Armour All is another good thing to use sometimes because it will clear away some crud and NOT damage any plastic parts inside- it probably won't dissolve any rust, but neither will it contribute to it.
On the other hand WD 40 was actually designed for electronic uses, but will short circuit electronic things out if it isn't dry-
In June of '08 I lost almost everything in 7 feet of flood waters- it was all not just wet- CAKED in mud that would not easily dissolve- I had to throw caution to the wind to try to save some electronic stuff- tearing much down to circuit boards- and plain old fashioned Windex glass cleaner bought back several things- including a laptop that had been under water for several hours- of course you have to let it dry thoroghly oif there's power involved.
I also had to wash my entire CD and DVD collections like dishes in the sink- and the damage was surprisingly minimal- although I had to throw out ALL the cases and lables soaking wet with stinking moldy slime waters.
I'd clean the speedo first with armour all and the use mostly WD 40 in small amounts- it dissolves rust, and isn't too bad on plastic.
