Hey guys, I just got my truck back from a long long month at the shop. She's pretty much ready for the insurance inspection to be passed now, took long enough though! haha

(1990 F150, 5.8 351 with the overdrive trans)

So she has to go back to the shop once more before its good to go, and this is because my speedometer is 19km/h out. Meaning when doing 100km/h your actually going 119km/h. They said that this is due to the way the truck is geared, which the previous owner told me was 4.11.

Now, from what I've heard this 4.11 is what a lot of guys use for racing, and I kinda want a fast truck haha. But the shop is talking about changing my gear ratio to correct my speedometer.

Is this the only way to go about doing things? Like am I losing out on power or anything by changing the ratio? I just want to fully understand what they are doing before they do it haha. Hopefully someone can shed some light upon my situation!

Thanks guys.

 

Your shop are either idiots or thieves. Rather than change the rear gears, simply correct the speedo. Your 91 has a cable driven speedo so correcting it costs about $7 in parts and takes 20 minutes.

 

haha! Alright! That I like the sound of..

Could you explain how to do it so I can tell them how to do it...? Or.. maybe even do myself?

 

All you do is detatch the speedo cable from the tranny tailshaft housing (one bolt) and slide it out. You remove the clip holding the colored plastic gear in place, slide the gear off, slide the new gear on and "assemble in reverse order."

The toughest part is figuring out what new gear you need. To do that you need three pieces of data.

1) What size tires do you have. The original tires were 235/75-15. If you're running something different, it changes the speedo calibration.

2) What was the original gearing? You can find this out by reading the code on the vehicle ID sticker on the door jamb. It should be 17, 19 or H9 but it could be something else; you need to know which.

3) How many teeth are on the existing gear? Just unbolt the cable and slide it out, count the teeth, slide it back in and re-tighten the bolt. If you're feeling really adventurous when you've got the cable out, shine a flashlight in the hole and count the number of teeth on the driving gear on the tailshaft. That number will make the process totally foolproof.

Post those three things and I'll show you how to get the correct tooth count and the Ford P/N for the gear you need.

BTW don't worry about running 4.10 gears with an OD trans. At 65 MPH you're only turning about 2280 RPM with stock tires. That's a little high for max highway fuel economy but it optimizes your towing capacity and should actually improve your around town mileage

 

Alright!

1) Tire size in the front: P235/45 R-17 and the rear: P275/40 R17

2) And as for the original gearing I'm unsure of which code to look at on the sticker. The tranny code is E if that helps? haha.

3) And I'm still currently trying to find the speedo cable haha..

 

The axle code. it will almost certainly be 17, 19 or H9

 

Haha kk, found it. It's actually 18 though.

 

18 is 3.08 gears. Makes no sense with a 351, at least not to me.
Those 275/40-17 tires are short. They roll at 810 per mile vs. the 720 per mile of the stock 235/75-15s. Get me a tooth count and we're ready to go. There is one minor problem on the horizon. The combination of bigger gears and smaller tires may get you into a speedo gear which doesn't exist. I'll know when you tell me what you've got

 

Also, are you absolutely positive that you have 4.11 gears?

Something here is just not adding up. The shorter tires would mean that you're going slower than indicated as would the allegedly bigger gears. But you say you're going 19% faster than indicated. How do you know this?

 

Well the previous owner mentioned that it had 4.11s, I'll give him a call tomorrow and recheck on it. I could very well be wrong haha.

And the reason we figured 19% was by using a GPS.

 

Forget the previous owner. Check the ratio for yourself. It's simple enough to do.

Assuming you have an open differential:

1) Jack up ONE rear wheel and put the tranny in neutral
2) Make a chalk mark on the driveshaft
3) Turn the raised wheel by hand through TWO complete revolutions and count the rotations of the driveshaft. A 4.11 will produce slightly more than 4 revolutions.

If you cannot turn the raised wheel by hand then you have a limited slip differential. to check its ratio:

1) Jack up BOTH rear wheels and put the tranny in neutral
2) Make a chalk mark on the driveshaft
3) Turn the raised wheel by hand through ONE complete revolution and count the rotations of the driveshaft.

It's a long way from rocket science

 

Well! You were right, 4.11s are incorrect. It spun about 3.5 revolutions.

 

So you have 3.55s. but with the small diameter tires, it acts like you have bigger gears. My best guess is that the previous owner changed to 3.55sand then swapped out the speedo gears but put the wrong gear in, one which raised rather than lowered the indicated KPH.

You really need to get the tooth count off of that speedo gear

 

And never go back to that shop again! Holy crap, change out gears...

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