whats wrong?

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Supporting Vendor
Feb 25, 2003
Seattle, WA
Can some one tell me why my speedometer says I am going 60mph when I am actually going 48mph. I own a 66 Fj40 stock drivetrain with 32" tires. I replaced the speedometer cable. Thought it might help but it is still the same. Any ideas? Thanks.
shouldn't I be going faster then. I used one of those rpm tables and it said at 2500rpms with 32" tires I should be going 60mph. Am i missing something.
No, I am. Sorry. I had the #s reversed. Hey, I've got it. All you need is a set of 38.5s and it should be pretty close.
check the drive gear on the t-case...it's plastic, and does wear out. Check the connection at the speedo as well, ensure it spins smoothly.
I will check out the drive gear. What about my speed though. Why am I going 48mph at 2500 rpms when the rpm table said i should be going 60mph?
isn't 32 stock size? i know the stock tyres on my 45 (the spare is from the factory) were those tyres with funny numbers, but they worked out about 32 inches. no idea what my 40 was though
are you sure you were using the correct diff ratio when you were filling out the table? In my 60 with 4.11's and 33's I'm probably doing 55 at 2500 rpm or so. with 32's I would Imagine it's not to far from 48.

out of curiousity What are you basing what your true speed of 48 mph on.
I now it's 48mph because i have seen the radar displays and my girlfriend driving in back of me said I was going less than 50mph. I did put in the right diff. ration of 4.11.
[quote author=bad_religion_au link=board=1;threadid=5167;start=msg40081#msg40081 date=1063246463]
isn't 32 stock size? i know the stock tyres on my 45 (the spare is from the factory) were those tyres with funny numbers, but they worked out about 32 inches. no idea what my 40 was though
Stock size on US 40 is a puny 29 inches.
ok then. i got no idea what we got
I didn't get my speed figured right until I calculated the actual speed by using the measured circumference of the tire by measuring how far it went in one revolution.
Just because a tire says it's a 31 or a 33 doesn't mean that exactly. It can mess up your calculation.
My speedo is within 2 mph from 10mph up to 90mph with my half worn out 31x10.50 tires.
all aussie cruisers and luxes got 7.50r16 as standard and wider metric (but same rolling diameter approximately) tyres for the newer high-spec models. The 7.50r16 came on the 5.5" rims and had a diameter a smidgen smaller than a 32". Prob similar to a 32x7.50r16. So 31x10.5r15 are smaller than stock ~!

At least you wont get a speeding ticket :)
Go looking for one of the roadside radar/sign units and drive by several times at varying speeds... record observed speed and speedo reading... see if you are always off by a set amount or a set percentage. If you have access to a gps you could also use that to verify speed. Are you sure your gf's speedo is right? My stock y2k dodge is off by 2-3mph at almost all speeds and this is not unusual for a new car... most are set to read a little fast.

Are you the original owner? If not, are you sure they are 4.11s? Did the PO have different tires (36-38s perhaps?) on in the past where s/he might have changed the drive-gear to correct their speedo error? or added an adapter on one end or the other for the same effect?

Do you have access to another gauge pod? If yes, swap them out and see if the problem persists. If it does, with numbers very close to the first set, then it is not likely to be the speedo itself. If the speeds are reflected more accurately w/ a different speedo, then it seems like an actual speedo problem... they can just go bad or could need a good internal cleaning and lubricating. If you don't have an extra one, maybe you can borrow one locally since they pretty easy to remove... might not be worth a shipping charge to borrow one.

I know this is a common problem on fj60/62s when the speedo head is at fault. It is, as I understand, likey a problem where dirt or contamination is interferring or partially coupling the rotating magnets causing a mechanical "boost" to the follower magnet attached to the needle. From a web site:


"Traditional automotive speedometers are driven by a flexible, sleeved cable that is rotated by a set of small gears in the tail shaft of a transmission. The speedometer itself is two rotating, barrel-shaped magnets. One magnet is connected the sleeved cable, and the other is connected to the speedometer needle. These magnets are calibrated such that a given revolution speed of the flexible cable corresponds to a specific speed indication on the speedometer."

If you are brave enough, disassemble the speedo head and clean. It may be that some bug has made a nest in there.

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