Top speed
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New User
Joined: Dec 2017
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Top speed
Ive got a 76 f-350 flat bed, 390 engine, 4 speed manual transmission, 373:1 rear end differential and 245/75/16 tires. My problem is that the highway speed is about 55-60 before the rpm’s are feeling too high.
From what I’ve read, 373 gearing should get me a little faster on the highway. Is it the transmission? 1st gear is a granny type gear....and it doesn’t really have great torque going up hill on the hwy.
Thoughts on how to get better hwy speed? Do I have another unidentified issue? Thank you.
From what I’ve read, 373 gearing should get me a little faster on the highway. Is it the transmission? 1st gear is a granny type gear....and it doesn’t really have great torque going up hill on the hwy.
Thoughts on how to get better hwy speed? Do I have another unidentified issue? Thank you.
Senior User
Joined: Oct 2017
Posts: 280
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From: british columbia, canada
Well its not like todays rides with 6/ 8/ or 10 speed trannys. Its 40 plus years old, things were different back then. 373 is a good all round gear unless you are running big rubber which you are not. The insulation or lack of it makes it sound like its reving way to high. I don,t have a calculator at my finger tips, but at 60 you will be reving about 2500. Do you have a tack to verify?
Elder User
Joined: Aug 2016
Posts: 723
Likes: 3
From: Cape Town, South Africa
I've got an F350 dually with the smaller 351c engine and a three speed FMX tranny, also with 3.73 gears.
Rear tyres are 215/80/16 and the fronts are the same as yours.
At 120kph (75mph) my revs are at 3,200, and it sounds sweet.
If there's no poh-leece around, and there's gas stations en route, I cruise at 3,800rpm. You can guess the speed. LOL
No problems with hills.
With a 390 and 4 speed tranny, you should have no problems cruising at all.
A tach would be very useful, as already mentioned !
If you can 'feel' the revs are too high at +/-60mph, then there's issues other than 390, tranny, or 3.73 gearing.
Rear tyres are 215/80/16 and the fronts are the same as yours.
At 120kph (75mph) my revs are at 3,200, and it sounds sweet.
If there's no poh-leece around, and there's gas stations en route, I cruise at 3,800rpm. You can guess the speed. LOL
No problems with hills.
With a 390 and 4 speed tranny, you should have no problems cruising at all.
A tach would be very useful, as already mentioned !
If you can 'feel' the revs are too high at +/-60mph, then there's issues other than 390, tranny, or 3.73 gearing.
Laughing Gas
Joined: Jan 2016
Posts: 828
Likes: 66
From: Hartford, WI
3.73/3.90 type gears and a 28”/30” tire your easily going to spin the motor right around 2700rpm at 60mph. Like has been said before these trucks and cars back then didn’t have a overdrive in the transmission. It’s not going to hurt the motor. Maybe your wallet for all the gas stations you will be stopping at.
My truck right now has 3:50 gears. To me a good comprosise for a street driven truck and has 33” tires. At 70mph it’s around 2500rpm. Still want to put a gear vendors overdrive unit on it though.
My one truck had a stock 460 and factory 4:10 gears. It got driven at freeway speeds all the time
My truck right now has 3:50 gears. To me a good comprosise for a street driven truck and has 33” tires. At 70mph it’s around 2500rpm. Still want to put a gear vendors overdrive unit on it though.
My one truck had a stock 460 and factory 4:10 gears. It got driven at freeway speeds all the time
Postmaster
Joined: Nov 2011
Posts: 2,553
Likes: 10
I run similar tire size with 4.10 rear on an F250. I have run it at 70-75 for longer distances w/o issue. With 1:1 (direct) final drive the engine will spin in the low or even mid 3,000 RPM range. That is quite a bit faster than new vehicles with slick aerodynamics and OD transmission. An engine in good shape can take it. My original and fairly low mile 400 takes it fine. Good mufffler(s) help keep the noise in check .
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