RPM at highway speeds

[Anonymous]

What you guys averaging at highway speeds? RPM wise. And what is your setup (tires, tranny, differential, etc.)?

 

[63Futura]

With my stock setup in my '63 Futura (170, Ford-o-matic, 3:20 rear end, stock 13" wheels and stock size tires (Can't remember what size)), I am running just about 3,000 at 65 mph.

A little high for me, but I only drive the car about once a week anyway.

 

[CobraSix]

According to the calculations (I have no Tach) I'm pulling around 22-2400 RPM at 65-70 MPH.

3.20:1 Rear
T5 Tranny
14" rims with 195/70 R14s

Slade

 

[Anonymous]

Cobrasix- What's your 5th gear? I'm curious to see if your calcs are showing the same as mine.

The numbers you posted line up really well with mine, assuming that you have a .79 5th gear.

--mikey

 

[CobraSix]

I'm pretty sure it's .79 also. Its the 4 cylinder T5 I'm running.

 

[Anonymous]

Cool. That means we're either both right, or both wrong.

I've built an Excel spreadsheet that you can enter wheel diameter, tire width, aspect ratio, trans & diff gears, and desired speed. It takes that info and spits out an RPM number.

It's easy enough to throw together, but I can e-mail it to anyone who wants it. It's handy in playing with different rear end, tranny, and wheel configurations. Good for determining theoretical shift points, too.

--mikey

 

[Anlushac11]

8)

My '80 Mustang was running a C4, 2.73 rear and P215/60-hr-14 tires. RPM' at 85mph was right about 2900rpm IIRC.

 

[Hot 6t Falcon]

The formula for figuring RPM is

RPM = (MPH x 12 x 5280 x GR) / (60 x TC)

RPM is Revolutions Per Minute
MPH is Miles Per Hour
GR is Gear Ratio -usually on a tag on a bolt on the rear end.
TC is Tire Circumference (easy way is to put a mark on the edge of the tire and on the cement. Then roll tire either direction until the mark touches the cement again and measure distance from orginal mark.)

My 175/80/13 tires have a 75.5 circumference.
I have a 3.50 gear ratio.
3000 rpm = 61.7 mph
3200 rpm = 65.8 mph
3650 rpm = 75 mph (speed limit in ND - at that rpm, your engine will not last long.

If you want to figure MPH the formula is
MPH = (60 x TC x RPM) / (12 x 5280 x GR)

Good Luck

 

[Anonymous]

Dennis,

Did any of your relations work in communications during the late 50's-mid 60's?

 

[ricksmol]

My Mustang..with a T5 (.79 ratio in 5th gear), 2.79 rear differential and 15" (24.6" diam.) wheels is turning exactly 2000 RPM at 65 MPH.

Rick

 

[Anonymous]

V8 T5, 205-70/14's, 3:20 rear ratio, 70 mph = 2000 rpm 8)

 

[Anonymous]

Okay let's here some numbers with automatics. 8)

 

[Anonymous]

My 175/80/13 tires have a 75.5 circumference.
I have a 3.50 gear ratio.
3000 rpm = 61.7 mph
3200 rpm = 65.8 mph
3650 rpm = 75 mph (speed limit in ND - at that rpm, your engine will not last long.

Same here, with a stock 63 Futura (170, Ford-o-matic, 3:50 gear ratio, stock 13" wheels and stock 175/80/13 size tires. According to the calculations (I too have no Tach) I am running just about 3,200 at 65 mph.
I try to stay down around 55 mph, the speed limit here in L.A. but "I can't drive 55!", nobody else does either.
I'm thinking a built C4 with a Gear Vendor OD in it.

 

[Anonymous]

The formula for figuring RPM is

RPM = (MPH x 12 x 5280 x GR) / (60 x TC)

this must be for an automatic with 1:1 final ratio because I don't see where it takes in to account your .79 5th gear, that is if GR is the rear.

Anyway using this formula mine is 2300rpm with the c5 @ 65.

don

 

[x21]

Going at about 70mph, with 2.83 stock rear (for a 6), a stock C4, running 205/70R14s, about a couple of notches or so below 3000 rpm... but then again it has been a long time since I last drove her long distance at that speed, so memory may not be up to par

 

[Anonymous]

Good.

This all confirms my calculations.

I have 215/60 R15's (TC=79.05), C4 with a GR=2.80, and cruising at 70MPH my RPM's should be somewhere around 2618...my tack says between 2600-2700 RPM.

I just thought my guage was alittle off or maybe I was figuring incorrectly.

Cool.

Now...do I want to go with a manual transmission and higher gear ratio? Hmmmm...would help me get quicker takeoffs. 8)

 

[68coupe200]

I'm surmising that with my upcoming T5 swap, i won't be turning more than 2300 rpms on the highway at 70, even with my 4.11's and 225/60/15's.

 

[Anonymous]

I'm thinking a built C4 with a Gear Vendor OD in it.

I was going to get a Gear Vendor OD until they told me that the driveshaft tunnel would have to be "massaged" in certain areas for the unit to fit. That's what they had to do in an installation in a round-body Falcon.