Speedo Correction (1 Viewer)

[cxh10]

I've recently confirmed what I already suspected, that my speedometer is off by about 5mph on my 82 which has some 33's on it. I read a few old threads but it seemed like no two people could agree on the best route to take, I was hoping since they were old threads maybe some people have actually tried these ideas by now and could give me some good advice.

 

[rjcruiser]

Just drive 5mph slower

 

[wantatlc]

Just drive 5mph slower

that is the cheap and easy and effective solution (and I use it daily)

the expensive solution - get a speedo made for you with the correct gearing on it

 

[Fast Eddy]

I've recently confirmed what I already suspected, that my speedometer is off by about 5mph on my 82 which has some 33's on it.

It can't be off by some mph. It has to be off by a percentage. Your 33s probably measure 32. It originally came with 28s, so it's probably off by around 4/28 or 14%. That would mean it says 35 when you're going 40, and 70 when you're going 80 (as if...).

 

[Poser]

It can't be off by some mph. It has to be off by a percentage. Your 33s probably measure 32. It originally came with 28s, so it's probably off by around 4/28 or 14%. That would mean it says 35 when you're going 40, and 70 when you're going 80 (as if...).

If indicated speed on the speedometer is 55 and you are really doing 60, wouldn’t the speedometer be off by five MPH?

 

[DiHrdBowTi]

I talked to a speedo shop here in Reno and they suggested an interface/conversion box. They calibrate it to your tires. If you change tire size they can change the gears..

 

[numtel]

Just disconnect the speedo or take it out. I haven't had a speedo in my cruiser ever and am okay with it.

 

[wantatlc]

Just disconnect the speedo or take it out. I haven't had a speedo in my cruiser ever and am okay with it.

but the DMV may not be down with that

 

[ar2stp48]

Several years ago after attracting a little blue light attention, I checked my actual speed against what seedo showed. A few trips past a radar unit showed speedo was off by 5 mph at both 55mph and 60 mph.
Drove 5mph slower than any posted limit from then on and never had any more trouble. Quick, simple and cheap.

cxh10---good to see another arkie; I'm in Magnolia area in sw corner of the state

 

[IDave]

I agree with fast eddy. It's an algebra problem. Ratio of the actual tire size vs. the original tire size multiplied by the registered speed equals the actual speed. At some point, yeah, that's 5 miles faster than the registered speed, but at slower speeds it's 2 mph and at some faster speed it's 10 mph.

I know that toyota made various different speedo gears (that go into the insertion of the speedo cable into the transfer case): 16, 17, 18, 19tooth gears and a couple of different diameters of speedometer driven gears to mate with two different speedometer drive gears 6x18 and 6x17. So, figure out what you need: they are available from SOR and probably Toyota. These are the part numbers and the corresponding ratios.

 

[Martin White]

There are lots of GPS units now that read out speed, and which are quite inexpensive. So you can either calibrate your speedo or just carry one of these units with you and read the speed off the GPS.

 

[fj40forlife]

I used a gps before. but then again it say the fast I gone was 210


Eric

 

[dgangle]

IDave, You're the man. many moon ago I read some of this info on the LCML but never saw this table before.

With 33's on my 75, my speedo was off about the same as the above example. I put in the smaller 16 tooth driven gear. The speedo was now considerably more accurate as verified with a GPS. The problem was that the driven gear was slightly smaller in OD than the 17 it replaced so it lasted all but 30 minutes before the drive gear wore the top of the driven gear's plastic teeth off and then it would not engage. Moral to the story? Gotta do both the drive and driven gears together. I'll have to check their availability w/ CDan and report back.

 

[IDave]

The problem was that the driven gear was slightly smaller in OD than the 17 it replaced so it lasted all but 30 minutes before the drive gear wore the top of the driven gear's plastic teeth off and then it would not engage. Moral to the story? Gotta do both the drive and driven gears together.

BTDT

 

[IDave]

Just for kicks and giggles, I've looked up the same stuff in a later Body and Chassis manual (unfortunately, they don't seem to have this info in the 1970s manuals, maybe someone could check for the 2F stuff.)

Anyway, in short, assuming a 4.11 diff gear ratio it appears you want a smaller gear ratio the larger the tire diameter (makes sense). I've listed the actual diameters (as best as I can determine from tables) in pencil to the left of the tire sizes from the table in the lower figure. They are recommending ratio of 3.0 for a 28 inch tire, and 2.83 for 29 to 30.8 inch tires. I would extrapolate and say that you'd want 2.67 for anything bigger, and beyond 33 or 34 you'll lose accuracy again.

Note the sidebar says you can only match 6x18 drive with 6x18 driven. But that would be for the 28 inch standard FJ40 tires most of our trucks came with (FJ45 lpbs came with 31 inch tires according to my owner's manual). In other words, for most bigger tires we need the 6x17 drive gear that didn't come with our truck as well as one of the fewer teeth driven gears.

I would match a 33403-60030 with a 33481-60030 for a ratio of 16/6 or 2.67 for 33 inch tires.

 

[Doug_S]

I use a GPS and feel very comfortable in traffic. A relatively cheap solution to the issue of 33x9.5 tires that I have on my rig.

 

[dgangle]

Well, I understand the #33481-60030 to be discountinued so never mind. I guess a GPS and a conservative right foot are the answer.

Here's another relevant past thread FWIIW:
https://forum.ih8mud.com/showthread.php?p=1530188&mode=threaded

 

[Klunky Chris]

what if you can't read your GPS due to windshield vibration at speed?????

not too practical to constantly hold it while driving......
(what I had to do )

 

[rjcruiser]

Okay...Okay....you guys are spending way too much time on this issue

Instead of replacing gears that are no longer available, why don't you just paint a new set of numbers on the dial. A little white out and a sharpie should do

 

[Suarez0523]

Okay...Okay....you guys are spending way too much time on this issue

Instead of replacing gears that are no longer available, why don't you just paint a new set of numbers on the dial. A little white out and a sharpie should do

Genious