4104 Tires

[RJ]

Can anyone tell me how many revs per mile with 315/80 r22.5 thanx

Bruce -

Go to the website for your tire manufacturer and look up the specs for your tire.  As Chessie says, each builder is slightly different.

Michelin XZA2- 315x80x22.5 is 492 per mile.  About 29 engine rpm faster.

Chessie -

I think you mean 29 fewer rpm at the same road speed.  492 is fewer revs/mile than the designed 495.

FWIW & HTH. . .

[chessie4905]

  Faster engine rpm compared to running 12:00xr22.5's. This sort of sucks for angle drive GM's. Maybe we need to have China manufacture some 3:70 or 3:90 ratio gear sets for us.


   Here's a Continental tire book with other brand equivalents and other general tire specs, of course on their own.

http://www.continental-truck.com/www/download/transport_us_en/continental_dataguide.pdf

[TomC]

I'm confident if China made new gear sets for us they would last at least 100 miles. Good Luck, TomC

[TomC]

On any tire size, the RPM (Revs Per Mile) can vary a few with different tread patterns. Drives will run slower then all position since typically drive tires have thicker rubber making them bigger in diameter.
On my bus, I run Michelin XZE 11R-24.5 16ply, or H rated at 476rpm. With my 4.56 rear end and .88 overdrive in the V730, I cruise at 60 turning right at 1900rpm. Top speed at 2,400rpm is 75mph-which is too fast for a transit. Good Luck, TomC

[Beesme]

So does that mean at say 60 mph I'm getting less mpg than if I were turning 495? Thanx for all the answers at least I now know what I have . The tires are 4 yrs old and less than 10 thou miles ... I'm stuck with them for a few years haha

[chessie4905]

   Why do you think that? Everything comes from China nowadays anyway. I doubt Chinese trucks have gear set longevity problems.

[RJ]

So does that mean at say 60 mph I'm getting less mpg than if I were turning 495?

Bruce -

Specifically for the 4106, the powertrain was set up for 1650 rpm @ 60 mph with tires that turn 495 revs/mile.  I have that info in one of the GMC sales brochures for this coach.

As I said earlier:

"Tires that turn MORE than 495/mile will lower your top speed and increase your fuel consumption.

Tires that turn LESS than 495 will increase your top speed and return better fuel mileage."

What I left out of those sentences is the word "slightly."  Depending on the tires, it could be as much as a 6% difference.  Perhaps a rough mathematical example will help.  I'll use a 5% difference +/- from the designed 495 to keep it simple:

Suppose you have enough fuel to go exactly 100 miles using the designed powertrain. A tire that turns 520 revs/mile will only make it 95 miles before you run out of fuel.  OTOH, a tire that turns 470 revs/mile will have gone 105 miles when the fuel's exhausted.

That's an extremely simple mathematical explanation, hopefully to bring this tire rpm thing into perspective for the GM guys.

Oh, I should also throw out this caveat:  All the above is based on the OEM manual gearbox!  If the coach has a V-730 automatic, that's a whole 'nother ballgame!

FWIW & HTH. . .

[Ed Hackenbruch]

I don't know RJ, you might have to find a simpler way to explain that to the GM guys.  After all, most of them think that an engine that sits sideways and tilted is normal!!!

[TomC]

The idea for gearing/tire size is to get the engine running in it's sweet spot for best fuel mileage when cruising. If I want maximum fuel mileage I'll slow a bit to cruise right at 1800rpm-which Don Fairchild tuned my engine to do. 1800rpm on my bus is right at 56.5 mph.

On new trucks (Freightliner) we're gearing higher and higher to be able to use direct drive transmissions. Next year, a new horsepower setting for the DD15 is coming-400hp at 1,900rpm max with 1750lb/ft torque @ 975rpm! Using a 2.21 rear axle ratio with 510rpm tires, that will give 1127rpm cruise at 60mph! Inching towards that 10mpg mark with 80,000lb trucks! Good luck, TomC

[eagle19952]

The idea for gearing/tire size is to get the engine running in it's sweet spot for best fuel mileage when cruising. If I want maximum fuel mileage I'll slow a bit to cruise right at 1800rpm-which Don Fairchild tuned my engine to do. 1800rpm on my bus is right at 56.5 mph.

On new trucks (Freightliner) we're gearing higher and higher to be able to use direct drive transmissions. Next year, a new horsepower setting for the DD15 is coming-400hp at 1,900rpm max with 1750lb/ft torque @ 975rpm! Using a 2.21 rear axle ratio with 510rpm tires, that will give 1127rpm cruise at 60mph! Inching towards that 10mpg mark with 80,000lb trucks! Good luck, TomC

that take two days longer to get there....