how much far off is your speedo? thanks!

 

Grab your calculator! Your speedometer will read 121.1% of your actual speed. 60 mph with 3.08 gears will read as 72.7 mph with 3.73 gears. You also must take into account the size of aftermarket tires that you have installed. If larger in diameter than the original tires, they will reduce your speedometer error.

(3.73/3.08) multiplied by (original tire diameter / new tire diameter)= current speedometer reading as a percentage of original speedometer reading.

So, lets say that you had 29" diameter tires on your truck and have replaced them with 33's. (3.73/3.08)*(29/33)=1.06.
Therefore, your speedometer would be reading 106% of the original reading or 63.8 mph @ 60 mph.

Once you have calculated the error percentage, you can multiply the number of teeth on your existing speedometer drive gear by this number to find the number of teeth that are required on the new gear. Using the example above, and assuming that you have 18 teeth on your existing gear (and this is only an assumption, you must pull yours out to find out for sure ) 18 multiplied by 1.06 = 19.08. You would therefore require a 19 tooth speedometer gear.

Hope this helps and good luck. GlennMc.

 

Hey I just had mine done last week and my speedo is correct but then again I did not do the install and the shop that did do the install also adjusted the speedo.

 

thanks a lot for the info guys! i knew my speedo had to be off a little bit, now i knopw which speedo gear to buy, thanks again!

 

or you just follow my previous post to this page......


http://www.bncoffroad.com/tire_gear_cal.html




but what do I know?

 

Ok..so on this same topic.

I have about 17k miles on my truck....all I do is drive back and forth to work in it ~45m each way~. Id like to swap out the rear 3.55s with the smaller 3.08s

Any of you have any low mileage 3.08s layin around you wanna "give" me for a nominal fee?

Otherwise Ill just order a new set from ford.

 

id give you mine, but theyre 12 years old and not exactly in great condition, so i would suggest getting new(er) ones if your truck is fairly new.

 

Yea...thats what I will wind up doing.

Just havent had much luck in finding a smaller gearset ~3.08 or lower~ from any online parts stores.

 

JaxHarley;
I have the 3.08's that I took out of my truck but they have 70,000 miles on them. Nothing wrong with them but if you are going to take it apart, you might as well put new ones in. If you look for Ford Motorsport on the internet, I think you might be able to search for dealers through their website. If you buy used gears, you will still need to buy a crush ring, pinion nut and possibly a pinion shim. Good luck with the change. If I may offer an opinion, I changed from 3.08's to 3.55's and I would not consider going back even if I didn't do some towing with it. The truck is far more responsive and easier to drive since the change. I have also seen no noticeable reduction in fuel mileage. I find that I can drive in a higher gear than I used to and still accelerate well. I think that has prevented any mileage drop. Good luck with your change. GlennMc.

 

So the new gears will come with a crush ring(Whatever this is), pinoin nut and pinion shims?Did you replace all of your bearings when you did the swap?The reason I am asking is that after reading you post on the gear swap you did, i am thinking about attempting it myself.I am trying to get together a parts list: 2 sets of fms gears, install kit or rebuild kit?, replacement fluid.Any special tools required?Also my truck is a 4x4 and would it hurt anything if I did the rear axle one weekend and the front the next weekend (as long as I don't use the 4x4 mode)Any helpfull hints are appreciated.

 

Lightningcrashz;
The new FMS gear set comes with crush ring, pinion nut and pinion shims. I did not replace the bearings. I did replace the pinion seal. I inspected all of the bearings to see if any required replacement but they were in good condition. Read the corral.net gear change information for a quick lesson on the role of the crush ring. It is very important.

In regards to doing the rears first and the fronts later, that won't hurt anything unless you engage 4WD.

I'm not sure if Ford sells gears for the front of your truck through Ford Motorsport. Look into that Good luck with your gear change and remember, like I said in my earlier post, to avoid surprises, make sure that you understand the whole job completely before you start!!!

 

The front differential is an 8.8 just like the rear so I am assuming that the same gears will fit.Maybe I am wrong.I haven't seen any gears labled specifically for the front 8.8 diff. on a 97 up f150.I plan on ordering up the Fordmotorsports 4.30 gear sets (so I can turn those 35s that are goin on after the lift).Before I order I will call Fms and find out for sure if the gears will fit in the front or not.Didn't GodogGo or someone just do this swap(except they used 4.10s) on their expy 4x4 (or was it a 4x2)?anyway my truck only has 20k on it so I doubt I will need to replace the bearings.Thanks for the tips and info.

 

The front and rear gears are totally different; the front ones are reverse cut.

 

well then,it looks like I need to see if I can get some front gears that are reverse cut.Maybe I will just end up going with 4.10s because I *know* I can get these for the front and rear (it is an available option right?) At any rate, after reading the install articles on coralle net I think maybe this is best left to a qualified installer.Does the hayes or chiltons service manuals have gear swap instructions in them? anyway thanks max &glenn

 

LightningcrashZ;
I can only speak for the Haynes manual for my truck, but it does not go inside the differential. If you have concerns about doing the change, have a shop do the work for you. GlennMc.