doubt it's a concern here, with an automatic, as most automatics select the correct gear for the situation.

[TheHubbardBus]

Originally Posted by CHEESE_WAGON

What I'm referring to is higher gearing making the engine work harder and spool the turbo for increased power under higher load, in situations such as acceleration from a dead stop or to keep the mass moving at a consistent speed (such as it were in this case) uphill or under other increased-load situations as load demand changes.

I don't know much about turbos.. this bus is the first vehicle I've ever owned that hasn't been naturally aspirated. But I thought turbos were driven by exhaust volume, which was in turn driven by RPM. So how does a turbo spin faster at the lower RPMs inherent in running higher gearing?

I agree that hilly country, stop & go driving, etc will burn more gas. But that's because you're running lower gears in those high-demand situations (per the tranny), not higher - which is burning fuel in direct proportion to engine RPMs.

What am I missing?

[CHEESE_WAGON]

Originally Posted by TheHubbardBus

I don't know much about turbos.. this bus is the first vehicle I've ever owned that hasn't been naturally aspirated. But I thought turbos were driven by exhaust volume, which was in turn driven by RPM. So how does a turbo spin faster at the lower RPMs inherent in running higher gearing?

I agree that hilly country, stop & go driving, etc will burn more gas. But that's because you're running lower gears in those high-demand situations (per the tranny), not higher - which is burning fuel in direct proportion to engine RPMs.

What am I missing?

Increased throttle (such as it were on a diesel) creates higher load, which in turn creates higher exhaust pressure, which is what spools the turbo. So when an engine experiences higher load on acceleration or going uphill, the higher exhaust pressure from increased rpm will spool the turbo to provide the increased power that is needed. That factors heavily into lower-speed driving on back roads and such, because hills will demand more power to climb them.

Pretty much, either you're burning more fuel at higher rpm without the turbo, or you need extra fuel at lower rpm under high load. Most diesels are going to be happiest at 1350-1650 rpm, BTW. They're not all the same, some may be a bit more high-strung than others.

[Brad_SwiftFur]

Originally Posted by CHEESE_WAGON

The lowest I've seen in a bus were 5.29s, but I think most will likely have something in the 3.73 - 4.88 range.

Mine has 5.38 gears.

Originally Posted by CHEESE_WAGON

Something that folks may understand already about gearing, but I'll explain it anyway. Higher numerically ratios are lower gearing, as most already know. The trade-off with higher gearing (numerically lower) is that while you gain top-end, you give up acceleration and put more strain on the transmission, as others have mentioned here.

You also are going to give up fuel economy, as the harder the engine works, the more fuel it will burn. Especially if it is turbocharged, higher gearing puts more load on the engine, making the turbo spool more, thereby requiring more fuel.

There's some truth to that, but let me pose this question to you: If you're parked for the night, does 600 RPM or 1200 RPM burn more fuel idling? Obviously the higher RPM will burn more fuel idling. For some (many?) buses the same logic will hold true with engine RPM at a certain highway speed, at a given speed a lower RPM will reduce fuel consumption....

Originally Posted by CHEESE_WAGON

So your major consideration is this... Are you going to be driving this thing mostly on the interstate, or on back roads? Mostly interstate will warrant changing to slightly higher, but I wouldn't get too drastic.

However, if you're going to be using more back roads than interstate, or perhaps a bit of mountain driving, I would leave it alone... If you re-gear, you'll likely give up 10-15 mph on hills, torque or no torque, and you will REALLY miss your old lower gearing then.

.... And this is where gear ratio selection comes into play. Ultimately the whole system works together - from the fuel ignition inside the cylinders, to the rods and crankshaft, through the clutch/torque converter, transmission, driveshaft, rear-gears, and tires ... at some point you reach a "one cylinder combustion results in x amount of forward movement". Whether it's with an overdrive gear, 5.38 rear-gears and 41" tall rubber, or a 1:1 final drive, 3.53 gears, and 32" tall rubber, or any other mathematical combination, the end result is the same.
(EDIT) Most of us end up measuring this as "X RPM at Y speed", an informal, yet reasonably accurate method.



It's reasonably well established that at speeds above 35-40, most of your fuel economy is lost due to air resistance, and the faster you go, the more energy (and fuel) is needed to overcome the air resistance and maintain that speed. And yes, ascending hills takes energy too, the more weight you have, the more energy is needed. The same above-mentioned math comes into play, your engine produces x amount of power (torque) at a given RPM, and no matter what gears and rear axle ratio you have, that engine will only move x amount of weight up a given hill at y speed. No amount of re-gearing will change that, it will only change what gear the transmission will use to do it. That's why trucks from years ago had 13-speed, 15-speed, and whatever-else-speed transmissions, so truckers could switch to whatever gear to keep the engine speed up when pulling a heavy load up a hill (and use a low gear on the downgrade to keep speed in check).


Since many buses only have 4 or 5 gears to choose from, it limits the selection when ascending a grade. A bus that might have pulled a given grade in 3rd gear with a 5.38 ratio, may not have enough torque to do the same with a 4.11 ratio (all other factors being the same). The engine still produces the same amount of torque, and if the transmission had more gears to choose from, it would (at least in theory) still ascend the hill at the same speed as before and the same combustion-in-the-cylinder to x amount of forward movement as before, just now in a different transmission gear.


I realize all this discussion does not take into consideration things like differences and variations in turning resistances of gears and driveshaft revolutions, but I expect those play a very small role in the big picture of my point.

[EastCoastCB]

Originally Posted by TheHubbardBus

And to further complicate the subject, what you're gaining on the top-end is theoretical... it's the speed you could go at a given rpm in a certain gear if your engine has the oomph to make it happen. But if your gearing is not well matched to your weight, speed, torque, and increased air resistance at speed, then you very well may not be able to reach it at all, or if you do, you accelerate so slowly that you'll never see it outside looooong stretches of flat or descending roadway. So you shouldn't just pick out a speed you want to go and pick the gearing that - on paper - will make it happen, without some idea of whether your power-plant is up the the task.



Either you mispoke, or I misread (most likely the latter), but unless you're lugging the engine way outside its efficiency range, higher gearing (lower engine rpm) should result in increased fuel economy. Or maybe you're referring to lugging?

In my experiences with school buses they have the gearing and oomph to get going. What they lack is long legs out on the highway.
Unless you get one geared high.
I've driven buses with 3.42 on up to 5.29 and I'll take highway gearing any day.

[MIKE N STEPH]

Well i have 2 buses thats why my signature has a diff bus listed. The bus will be used for a mix of driving both on and off highway i do understand switching to a lower numbered ratio i will give up a slight bit of off the line take off and im ok with that i was more looking to find the best ratio for best of both worlds lowering my rpm on the highway a bit but still having the back road manners with regards to acceleration