Gear ratio and mph

[RonD]

Transmission output shaft was used for speedometers since the early days of speedometers in vehicles, motorcycles and some early cars did use front wheel instead

Mechanical cable was used until 1980's, then VSS, vehicle speed sensors, were added but just for computer and cruise control use
In mid-1990's speedometers switched to electric so no more mechanical "speedo cable", but the VSS was driven by same "changeable" speedo cable gear setup as the cable was
Ford uses 8,000ppm(pulse per mile) calibration, the gear driven VSS had that output

Any vehicle that used a transmission driven speed signal, mechanical or electronic, needed to be adjust for rear axle ratio and tire size to maintain the 8,000ppm calibration, which is why they had "changeable" driven gears

Rear axle ABS runs about 20,000ppm, so the signal needed to be converted to 8,000ppm to be used by speedometer
As far as I know only the Rangers 1998-2000 ever used rear axle ABS sensor via GEM module as a faux VSS, GEM converted it to 8,000ppm, and could be programmed for tire size, ratio didn't matter because speed signal was after rear axle ratio in drivetrain
The Explorers and other Ford vehicles with 4 wheel ABS also did this for a few years but the 4WABS module did the conversion instead of the GEM Module, 4WABS module could also be programmed for tire size, ratio didn't matter

In the late 1990s Ford started adding OSS(output shaft speed) sensors to transmissions, over a few years
These were not gear driven but used tone ring added inside all the transmissions, manual or auto, and an external OSS sensor to read it

These were also higher PPM, 25kppm, they were connected directly to the computer, and the computer did the conversion and sent out the 8,000ppm signal to speedo and cruise
Since this was transmission driven speed signal rear axle ratio now mattered again
Computer could be programmed for rear axle ratio and tire size
I think Forscan software can do this

So 2004 Ford would most likely use OSS sensor on transmission, so you would need to correct Computer's data on rear axle ratio to get correct calibration, and tire size if not stock

 

[cbxer55]

Thanks Ron. Knew you would know the answer. I will wait and see if the taller rear tires get it back to close enough for government work, before doing anything else. It was off by 10% before, the difference between 3.73 and 4.10 is .37 or 10%. Don't know what affect one inch taller rear tires will have. Will find out tomorrow. Raining all day today.

I know, on my 04 Lightning, it's as simple as getting one of those handheld tuners that plug into the OBD II port. Can set axle ration and tire diameter there, and fix it in one fell swoop. 98 Ranger not so simple?

 

[RonD]

You can use the "new tire size" calculator: https://www.therangerstation.com/how-to_pages/gears_needed_after_adding_larger_tires.shtml

Looks like 1" more diameter will make a 4.10 go down to 3.96 so 3rd of the way there, lol
If you could fit 3" more diameter it would be calibrated, 3.72

Never tried changing GEM or 4WABS to calibrate speed signal so not sure whats involved, but if Ford can do it so can we

 

[joeweger]

“technically “ it would go faster, but be slower getting to a given speed. (0-60) I say technically because if you have a small engine like a 2.3 lima it might not have the horsepower to overcome the increased resistance to get to 90. I went from 3.45 to 4.10 and rpms are higher at certain speeds but it gets to 60 quicker and has much more bottom end pull. 85 is all it has overall, it just runs out of tourque at a certain rpm and wont go any faster. With 3.45’s it probably would max out at a similar speed but take longer to get there. (wind resistance).I never drove it hard with the 3.45’s, the axle swap was one of the first things I did. 3.73 is probably the best balance of power vs economy, I just always wanted to try 4.10’s with a 2.3 so thats what I went with.

Yeah I do know that and I know how to calculate my speed. Want be a problem. It came out 94 ranger with the same setup except for the rear end. I think since I went 10 inch backing plate to 9 inch backing plate my emergency brake cable is a little bit short makes sense.