Speedometer and Gearing

[etniesbmx1]

So according to a GPS my speedometer is 19% out. So at 100km/h the truck is actually doing 119km/h. The shop doing my insurance inspection is saying the easiest way to fix this would be to change the gear ratio. Now, I really dont understand some things here and I'm hoping you guys can help!

The truck is a 1990 F150, 5.8 351, 2wd, and I have the overdrive trans.

Now, the previous owner told me the truck has a 4.11 ratio, he mentioned that guys will use this for racing. I want a truck that is fast and definitely race-able haha and the mechanic tells me that my truck never came factory with a 4.11. So could someone here kind of shed some light on what exactly changing my ratio is going to do. They explained it but I'm not really good with technical terms haha.

I'm hoping you guys can help me out here, you've been awesome in the past! haha

Thanks!

[taisa899]

you have to change the gear in the tailstock of your tranny that runs the speedo.

[Lone Ranger]

Your truck most likely came with anywhere from 3.08 to 3.73 gears, not 4.11s. Having stock size tires and deeper gears with make your speedo show faster than you're actually going, not slower. Larger than stock tires on stocks gear will however.

19 km/h at freeway speed is a huge change, somewhere to the effect of having a truck with 31" tires stock and throwing 35" tires on without changing anything else. Speedo gear in the tranny is the best and simplest way as gears and the setup required is expensive. The only way I would change my gears is if the engine isn't turning at the proper RPM on the freeway, and for a 351 you'll want it around 1700 - 2000 rpm at 100 km/h in top gear. LR

[etniesbmx1]

Hmm. Alrighty. But someone suggested this:

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.

Is this information incorrect? Because I'll save a lot more money if I go this route haha.

[taisa899]

drivers side of the tranny tailstock you'll see a covered cable plugged into a small black plastic piece. That's the speedo gear! Unplug it, remove 3/8 nut and pull it out and install new speedo gear

[SKRWZ]

Hmm. Alrighty. But someone suggested this:



Is this information incorrect? Because I'll save a lot more money if I go this route haha.


Yes but in order to help you you will need to tell us what tire size you are going to use and what gear ratio as well.

[etniesbmx1]

Well my tire size is:

Front:P235/45 R-17

Rear:P275/40 R-17

And from what the previous owner tells me is that the gearing is 4.11.

And thank you taisa. I misread your first post! haha

[fordfan22]

PERFECT! i have the same problem with my mustang. previous owner threw in 3.73's and never changed the speedo gear. i brought it to a shop and asked how to fix it, he said they had to take the whole diff apart send in a certain piece to a certain shop get it changed somehow and then put it all back together. then a guy at work told me it was just a like, 6$ sensor that just plugs into the tranny. so its the sensor? how do i know which one to get? i do like taking people for rides and stepping on it and having the speedo show 140 and passenger say my car is so fast but really, i'm only doing 100..haha

[taisa899]

just tell your parts guy you need a speedo gear for the type of tranny you have and the rear end gear ratio you have.

[fordfan22]

sure sounds simple. i'll have to do that. thanks. but then when i get pulled over i can't use the excuse that my speedo doesnt work properly anymore and that its getting fixed tomorrow. haha

[SKRWZ]

just tell your parts guy you need a speedo gear for the type of tranny you have and the rear end gear ratio you have.


Good thought but since most parts guys just look up a part # and go fetch it they would most likely look at you like you have a third eye when you ask them what speedometer gear you should put in, give these guys a call they specialize in speedometers and should be able to hook you up with no issues http://www.transmissioncenter.net/speedometer_calibration_______va.htm

[etniesbmx1]

Awesome! Well thanks for the help there guys! Really appreciate it.

[taisa899]

it'll fix it for gear ratio. Speedo will still be out cause of tire size just not as bad

[SKRWZ]

Let us know how you make out

[etniesbmx1]

Will do! Might be a little while, as my "Need Fixing" list keeps multiplying. haha but thanks!