My '88 CRX DX has a brand new clutch in it and it runs and works perfectly. Doesn't slip at all, doesn't chatter, or do anything else unexpected.

But in the mornings when I drive to work (slightly uphill on the highway), I'm running 3500 rpm at 70 mph according to the cluster, and in the evening when I come home (slightly downhill on the highway), I'm running about 3250 rpm at 70 mph according to the cluster.... This pattern is so rock solid reliable that I'm at a loss to explain it.

Has anybody run into this? I'm thinking that either the speedo or tach are sensitive to temperature or angle (uphill vs downhill). I lubed the speedo cable not too long ago so it shouldn't be gummy, and the tach is electronic isn't it?

Anybody have any ideas?

 

Do you go home with the headlights / running lights on? The tach is electrically driven, and electrical loads can affect its readings.

 

The stock guage cluster is notoriously inaccurate in terms of precision. if you want really accurate readings, you need an aftermarket tach or speedometer.

The tach is driven off a sensor in the distributor, and the speedometer is driven off a gear in the transmission and mechanical cable that is then converted to an electronic signal in the gauge cluster and sent to the ecu.

as far as what you are seeing, i would say the real issue is the lack of precision of the instrument to perform the measurement. it would be impossible for the transmission with clutch engaged and working properly to spin faster (or slower) than the engine when the engine is at a given rpm in the same gear, all other variables being equal.

Finally, my only guess given the info is your tire pressure is a touch higher in the afternoon when you drive home, causing the radius of the tire to be slightly higher, thus reducing your rpm. I would say you could try the 100% nitrogen tire fill to see if that changes anything, but that would almost be a waste of money given that normal air is nearly 70% nitrogen anyway, and your tires should not be getting THAT hot on the highway.

 

HF2ZC, I usually go home around 5:30-6pm. No headlights this time of year. The only differences I can identify are time of day, temp, and angle of the road (I gain/lose about 200 ft in 6 miles of driving).

sanimalp, If the cluster is that innacurate, then it's among the worst I've ever seen. As far as I know, it hasn't been acceptable to have 7-10% error in auto instruments since the 60's and 70's. Are the clusters really that bad? I'd expect the results to be less precise and repeatable if it was just a matter of poor quality instruments. This is exactly the same day-in and day-out.

None of my other cars have this problem, so that kind of eliminates the tire pressure idea since they'd all suffer the same effect.