Top speed

[ZX3ST]

I went full-ham on this thought exercise a few months back.

Depending on the rear tire size (305/35/19 or 315/30/19 assuming you're running a OEM size) your top of 4th gear at 8200rpm would be 144.5/139.5. 5th gear would be 179.2/172.9.

Making some estimates on coefficient of drag and tire rolling force........

At 175mph and standard air density you will need approximately 360whp just to overcome aero and tire drag. Likely just a few HP more to account for bearing drag and other stuff, this is just back-of-napkin.

390whp for 180mph. 422whp for 185mph. Again, real world would likely need a bit more HP.

I suspect anyone seeing much over 180mph indicated speed either has significant power mods, or their speedo is reading a bit higher than what's real.

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[ZX3ST]

I put together some neat Excel sheets to look at this stuff cuz I'm weird like that.

 

[Spart]

Depending on the rear tire size (305/35/19 or 315/30/19 assuming you're running a OEM size) your top of 4th gear at 8200rpm would be 144.5/139.5. 5th gear would be 179.2/172.9.

So about this 144.5/139.5 thing. I think people calculate that using the nominal tire sizes - 305/35R19 and 315/30R19 - for the GT350 and GT350R.

A 305/35R19 tire nominally is 2189mm in circumference, meaning you get 735.3 revolutions per mile.

The MPSS 305/35R19 is not 735.3 revolutions per mile though. It's 758 revolutions per mile. Same with the PSC2, the nominal 315/30R19 size is 761.6 revolutions per mile, but the actual Michelin is 785 revolutions per mile.

Using the correct number for the tire model instead of a generic number for the nominal tire size, you get 141.2mph/136.3mph for the GT350 and GT350R OEM tires.

 

[ZX3ST]

So about this 144.5/139.5 thing. I think people calculate that using the nominal tire sizes - 305/35R19 and 315/30R19 - for the GT350 and GT350R.

A 305/35R19 tire nominally is 2189mm in circumference, meaning you get 735.3 revolutions per mile.

The MPSS 305/35R19 is not 735.3 revolutions per mile though. It's 758 revolutions per mile. Same with the PSC2, the nominal 315/30R19 size is 761.6 revolutions per mile, but the actual Michelin is 785 revolutions per mile.

Using the correct number for the tire model instead of a generic number for the nominal tire size, you get 141.2mph/136.3mph for the GT350 and GT350R OEM tires.

You're correct and that's why I added a "override" input to my spreadsheet so that one could input a specific tire circumference in cases where it differs from nominal. It appears I accidentally cleared that box when I made my original comment.

For a 305/35 MPSS I'm coming up with 140.3 and 174.0 at 8200 RPM for 4th and 5th.

 

[Spart]

You're correct and that's why I added a "override" input to my spreadsheet so that one could input a specific tire circumference in cases where it differs from nominal. It appears I accidentally cleared that box when I made my original comment.

For a 305/35 MPSS I'm coming up with 140.3 and 174.0 at 8200 RPM for 4th and 5th.

Why 8200? I'm sure most of us don't shift at 8250, but 8200 seems like an arbitrary number.

 

[ZX3ST]

Why 8200? I'm sure most of us don't shift at 8250, but 8200 seems like an arbitrary number.

I built my spreadsheet to do the math in 100rpm increments so there isn't an entry there for 8250. And I thought we were just talking about max speed attainable in each gear. I could be wrong on that point.

 

[Tomster]

Pretty sure the valves open in 6th gear at WOT regardless of speed.

Yes, you have to put it to the floor to acomplish that. The gearing is such that I wouldn't put it to the floor in 6th. But thats just me.

 

[Tomster]

The fact that it takes 8 times the horsepower to go 2 times faster under the same conditions. The stock GT350 has a hard time hitting maximum speed in 5th gear. Speed limitations in this car are the weight and drag. Gearing indicates 178 mph @ 8250 RPM, but it doesn't have the power to get there.

It has the power, the deciding factor is real estate. Like i said, the end of the back stretch at daytona running the oval, i was right at max speed.

 

[Tomster]

At an event we had 3 16-17 GT350" all do 177 mph, The r only did 173 because of the rear wing.

It was because the tire/wheel size. Its a gearing issue.

Do you have GPS on those 177 mph numbers? (With a high GPS accuracy)

 

[Tomster]

the R will do 173 MPH

Ask me how I know.

 

[DrumReaper]

I think this guy hit close to 8250rpm in 5th and IIRC, there was a YouTube vid circulating with a 350R following a 500 and that R hit mid 170’s on the data.

 

[16Kobra]

It was because the tire/wheel size. Its a gearing issue.

Do you have GPS on those 177 mph numbers? (With a high GPS accuracy)

Yes sir

 

[The Rooster]

So I wonder how much more power is needed to use 6th.. I haven't tried yet; but Ive seen 280kph ish in 5th.
Mine gets there pretty quick.

 

[Spart]

So I wonder how much more power is needed to use 6th.. I haven't tried yet; but Ive seen 280kph ish in 5th.
Mine gets there pretty quick.

Probably a lot, as it's not designed to be a power gear. It's more of a double overdrive.

• 2nd (2.233) is 67% of 1st (3.253)
• 3rd (1.611) is 72% of 2nd (+5%)
• 4th (1.24) is 77% of 3rd (+5%)
• 5th (1.00) is 81% of 4th (+4%)
• 6th (0.629) is 63% of 5th (-18%)

You see how each successive gear is a bit closer in ratio to the one before it to give you good power as you shift gears? Until you get to 6th. In order for 6th to be a good power gear it would probably need to be around 0.85 instead of 0.629. As it is, shifting at redline in 5th drops you down to 5190 rpm. With a 0.85 6th gear, you'd be at 7013rpm, much closer to peak HP which is at 7500rpm. And you'd be geared for a top speed of 206mph, which isn't too shabby. As it is, 6th is geared for a top speed of 278.3mph which is ludicrous.

The 6th gear ratio has always confused me in this car. I mean, it already had the gas guzzler tax. Produced in low quantities. Ford must have been pretty desperate to save a few million on CAFE credits to have hamstrung it with such a disadvantaged double overdrive. I only ever use 6th on the interstate and even then it seems a bit lugged for the power curve. With a 0.85 6th gear, you'd be doing 2964rpm @ 74mph in 6th gear instead of 2193rpm with the 0.629 6th gear. To me, 3000rpm is about where the Voodoo starts to wake up so you'd probably have much more torque available for passing and such.

 

[460Fred]

The only time I use 6th gear is on an open highway with nobody around me. I always like to have instant power when I’m surrounded by those who might do me harm. That mindset is active around crowds as well ;-)

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