Speedometer not working after going to 16 x 7 from 15 x 6

[privatepang]

Hey guys
Recently upgraded by 15 x 6 rims to 16 x 7 all round on my 75 911s and now it seems my speedometer isnt working correctly. Would stay at zero then jump up to 30 mph and seem to climb as I go faster but it seems very gradual. Even if i hard brake, takes a bit of time to get back to zero.

Wondering if i knocked the sensor when I mounted the wheels or if its rubbing somewhere?

Did a search and read up about cleaning the magnets but i'm unsure if this is for g50 trannies only or if it relates to my car?

Any insight would be appreciated.

[twistoffat]

your speedo will need recalibration. Heres a thread with tips

Speedometer recalibration?

[schumicat]

first determine whether you have a mechanical/cable driven speedometer or the electronic one with magnetic sensor. I'm not sure when it changed but you may have the cable. Look at the back of your speedometer and see if there's a cable.

[ossiblue]

Your 75 should have a cable drive unless the tranny and speedo were changed out by a PO. Check the routing of the cable as it leaves the center tunnel (behind the drivers floor board) and moves up through the bulkhead into the trunk--it should be a smooth curve. Inside the trunk, look at the routing from the grommet to the speedo itself--again, a long, smooth curve. Finally, pull the speedo and check the connection to the back, make sure it is finger tight.

If all the above checks out, you may wish to disconnect the cable from the speedo and shoot some lube down the shaft. Also, you can use a variable speed drill motor with a small square bit and insert it into the speedo where the cable fits. Turn on the motor and see if the speedo moves smoothly. If not, you will need a refurbishing of the gauge.

Changing the wheels caused the cable to turn slower which may be the reason the irregularity is now showing up. The cable may be old and the lube inside a bit grungy, or the cable had a bit of a sharp bend, but the rapid rotation with the old diameter wheels was enough to override any symptoms. With the new wheels and slower rotation, the symptoms now appear.

[privatepang]

Ok thanks for the tips ! I'll give these steps a go. Fingers crossed

[donporfi]

What about a frayed cable ? Most probably the cable inside is frayed, the loose wires are slowing the response of the speedo because these small wires are rubing against the sheath acting as a damper. It jumps from zero to a speed because the cable is not moving until there is enough force to loosen the inside cable.
Also, what size tires were you using with the 15 inch wheels and what size tire do you use with the 16 inch wheels ? Without this data you cannot tell if the cable is rotating faster or slower.

[privatepang]

Its very similar in size as i went to a lower profile tire.

Think it was 195 x 60 x r15 to a 205 x 50 x r16

[donporfi]

The 15 inch tires have a theoretical diameter of 615mm, the 16 inch tires are a tad smaller at 611mm theoretical diameter. With the 16 inch tires the cable should run faster, meaning the speedo should show a faster speed.
But that was not your question, I again think the cable inside the sheath is frayed or lack lubrication or there is dirt inside or there is a sharp bend in it. Not related with the change of tire and wheel, to check if this is the problem, return to your old tires and wheels and see what happens.

[Joe Bob]

Get a GPS and check the read once you fix the cable. North Hollywood, Palo Alto or most any competent speedo shop will do a fix.

My SC has lower profile tires, a 964 engineg and 993 gauges. The speedo is dead nutz on w/o any mods.