Speedometer question

[Yooper truck]

Last weekend I swapped out my speedometer with another factory speedometer. It had been making grinding noises behind the gauge cluster and I had already changed the speedometer cable.

The newly swapped in speedometer took care of the grinding noise, but now is off by about 5 mpg. It is reading 50 mph when I am going 55 mph which I confirmed by GPS and timing between miles. It was just about dead on before the former speedometer started acting up.

I have looked up several posts and everyone references the nylon gear on the transmission end to change the speedometer reading. I didn't change that, all I did was took off the old cable and put on a new one and then swapped out the speedometer.

The new (used) speedometer came out of a 1972 four wheel drive suburban and I have a 1971 2 wheel drive. Is there a difference on the speedometer end between 2 and 4 wheel drive?

Anybody have any thoughts on this?

[Aussie Ol' Blue]

Just a thought, and I am by no means an expert but I would be thinking that the problem would be in your new speedo. My first thought is, aren't the tyres on a 4 x 4 bigger than a 4 x 2? This would be enough to make a difference.

[AirSpeed]

I would think it's the gear since that speedo came out of a 4X4, I've seen them for sale in LMC and Classic Parts, looks like a cheap and way fix.

[geezer#99]

You need to change the driven gear.
Look here.
http://www.transmissioncenter.net/sp...n_______va.htm

[Yooper truck]

Thanks for the feedback..... Sorry to beat a dead horse, but I am still puzzled.

I understand there can be a difference in drive ratios to the transmission to accommodate rear gears and tire sizes. I also understand that adjustments can be made using a different speedometer gear where the speedometer cable joins the transmission. However, I did not change the speedometer gear and I am getting a different speed reading now.

So that leaves me wondering if the actual speedometer unit is calibrated different? (however looking at new speedometers from vendors there appears to be only one gauge option)

I am thinking if GM can calibrate the speedometer using a speedometer gear at the transmission, why would they need a speedometer unit that is calibrated differently at the gauge? Logically it makes no sense.

If they did not calibrate the gauges differently, then why am I getting a 5 mph difference when I changed the gauge but did not change the speedometer gear?

I'm just trying to figure this out before I throw money at something that may not be the issue.

[AirSpeed]

Took me a minute but I totally see what you're saying. I don't know why that didn't occur to me. Maybe the speedos were calibrated different then but new vendors sell just one calibration and sells gears to make them correct for each trans?

[Johnnyhotrod]

I only see 2 possible answers, 1) the original meter would also be +5 if it was in good working order or 2) the 2nd meter has something wrong with it causing it to read +5. One of the things you can do when troubleshooting is "insert known good part". Neither of your meters seem to fit the bill. You can try and confirm your reading with yet another "known good" meter to see if it matches or you can just go with it and tune the trans gears to correct the +5...