I takes me 1,900 RPM to do 70 MPH in my SCREW.

I would like to know what rpm it takes everyone to do EXACTLY 70 MPH.

My theory is, that with all these different MODS (air intake, computer chip, exaust, etc.), the RPM required to do 70 MPH should be less then my stock SCREW's 1,900 RPM.

In my last truck, 87 F150 4 Speed, it took 3,000 RPM to do 65 MPH when it was new. Before I sold it this year, it was taking 3,300 RPM to do 65 MPH. I never did any mods to it what so ever.

The way I see it, if these mods can decrease the RPM required to do a given speed, then in fact, you will be getting more HP at that RPM range and a resulting greater MPG.

Any and all thoughts/comments are appreciated. Thanks.

 

Sorry to tell you, but you are way off. Gearing and tire circumference are the only things that affect RPM vs. Speed. I have the 3.55 diff with 265/70R16 tires. I am doing between 1900 and 2000 RPM at 70 MPH. The trucks with 17 inch tires are getting 200-300 RPM lower because of the larger circumference tires.
You can actually calculate the circumference of your tire and divide 5280 feet by that value. This will tell you how many revolutions per mile your tire does. Knowing the final drive ratio (3.55) and top gear ratio (overdrive transmissions have a final drive value less than 1.00), you can multiply a few ratios and calculate your actual RPM (at any speed). All of this without leaving your desktop.

 

Mine is right at 1900 rpm also at 70 mph. There was a thread on this subject a few months back that you could do a search on to find. By the way, it looks like we have identical trucks.

 

How accurate is the stock speedometer? And how does the speedometer measure speed? If the speedometer doesn't have a way of compensating for different mods, the mods might throw off an already (possibly) inaccurate speedometer.

 

Not true. Engine RPM is entirely a function of gear ratios. If you have your torque converter locked up in 4th gear, and you have the same rear end, same tires, then your RPM's must be the same, no matter what performance mods you put on it.

 

The only mods that will effect what RPM your engine turns @ a given speed are those that will change the final effective gearing. Things such as changing the overall tire diameter, changing your gear ratio, etc can effect your final effective gearing. Intake mods, exhaust, pulleys, etc have no effect on your final gearing, so won't change the RPM's you take to do 70MPH.

FWIW, it takes just about 1900 RPM for me to turn 70. I have slightly taller tires than stock. There are likely some inaccuracies in the speedo & tach themselves (+/- some %) - there usually are in gauges of that type.

 

chaean - my speedo did change when I put my 18" wheels on. It reads about 5 mph slower than actual speed. I have heard that you can have them reprogram the computer to compensate for the new tire/ wheel size, but mine has not changed enough to pay someone to do this.

 

Mine gearing is stock and so are my tires. I am at 1900 RPMS almost exactly on a flat strech of highway at 70MPH. I have a FIPK and a cat back. The performance mods will affect your shift points but at that speed just crusing, you will be in 4th (OD) and your engin may not have to work as hard as it can breath easier (ie you may not be using as much gas) but the RPM will be the same.

The tire size is just like changing the gear ratio, and yes it will throw off your spedometer, as I belive a gear ratio change would as well, not as sure about the latter.

 

i have 35's on my truck with 3.55 gears. i dont know how much the larger tire has thrown off my speedometer but at 70 it's about 1900 rpm also

 

I agree with the previous people who say the RPM vs. MPH will not change as long as the torque converter is locked up.
To answer the one question regarding the accuracy of the speedo; mine reads on the fast side. I have been comparing it to a GPS mounted to the dashboard and at 65 MPH on the speedo, the GPS is showing 62.7. This is on a Screw with the off-road package.

 

I never thought to check mine at 70 but on mine the tach indicates 2000 r/min at 75 mph. This is with a 4WD with 17" tires.

I noticed something strange with the speedo though. If you put the instrument cluster into HEC test mode (hold down the select/reset button for 5 seconds when you turn the ignition on) and go to the "ENG" test the speed shown there is slightly lower than what the needle shows. I was able to duplicate this on my dad's '99 Expedition as well.

matteo, can you check this against your GPS and see if it is more accurate. It looks like it may be.

 

At 70 mph with OD engaged 1900 rpm is correct with stock set up .