91 with rearend change, now speedometer is off.

[mjs2011]

Ive got a 91 7.3 NA with a zf 5spd, it originally had a 4.10 rear end, and since I don't tow heavy with it, i wanted to swap the rear end to gain some mpgs and speed on the highway. My dad had 3.55 put in it, and when he got it back, he did an rpm check. The numbers he reported to me were the exact same as with the 4.10. 2200rpm at 65 and 2500 rpm at 75. I didnt think that sounded right for the 3.55, so today he went back out and checked with gps. GPS speed at 65, speedo at 58, and 1800 RPM. GPS Speed at 75, speedo at 66 and RPM at 2100. Good those are the RPMs I wanted to see at those speeds.

(Funny that his initial check of 75mph at the speedo was actually close to 90 mph with the old girl, on a 65 mph highway!!).

So anyways, is there a way to hone in the speedometer so that the reading is accurate?

 

[sle2115]

Mine is only an 87, but I had to change the speedo gear, easy to do, finding the speedo gear, not so easy...local stealership said only 3.55 and 4.10's were put in, not a problem you say, you have 3.55's...well, my local said they were no longer available. Again, this is for an 87...not sure about your 91. Try to find one for a 3.73!

 

[460mudsports]

If you can't find the correct transmission speedometer gear, you can always get a gear adapter made that goes in the speedometer cable. GM (Chevy, GMC) used gear adapters on some of their pickups in the 70's and 80's. The GM adapters connected to the transmission and then the cable connected to the adapter.

Abbott Tachograph in Pine Bluff, AR still makes these adapters, geared to your specs (they can help you determine the gear ratio needed). To put one of this style adapter on a Ford, you have to put it inline in the cable. AT makes new cables with Ford connections on both ends and GM connectors in the middle for the adapter -- you basically get 2 cables connected with the adapter. AT's website does not show this adapter anymore, but it is mentioned. They also have adapters for electronic speedometers and does repairs on any speedometers. They are only about 60 from me and I have had them make a couple of these adapters for my older GM cars and trucks and they rebuilt a 100mph speedometer from a 74ish F150 for me to go in my 79 F150. I haven't done any business with them in about 10 years, but I believe they are still a small family owned business and trustworthy.
Hope this helps.

 

[fordf350man]

here is a place that sells cables and gears for the speedo, i have the page up so it will take you right to it

http://broncograveyard.com/bronco/c-843

 

[Shadetreemechanic]

The dealer can also help you figure out which gear. Make sure you pull your current one and take it with you. Speedo gears at my local dealership are about $6 a piece, or were last year when I was trying to get mine right.

 

[redneckaggie]

I know I also saw different speedo gears for our trucks in lmc last night when I was finishing up my parts list

 

[sle2115]

Need one for a 1345 Tcase with 3.73 gears...never did find one, just never took the time!

 

[mjs2011]

I think I'll take mine in to the dealership and see if they can get me another one. Are they hard to get at? Any tips for removal?