The Stovebolt Site Forums The Shop Area Driveline Re calibration of speedo after diff change to 3.55

Did the diff change and what a great difference but now of course the speedo is off....I'm going 58 when it says 50 on the speedo. I had planned to change the gear on the end of the cable that goes into the tranny and my OEM one has 12 teeth ... Lady at the speedo place says she's never seen on with only 12...most are 18 or more.

So...what's the usual amount of teeth on this gear and what do I need to change to.

I did contact two places that sell adapters and both are sold out with no forecast on time they get them in.

Thanks for what ever help you all can provide...

Mark

** Have you contacted Jim Carter?? They advertise they can build what ever adapter for speed correction you need. All you have to do is tell them what your speedometer is reading at 50 miles per hour. **

Mark,

You should consider buying an adapter that matches your needs (different rear-end and most likely different tire diameter).

As marzach posted, it looks Jim Carter now make an adapter to match specific needs. The Speedometer Shop will also make an adapter to match your set-up.

I'm just beginning to understand this stuff...drive gear, driven gear and all that. I talked to a guy at the Speedo Shop today and he wants me to test at two more speeds and call him back... Yes, my tires/wheels are different too.

Thanks for the replies

Testing at two more different speeds is an excellent idea. That will give you an idea of how the error is distributed.

Use a Garmin or a TomTom GPS and record the speedo reading at 10 MPH increments from 20 to 60 MPH. That should give the speedo shop plenty of info to work with. When I switched from 12" wheels to 14" on the GEO Metro I drive about 1200 miles a week, the change was about 10% over the entire speed range and the odometer readings. I'm indicating a pretty consistent 10% slower on the speedo than the GPS. The error increases as the speed goes up.
Jerry

Hi Jerry,
I have tested it with my GPS Speed app on iPhone and against the radar speed signs on our local high ways... both agree so the iPhone app seems to be correct. I also tested in in my Jeep and it's correct. I'll get several more readings today and call them in. The shop that's worked on my speedo is a block away from Bob's Classic Chevy...they know what to do.. They build the adaptors for them. Best to have several readings, I agree. Maybe it's Jim Carter...I forget

Ok...got numbers from 30mpg thru 60. Problem is (arrrrgh) you can never reach the Metro Speedo place by phone and they don't have an email addy so I can give them the numbers. Just emailed Jim Carter and gave him the numbers so maybe he can walk up the block and tell them If this gets much more difficult I'll just call Speedometer Shop....they seem to know how to get this done and they always answer the darn phone

Mark,
Call Jim at PATC (Performance Automotive and Transmission Center) 318-742-7754.

I called him and he had me go out, using either a GPS or an iphone App and note the speeds at 30, 40, 50 & 60 of both the truck's speedo and the GPS. According to my iphone ap, the truck was reading about 21% low, he had a 20% speed up adapter in stock. Close enough for me. Works perfectly and very accurate. A whopping $62 smackers. Takes PayPal or credit card.

Thanks for the "review" and the contact information, Craig.

The one one I got from the Speedometer Shop was more "accurate" (1% error vs your 5% error), but it cost double what yours cost. Your unit would also be "Close enough for me."

Been off the forum for a bit and STILL dealing with this speedo issue. It seems ? to be about 16% off from the diff change to 3.55. I started with Jim Carter and they had me send my speedo in to the shop down the street from them to calibrate...they ended up rebuilding it. Then told me to get some numbers from Speedo and compare to GPS or radar. Ok..16%. They built an adapter but once installed it was still wrong, plus they seem to be going out of business, never answer their phone, etc. So I'm PO'd and call The Speedometer Shop. Fine, they say send it to us and we can fix it.

That was three or for weeks ago and I still don't have it..but they say it's done. I guess the old guy there, Rick, has been going thru cancer treatment so I do feel sad for him but no idea when I'll get my part back... This whole issue has been going on for months. Jim Carter gave me $70 store credit but not a refund which would have been more fair.

Advice: Don't use Jim Carter for this or Metro Speedometer in the same town has Carter... Also, good luck with The Speedo Shop.... Maybe there are other/better places.

Mark

Have you tried calling PATRICK? He may be able to help. He has speedometer gears and adapters listed on page 13.

I tried Patrick a LONG time ago and they said they'd put me on a wait list until they got 70 orders.... That was back in the Fall of last year. Never heard from them. Forget them

Check here, don't think this was mentioned in any of the above posts. http://www.partshp.com/speedometer_gears.htm Select ratio adapters (mid to lower left of the homepage) You can order a stock adapter or special order. Haven't used these guys but will soon as I recently completed a 5:14 to 4:11 swap.

I actually used a different technique to figure out the conversion on my '46 1/2 ton with 3.55 ring and pinion and 16 x 7.00 tires. I marked an exact 50 mile run using my car that has an accurate speedometer/odometer. Ran the truck on the same route and noted the mileage recorded. Did the math and got a converter made (many years ago) based on that info. Worked just fine and is accurate.

David,
Sounds a bit like overkill to me but, hey, it worked. It's all percentages I guess so how ever you determine the error is fine.

I am, BTW, STILL waiting for the ratio adapter I sent to The Speedometer Shop. They don't have a working email but do have a phone recorder. If they ever listen to the messages is still a mystery sometimes. The first shop, that did it backwards, didn't ever return calls or have email either.

I did manage to talk to a guy yesterday at the Speedo Shop in SoCal. One of them is going in for cancer surgery today. Good luck to him..I hope he does well. The other guy is, I guess, trying to run the place by himself. My probs seem pretty insignificant in comparison...

Overkill maybe, but a good excuse for a nice drive.

True about the health issues - kind of puts things in perspective. That said, I can understand your frustration.

Just got a call from this gent. He got good news on his Cancer scare and sounds VERY relieved. Turns out he's 81 years old has five dogs and a mentally retarded son he takes care of.

Sending out my ratio adapter today... I'll be happy to finally have it but more happy for him... He must have been on heavy drugs or chemo when I first spoke to him...he sounded more than a bit toasted. Now he sounds really good. Of course I had no idea until much later about his life trials

Mark