speedo issues

[johnscf]

My speedo is reading way high, says I’m doing 80 when I’m really at 60. This is a new to me 67.5 1600 with a 2liter and 5 speed swap. Tires are 195/55/15 which I would think would make it be off in the other direction and read lower than actual. Also at times needle is pretty bouncy especially at low speeds. Thoughts?

[spl310]

Have the speedometer rebuilt, and get a different drive pinion gear

[Habitat.pat]

Could be several things. The speedo cable needs lubricant causing the bounce, bearings in the speedo head are worn (try lubricating then have it repaired). Once you get it working at low speed if it still reads high at high speeds, different drive gears are available.

Peace,
Pat

[johnscf]

Have the speedometer rebuilt, and get a different drive pinion gear

Speedometer was recently rebuilt while with the prior owner. 5 speed is said to be from a 69

[Bwk2000]

My $0.02 … You probably still have the stock 1600 3.90:1 ratio diff in there. Essentially, you will likely have to swap out the driven gear pinion (blue or yellow) with one that has more teeth (red or purple) and modify the carrier a bit to properly calibrate the speedo (assuming it has not already been done and this is not just a binding/lubrication issue as already mentioned).


Here’s the wiki write up:
http://www.311s.org/pmwiki-311/pmwiki.p ... inionGears

[rwmann]

Red 20T pinion, unless the tire diameter is smaller than stock. Purple 23T pinion will be a tough find.

A U20 / 5-speed with 3.90 rear end ratio is a peppy combination.

[redroadster]

If the pinion is tight to the ring gear, it will read high too
I always gently see where that is and back off 1/8 "

[spl310]

If the speedo was rebuilt, check the cable routing. It needs to have gentle bends. If it has tight bends, that can result in the jumping. The info above should help you with the accuracy.

[Gregs672000]

I think the speedos were created to reflect the "fun factor"... at 60 you feel like you're doing 80!

I have a Mazda LDS with a 3.9. The speedo is pretty accurate under 50 but creeps up and off the higher I go to about 7-10mph off... kinda hard to tell as it naturally jumps with bumps too, so sometimes I'm going 70 then 85, then... Stock, 3200rpms in 5th gear was 70mph, now it's about 3500. To get it right requires the largest speedo gear and a special adapter. I gave up.

"Mr. Scott, warp 9... or so"

[rwmann]

The 20T and larger pinions (larger diameter) require a differently clocked carrier (part that the pinion is pressed into the end of and is retained by the roll pin).

[johnscf]

The 20T and larger pinions (larger diameter) require a differently clocked carrier (part that the pinion is pressed into the end of and is retained by the roll pin).

I’m going to try the 19T which I think was stock for the 3.9

[Bwk2000]

The 20T and larger pinions (larger diameter) require a differently clocked carrier (part that the pinion is pressed into the end of and is retained by the roll pin).

I’m going to try the 19T which I think was stock for the 3.9

The rebuild of the speedometer can also introduce accuracy/calibration errors in addition to custom tire size and wrong pinion for your diff ratio.

What I recommend is this:
Take the car on the fwy and have a buddy drive exactly 50 mph. Pull up beside him, match his speed and record what your speedo says. Now, as a general rule, going up 1-tooth on the pinion REDUCES your displayed speed by 5% (2.5 mph) @ 50 mph

So, if yours reads 55 mph when you’re cruising beside him, you need to pick a pinion that has TWO more teeth than whatever you have in there now. Once you pull it out and count them, you’ll know what new pinion you need.

[mraitch]

check the tech wiki - there are two sections devoted to this. You likely can't find the ROADSTER part number, but later cars can be used but have to ba careful with the clocking of the carrier.

[DAC21]

The 20T and larger pinions (larger diameter) require a differently clocked carrier (part that the pinion is pressed into the end of and is retained by the roll pin).

I’m going to try the 19T which I think was stock for the 3.9

The rebuild of the speedometer can also introduce accuracy/calibration errors in addition to custom tire size and wrong pinion for your diff ratio.

What I recommend is this:
Take the car on the fwy and have a buddy drive exactly 50 mph. Pull up beside him, match his speed and record what your speedo says. Now, as a general rule, going up 1-tooth on the pinion REDUCES your displayed speed by 5% (2.5 mph) @ 50 mph

So, if yours reads 55 mph when you’re cruising beside him, you need to pick a pinion that has TWO more teeth than whatever you have in there now. Once you pull it out and count them, you’ll know what new pinion you need.

Or if you don't already have one, borrow a Car GPS. They are accurate with the MPH reading. No buddy required.

[Bwk2000]

+1 What he said