I didn't think there was any need since the speedo is set by the ring on the carrier in the rear end. I thought I would only need to reset the speedo when I up the tire size 32's are only slightly larger than stock Odo and speedo are reading correctly. Am I wrong about the sensor ring? Sorry to get off subject of post.

 

well for instance, if your truck was factory equiped with p235/75/15's then your constant conversion number is either 9.72 or 9.67. If it came stock with the 265/75/15's then it would be 9.18. 31" tires would be 9.18, which would make 32" tires approximately 8.7 to 8.9 if I could remember the math conversion. so with that said, if you had the 235's then you would be off by almost a full point. and if you had the 265's then your off by .3 to .5. If you have not reset your PSOM then your mileage and speed will be off. You are correct that the ring gear is the same constant as long as no one has set that constant to any other size ring gear. tooth count is either 108 or 120. look at your door tag and see what the tag says was stock for your truck. For instance, mine came stock with 235/75 R15's and now has 33/12.50/15's on it. I originally set my PSOM for my old 31's and now I am about 5 miles off at 60 mph with the 33's. So for now, I watch my speed and dont worry about the mileage except for big fluctuations. I need to get off my lazy ---- and reprogram my PSOM for the 33's.

 

About how Hard is it to do (recalibratePSOM/speedometer) I have
a '96 Bronco with 302 and Autotranny

 

Thanks for the info ponyracer. Mine came stock with the 265's, the 32's i have are actualy just a hair over 31 1/2 Mfg calls them 32 That would be why my odo and sped are correct well almost. Im off by about 1/2 a tenth over a 10 mi distance. Checked between mile markers.

 

PINPOINT TEST F: SPEEDOMETER/ODOMETER READS INACCURATELY


F5 RESET CONVERSION CONSTANT


l Reprogram speed conversion constant using the following procedure:

-- Locate the dealer mode programming enable single-terminal connector. The connector is located at the bottom of the instrument panel below the center of the glove box. On all vehicles, the dealer mode enable wire is Circuit #567 (LB/Y).

-- Insert metal clip in service wire panel connector. Clip other end to vehicle ground while the key is in the OFF position.

-- Turn key to RUN (not crank) position while holding down the trip odometer RESET button on speedometer. Release RESET button.

-- Speedometer will prove out (sweep across dial and back). The English/Metric and revision levels will be displayed. The last number of the display is the dealer mode programming lockout count. This count shows the number of allowable conversion constant changes remaining. When the count is zero (0), no additional changes can be made to this instrument cluster. If a conversion constant change is required, a service instrument cluster must be ordered. Refer to this section for ordering information.

-- Press and release RESET button again and PSOM conversion constant (without the decimal point) followed by the word CAL.

-- Press and release the odometer SELECT button as many times as necessary to change conversion constant to the correct value.

NOTE:
Pressing and releasing the RESET button to lock in the new Conversion Constant reduces the number of times the PSOM can be reprogrammed by one count. The module can only be reprogrammed six times. If you change your mind and want to go back to the old conversion constant, turn the ignition switch off before you lock in the new constant.

-- When the correct new value is displayed in odometer window, press and release the RESET button to lock in the new conversion constant.

-- Turn the key off and remove the ground wire. Verify proper speedometer operation.

-- Is conversion constant now set to the desired value?

NOTE:
The module can only be reprogrammed six (6) times.

I dont have the coversion constants for anything above 31" tires. for 265/75/15 the constant is 9.18 and for 31"-10.50/15 tires it is also 9.18. Do a search on this site as I think that there is more information than I have for taller tires if you need it.

 

All I know is it costs too much to fill it up, and I have to do that too often. But everytime I get behind the wheel, I don't care what it costs.
D

 

Does 6.8mpg sound right for a 1979 Bronco Ranger XLT with a 3spd auto and a 351m?

 

Is that around town? On the interstate?
Short cold trips?
What's the current ambient temperature?


My 300 six with overdrive can range from 8mpg to 18mpg depending on those conditions.

 

Its been a combination of both the last few weeks and some of the highway trips used about 1/4 of a tank per round trip.

I've done a few short around town trips to visit my parents but not that many and the average temperature has prob been in the 40s the last week or so but I always let it warm up first.

I know only having a 3spd does effect it but I wasn't sure if it should be that bad.

 

90 with a 95 roller motor e4od 355s 32 inch tires 16-18 on the hiway as little as 10 with a trailer or a lot of stop and go

 

Another question is, is your speedometer correct? Are there larger tires on it than stock? If so, has the speedometer gear been recalibrated? If your speedometer is off, you could be traveling 150 miles while your tripometer only reads 120. That'll throw your calculations off.

Other than that, if you have some highway in there, are driving normally (no lead food) and don't have anything really all too radical about the Bronco, I'd say that's a little low. 8 - 11 is the average I generally hear for the '78/'79 Broncos.

May be time for a full tuneup?

 

It has regular tread tires, no lift kit, or any other mods aside from a K&N filter that I put on. The speedometer is as correct as I can tell after driving by one of those stand alone radar detectors that the police sometimes put up. I don't have a tripometer so I am going off the odometer as much as I can but I have been taking pictures of the odometer with my phone each time I fill up.

I only bought it about a month ago so I haven't done much of a tune up to it other than the air cleaner, a new master cylinder, and a new steering arm.

Have you ever heard of anyone putting a 4 speed auto into a Bronco like mine?

 

If that's the case, I'd just do a full tuneup on it and see how much it changes. New spark plugs, spark plug wires, cap/rotor on the distributor. New fuel filter (be sure to check for one back near the gas tank).
Check your timing and idle mixture with a vacuum gauge. That sort of thing. Lots of those little things can add up to poor mileage if they're out of tune.

Not sure on the automatic. I know some were made for the smallblock engines (AOD and E4OD), but I don't know if they did for the big blocks.

 

Well they only offered a 351 or 400 in 79 and it was a 351m which means it was modified because it was during their transition from the 351c to the 351w so I haven't found any stock transmissions that were for any Broncos that year.

I'll try doing a full tune up. My dad was a mechanic all his life and he's taught me a lot but I wasn't sure if there was anything that anyone else could tell me about it. Thanks for your help.

 

Yeah, that's why I was saying I wasn't sure if there are any available for the big blocks, since that's all that'd bolt up to yours.

Those tuneup things are a great place to start. If, from there, that doesn't help, it's time to look into carburetor tune. Beyond that it can be any number of things like dragging brakes, dry wheel bearings, etc. etc.