Gear Ratio ??

[miker]

when looking into a straight truck, what is a good gear ratio to have that will provide a good fuel mpg while also providing a good enough torque while under a heavy load? I notice a lot of straights have ratios in the 4.00 and up while a lot of tractors have ratios in the 3.70 range.
also will it matter as far as the ratio whether you have an auto or manual?

 

[nightcreacher]

Gear ratio in fact has a lot to do with what kind of transmission,and also the type of motor.If your going to have a high torque low rpm motor or a high rpm low torque.The high rpm motors use a low speed rear end ratio,the low rpm motors use a high speed rear end.The high speed gears are lower numbers than the low speed numbers.In either truck,you want a transmission with an over drive final gear ratio.If you run a direct drive final gear,that is going to make the motor use to many RPM's while cruising.
When I ran the Cat motor in my tractor,it had a dbl over drive trans which gave it a 27% over drive,and I ran a 3.42 gear ratio.At 1200 RPM the truck would go 60 mph,and it got 7 mpg.The truck I have now has a Detroit 14 ltr 500 hp it has a decent torque rating,but not as good as the Cat I'm running a 10 speed auto shift and 3.70 gears,the autoshift final gear ratio is a 27% overdrive.The lower gears help on take off and in the hills.Tire sizes make a difference also.By the way,my truck now gets about 7.5 mpg,as long as im around 60 mph.Smaller tires you would run a faster gear ratio to equal the bigger tires

 

[Moot]

Miker, do a little internet research. I believe Eaton and or Meritor have online axle ratio calculators. But like Creach said, a lot of factors affect gear ratio selection. You need some real life information regarding horsepower, torque, transmission, tire size etc. to use the calculator.

 

[guido4475]

i once had a 2002 fl-70 with a isb cummins 260 hp engine with a 6-speed direct(no overdrive) trans, and 3.90 gears. it topped out at 78mph wide open,and it gave me 10 mpg all day long.this was a extended-cab fl-70.it was great around town as well,i should of went with a 3;55 ratio instaed to bring the rpms down, but that would of made the engine lug too hard going through the moutains. the 3.90 ratio did very well. it had lo pro 22.5's on it also.the truck gave me good life.if i had to go back to a 6-wheeler from this 10-wheeler, that is what i would go back to.

 

[piper1]

Miker, if you are looking at a specific truck, PM me the engine,transmission, tire size and gear ratio. I'll run the math for you. It's one of the things I miss about having a fleet job behind a desk.

 

[miker]

i once had a 2002 fl-70 with a isb cummins 260 hp engine with a 6-speed direct(no overdrive) trans, and 3.90 gears. it topped out at 78mph wide open,and it gave me 10 mpg all day long.this was a extended-cab fl-70.it was great around town as well,i should of went with a 3;55 ratio instaed to bring the rpms down, but that would of made the engine lug too hard going through the moutains. the 3.90 ratio did very well. it had lo pro 22.5's on it also.the truck gave me good life.if i had to go back to a 6-wheeler from this 10-wheeler, that is what i would go back to.

this is what caused me to ask the question, I dont have a specific truck in mind yet . But will send you the info when I do Piper,thanks
it seemed odd to me that most of the st have a lower gear ratio that looked like it would cause them to top out quickly.
Thanks for the info when I find one that I'm interested in, I get with you on it

 

[Moot]

I notice a lot of straights have ratios in the 4.00 and up while a lot of tractors have ratios in the 3.70 range.

Miker, this could be that many expedite straight trucks are built on a class 7 platform. I sent you a p.m.