rpm question

I have a 1990 ford bronco with a 351 engine. What rpm should I be running at 55? Everything appears to be stock and the tires are normal size. I am running about 2300 at 55 and I don't think it should be that high. It is shifting into od too.

Thanks

That does seem high, but depends on your gearing.

At 55 the beast is usually running at about 1650 rpm.

Thats what I thought. I'm gonna check the gear ratio today.

Probably the previous owner geared it for power. Here is a gear chart from Jeff's that is calibrated for 55 MPH cruise. It is calibrated for 3rd gear not overdrive. So you can select the cruise RPM you are comfortable with at 55 MPH based on tire size. 3rd, not in overdrive, is a 1:1 engine to drive shaft ratio. Overdrive will help reduce the cruise RPM for economy purposes but isn't considered when selecting gears.

Good luck,

Also, if you change your gears you will have to recalibrate your speedometer to get an accurate speed reading. An easy way to check the speedometer is to check your 1/10s against the mile markers on a major highway, like an Interstate. Those posts are very accurately placed. Mark the miles and 1/10s on your odometer at a mile marker post and drive until the mile marker posts indicates that you have gone 10 miles. Then note the miles and 1/10s reading again at the end, it should indicate you went 10 miles. Any variation will tell you how far off your speedometer is since it works off the same input. The number of 1/10s indicates what % your speedometer indication is off, each 1/10 = 1%, ie. 3/10s is 3%. You can do it by holding 60 MPH for several miles, but that is not always possible and by using 1/10s method, it will work at any speed and speed variation. Vehicles with mechanical speedo cables can only adjust by changing the drive gear with more or less teeth which adjusts the speedometer and odometer by about 7%. So in that case you would select the gear that would make it the most accurate.

Checked the gears today and it has 355's. Any other ideas

Sound like it's not going into OD.

How are you sure it is? Does it have a OD off switch?

What diameter tire are you running?

It is shifting 3 times and it does have an od switch. When u press the button it kicks down a gear. and i have 31's

maybe the torque converter is not locking up after shifting into OD

yo,

So we're talking 'bout the E4OD?

Here is the Transmission Control Indicator Light & Switch (Overdrive) Location pic in dash, to right of steering column in a 90



and the Shift Indicator pic



Source: by Chris A

Preliminary Testing and Diagnosis

Before any diagnostic testing is done on a vehicle some preliminary checks must be performed, as outlined below. Be sure to write down your findings, especially any DTCs found, for future reference.

Check transmission fluid level and condition.

Check for add-on items (phones, computers, CB radio etc).

Visually inspect wiring harness and connectors.

Check for vehicle modifications.

Verify that the shift linkage is properly adjusted in OVERDRIVE.

Verify that Transmission Range (TR) sensor is properly adjusted in NEUTRAL.

Verify customer concern; upshift, downshift, coasting, engagement, noise vibration.

Vehicle must be at a normal operating temperature.

Perform vehicle On-Board Diagnostics.

Record all DTCs.

Service all nontransmission D

Torque Converter Operation Test

This test verifies that the torque converter clutch control system and the torque converter are operating properly.

Carry out Quick Test; refer to On-Board Diagnostics with scan tool in this section. Check for DTCs; Refer to the Diagnostic Trouble Code Chart.

Do a Self Test for Diagnostic Trouble Codes (DTC)s by my pal, BroncoJoe19

The engine temperature must be greater than 50° F for the Key On Engine Off (KOEO) Self-Test and greater than 180° F for the Key On Engine Running (KOER) Self-Test.

Run it around to heat the engine up and shift thru all gears incl Reverse. Then turn off all accessories/lights, etc.

Make sure A/C is off and transmission is in Park (automatic) or in Neutral for a Manual & release clutch.

Do Key On Engine Off (KOEO) portion first.

Look Codes up in my broncolinks.com site using the new Search function.

And Post em here according to:

KOEO

&

KOER

Connect a tachometer to the engine.

Bring the engine to normal operating temperature by driving the vehicle at highway speeds for approximately 15 minutes in (D) position.

After normal operating temperature is reached, maintain a constant vehicle speed of about 80 km/h (50 mph) and tap brake pedal with the left foot.

Engine rpm should increase when brake pedal is tapped and decrease about five seconds after pedal is released. If this does not occur, refer to Diagnosis By Symptom; Torque Converter ClutchOperation Concerns in this section.

Hopefully, we'll see a Code for MLPS Manual Lever Position Sensor; also described as Transmission Range (TR) Sensor

I'm running 31s and have 3.54s Just had a highway run, and checked rpm, 3rd gear OD off ,almost exactly 2110 @ 55 just as Seabroncs chart shows.

You must not have 3.54s or something is definitly wrong with your tranny.

I definitely have 355's so I guess I'll look at the tranny. It is shifting fine and not vibrating so who knows??? I'm getting frustrated..

the only way you could be doing 2300 rpm in OD with 31's without a really badly slipping trans is if the previous owner put 4.56, or 4.11 gears in it. how did you check your ratio?

355 is stamped on the ring gear

take it to a transmission specialist

It looks like I'm gonna have to

Well before you take your truck to a shop and pay out the nose for labor, I have been trying to figure out a similar thing. I have had 3 broncos and and F 150 in the past ten years. A 1992 Bronco a 1995 Bronco, A 1995 F 150 and now a 1993 Bronco all with 351's and e4ods and 3.55 gears and limited slip diff. Now to my under standing the gear ratios in the transmission were the same though the the whole production of the e4od. But the 2 95's were much different in the in several ways which i believe was all related to the the programming in the newer model trucks for better fuel economy. They also accelerated quicker through entire rpm range especially 1500 to 2500, and cruised at about 500 or so lower rpm at highway speeds. I saw somewhere about swapping a 94-96 transmision computer into an older truck. I can't seem to find the thread where it was mentioned. This is with my personal experience with 4 different trucks that I had for a long time and was very familiar with. They all shifted good and to all 4 gears and would shift back to 3rd if you turned off the over drive. It is my experience that the earlier e40ds were a little different and caused you to turn more rpm. I'm thinking it may be something to do with when the torque converter locks up Were your rpms lower at speed before and this is a recent occurrence? The guy that posted further up said his at 1650 at 55 and according to his sig he has a 96 thats right where my 95s were too. I'm going to start a thread about the transmission computer since I can't find were i saw i posted before. I didn't meant to hijack your thread just trying to help and get answer or too as well but I saw your numbers are pretty similar to what mine are and were in the 92, and I find it hard to belive that 2 of my trucks both had the same problem and drove the same at the same rpm so many years apart. sorry for the long post but maybe it will help.