Top Speed Question
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Top Speed Question
This might be a question that Darylhunter would need to apply a scientific equation. Which ratio rear axle would have a higher top speed if there was no speed limiter on a 2005 superduty 5.4 engine, 6 speed manual - 3.73 or 4.10? the 6th gear overdrive ratio is 0.72, would the 3.73 be geared too high to overcome wind resistance or would the 4.10 rev too high? I think the 2005 sd's with 6.8, only has 4.10 or 4.30, what be the speed on both these engines and ratios with stock tires? If a tuner was used of course they would both have more power and torque. Anyone have a formula or take a wild guess?
Elder User
Joined: Dec 2003
Posts: 848
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From: Kokomo
These trucks block a lot of wind, I think I would rather run out of gear then motor, it takes BIG power to fight wind resistance at high speeds, I bet my truck with 3.73s would fall on it's face long before it got near red-line in OD, my Crown VIc on the other hand with not much wind resistance, 3.27s, and a hot chip will shift pretty firm as it shifts at 5600 rpm on the 3rd to 4th shift at 125mph and it just keeps on climbing.
Cargo Master
Joined: Jun 2003
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From: Carlsbad, California
I don't have the torque vs RPM curves for the 5.4 L motor but I think that it is a pretty safe bet that it won't pull red line in high gear with the 3.73 rear end.
Without resorting any further analysis though, I'm sure that it won't do it with the 4.10 either but it's likely that it will go a bit faster with the 4.10 than the 3.73s.
I had a '98 F-150 with the 5.4 L motor and it ran well but even with intake and exhaust work and a SC module, with 3.73 gears I could tell that it would run of steam long before red line. The 4.10 rear end would only raise the revs by 386 RPM at 135 MPH.
The formula for RPM for a given speed is:
RPM = (speed x trans x rear end x 1056) / (tiredia x Pi)
i.e., (140 x 0.72 x 4.10 x 1056) / (31 x 3.14) = 4483.5 RPM
The formula for Speed for a given RPM is:
Speed = (RPM x tiredia x Pi) / (trans x rear end x 1056)
i.e., (4483.5 x 31 x 3.14) / (0.72 x 4.10 x 1056) = 140 MPH
Where trans is the gear ratio for the selected gear, 0.72 for 6th gear in you case, and rear end is 3.73, 4.10, etc., and tire dia is the diameter of the tire in inches. 1056 is the conversion factor to square up inche/mile and hour/min units business.
Without resorting any further analysis though, I'm sure that it won't do it with the 4.10 either but it's likely that it will go a bit faster with the 4.10 than the 3.73s.
I had a '98 F-150 with the 5.4 L motor and it ran well but even with intake and exhaust work and a SC module, with 3.73 gears I could tell that it would run of steam long before red line. The 4.10 rear end would only raise the revs by 386 RPM at 135 MPH.
The formula for RPM for a given speed is:
RPM = (speed x trans x rear end x 1056) / (tiredia x Pi)
i.e., (140 x 0.72 x 4.10 x 1056) / (31 x 3.14) = 4483.5 RPM
The formula for Speed for a given RPM is:
Speed = (RPM x tiredia x Pi) / (trans x rear end x 1056)
i.e., (4483.5 x 31 x 3.14) / (0.72 x 4.10 x 1056) = 140 MPH
Where trans is the gear ratio for the selected gear, 0.72 for 6th gear in you case, and rear end is 3.73, 4.10, etc., and tire dia is the diameter of the tire in inches. 1056 is the conversion factor to square up inche/mile and hour/min units business.
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Thanks Darylhunter for your information. Glad to have you here on the forum. It would like 4.10 gears are the best choice overall. At 70 mph the rpm difference is less than 200 rpm. The difference in gas mileage between 3.73's and 4.10's would likely be very small.
Postmaster
Joined: Mar 2001
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From: So Cal
Our trucks are essentially barn doors as far as wind goes. So don't expect to win any topend races against any resonable car.
My 94F250 4x4, E4OD, supercab Long bed, 4.10 gears and 460 engine will get up over 100 before I have had enough. Stupidly I can get it up over 80 with an 8,000# trailer, normal comfort with trailer is maybe 70.
Either way, the acceleration is not as good when you get up over say 80 or 90. Compared to my car which is very comfy accelerating well past 100, the truck is a slug. Off the line the truck is very good.
And BTW there are places where you almost have to do 80-90, otherwise you are a rolling road block and traffic hazzard.
Just a few weeks back I was in my car and did a steady 90mph for multiple 10's of minutes at a time, flowing with the rest of the traffic and being passed by many others. Sure cut my 1000 mile drive time down compared to the times I have done similar drives at 55. I probably would not go over 80mph steady with my truck(without a trailer) because it feels different than my car, not sure how to describe it other than the car is designed to cruise well over 100 while the truck is designed to haul at normal speeds.
Just my opinion.
Jim Henderson
My 94F250 4x4, E4OD, supercab Long bed, 4.10 gears and 460 engine will get up over 100 before I have had enough. Stupidly I can get it up over 80 with an 8,000# trailer, normal comfort with trailer is maybe 70.
Either way, the acceleration is not as good when you get up over say 80 or 90. Compared to my car which is very comfy accelerating well past 100, the truck is a slug. Off the line the truck is very good.
And BTW there are places where you almost have to do 80-90, otherwise you are a rolling road block and traffic hazzard.
Just a few weeks back I was in my car and did a steady 90mph for multiple 10's of minutes at a time, flowing with the rest of the traffic and being passed by many others. Sure cut my 1000 mile drive time down compared to the times I have done similar drives at 55. I probably would not go over 80mph steady with my truck(without a trailer) because it feels different than my car, not sure how to describe it other than the car is designed to cruise well over 100 while the truck is designed to haul at normal speeds.
Just my opinion.
Jim Henderson
Last edited by jim henderson; Dec 12, 2004 at 02:26 PM.
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