347 Mustang With 3.73 Gears Highway Cruising Speed

Joe Scrivens

Member
Aug 27, 2014
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I was thinking of putting 3.73 gears in my fox hatch with a 347 stroker.

what would highway driving be like?

I don't plan on doing highway which is why i was realistically thinking 4.10 or 4.55 and keep it strictly track.

BUT

if I really wanted to get on the highway from time to time how horrible would the cruising speed/gear shifting be with 3.73 gear ratio reared on a 347 stroker?
 
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90lxcoupe

15 Year Member
Oct 7, 2003
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The cruising RPM doesnt really have anything to do with the engine. The gear ratio in the trans, tire size and vehicle speed will tell the whole story and there are multiple calculators to see what the actual RPM will be.

For my car i have a 4.30 gear with a .68 overdrive and a 28" tall tire. This puts me right about 2600 rpms at 75 mph, which is right where my combination likes to cruise. For example, if i had a 3.73 hear i would be right under 2300 rpms at 75 mph, which is a good engine speed for my motor to cruise at also but either way its a matter of 300 rpms so its really not that big of a deal.

Just ran the numbers thru my excel sheet and at 75mph with a 26" tire and a .68 OD gear you would be at
Gear RPM
3.73 2458
4.10 2702
4.30 2834

Dont fear the gear!
 
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A5literMan

At least it is lumpy...
5 Year Member
Jul 30, 2011
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With an over drive trans-t5/AOD auto,etc you're only cruising @ around 22-2400rpm on high way with 73's. 4.10 only add a little more. So really no big deal either way. Especially if it's just an occasional trip/toy. What's your max rpm on 347?? Ie rev range? Unless it's pulling more than 6000 rpm shift point go with 4.10's
 
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MikeH686

Mine is only two inches though.
5 Year Member
Oct 11, 2011
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I did this with my car since I'm changing over to auto to get the right stall converter, my 3.55 in a t5 in 4th which is the same ratio as 3rd in a c4 and 26" tall tires was putting me at 2700 at 60 mph
 
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89stang1

Stroked and Juiced
10 Year Member
Nov 5, 2008
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im running 3.73 gears with a 333 stroker and i think its perfect you can cruise 80mph in 5th gear, i have to check rpm but i know its a nice rpm, best of all you can roll launch in 2nd gear at 40mph with 4000rpms real fn nice!
 
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Joe Scrivens

Member
Aug 27, 2014
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I did this with my car since I'm changing over to auto to get the right stall converter, my 3.55 in a t5 in 4th which is the same ratio as 3rd in a c4 and 26" tall tires was putting me at 2700 at 60 mph
Not bad

I guess ill go with the 4.55 after all :)

I gotta weld my torque boxes first lol
 

Joe Scrivens

Member
Aug 27, 2014
42
1
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35
im running 3.73 gears with a 333 stroker and i think its perfect you can cruise 80mph in 5th gear, i have to check rpm but i know its a nice rpm, best of all you can roll launch in 2nd gear at 40mph with 4000rpms real fn nice!
Id love to hear this!!
 

Joe Scrivens

Member
Aug 27, 2014
42
1
6
35
The cruising RPM doesnt really have anything to do with the engine. The gear ratio in the trans, tire size and vehicle speed will tell the whole story and there are multiple calculators to see what the actual RPM will be.

For my car i have a 4.30 gear with a .68 overdrive and a 28" tall tire. This puts me right about 2600 rpms at 75 mph, which is right where my combination likes to cruise. For example, if i had a 3.73 hear i would be right under 2300 rpms at 75 mph, which is a good engine speed for my motor to cruise at also but either way its a matter of 300 rpms so its really not that big of a deal.

Just ran the numbers thru my excel sheet and at 75mph with a 26" tire and a .68 OD gear you would be at
Gear RPM
3.73 2458
4.10 2702
4.30 2834

Dont fear the gear!
Im going 4.55 ;) im sold lol thanks forinput
 

A5literMan

At least it is lumpy...
5 Year Member
Jul 30, 2011
4,782
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I don't know the details on your 347. I just an FYI if it doesn't pull a lot of rpms 4.56 gears will be to steep and actually slow your car down in the 1/4. If 1/8 mile track not as big a deal but you really need to match your powerband to the gear more so than worrying about cruise rpm. Especially if it's mostly a track/weekend toy. For an example 90lxcoupe is turning around 7000 rpms at the top of his powerband. If your car is still rocking the ecu's 6250 rev limiter 4.56 is to much gear go with 4.10's or so
 
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Joe Scrivens

Member
Aug 27, 2014
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I don't know the details on your 347. I just an FYI if it doesn't pull a lot of rpms 4.56 gears will be to steep and actually slow your car down in the 1/4. If 1/8 mile track not as big a deal but you really need to match your powerband to the gear more so than worrying about cruise rpm. Especially if it's mostly a track/weekend toy. For an example 90lxcoupe is turning around 7000 rpms at the top of his powerband. If your car is still rocking the ecu's 6250 rev limiter 4.56 is to much gear go with 4.10's or so
Makes sense however with my application im carbureted no ecu. The engine assbly is rated at 7500 rpm most my power is made between. 3 and 5 torque wise hp not sure yet

I have a big cam full roller valvetrain, portrd and milled cast iron heads.
 

A5literMan

At least it is lumpy...
5 Year Member
Jul 30, 2011
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Makes sense however with my application im carbureted no ecu. The engine assbly is rated at 7500 rpm most my power is made between. 3 and 5 torque wise hp not sure yet

I have a big cam full roller valvetrain, portrd and milled cast iron heads.
You really need either 3.73-4.10 any more and you'll actually slow down. It will scream through the rpm band to fast. With a t5 1st gear is really steep and torque multiplication with those gears are more than enough. Your internals may be capable of 7500,I guarantee the block isn't and doubt the valvetrain is capable of staying stable at those speeds. What iron heads are you using? Stock? World products?gt40's? All those heads don't flow enough to spin the engine beyond 6-6500(unless they are highly worked over)
 
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Joe Scrivens

Member
Aug 27, 2014
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You really need either 3.73-4.10 any more and you'll actually slow down. It will scream through the rpm band to fast. With a t5 1st gear is really steep and torque multiplication with those gears are more than enough. Your internals may be capable of 7500,I guarantee the block isn't and doubt the valvetrain is capable of staying stable at those speeds. What iron heads are you using? Stock? World products?gt40's? All those heads don't flow enough to spin the engine beyond 6-6500(unless they are highly worked over)
Oh im not gonna spin up thst high. I actually have a limiter on the msd at 6k.

Its gt40 heads i believe with larger valves and ports.
 

Joe Scrivens

Member
Aug 27, 2014
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You have no use for a 4.56 based on what you just told us then. Go 3.73. My car came with 4.10s and I can't wait to get them out.
Personally i was gonna do 3.73 but my friend keeps pushing the 4.56 's i think cars wind up too quick. 410 is max but i think 3.73 would b appropriate too