speedo question

[dimitri]

i was running the monte last night and had my friend use his smart phone to make sure my speedo was accurate and i found out that at 70 mph my speedo reads 62-63

the only difference on the car is the tranny (had a th350 and now a th400 )

what can i do to correct this (annoying to have to calculate in my head on how fast im going)

[Slaman]

Sounds like you need to change the gears for the speedo. These are located where the speedo cable is attached to the tranny. I've never done it yet but understand it is not too hard.

[VE3HZZ]

Before the change over of the transmission did you check this then and it was accurate and used a GPS to verify this. Remember that even GPS is not going to give you 100% accuracy on your speed/distance calculations all the time. There is an error correction of about 5%

So even though there is a different gear for the transmission to solve speedo errors first before you go and replace it need to know the rear end gear ratio, factor in the tire size and diameter that you have installed over stock to get what error correction is needed. I would also if possible check this speed with a dedicated GPS unit not just an APP on a smart phone (IMO) and check again to ensure of the amount of error.

 

Also remember FROM factory the speedo could be off as well as there is an allowable variance and most (even today) speedo's are slightly off in the lower end meaning if your car speedo says 60 MPH it could be only 56 etc. Manufacturer's normally error on the LOW side to avoid any possible suits over someone pointing to them for why someone got a ticket etc.

 

In newer electronic vehicles they have SPEED healers to compensate for changes to tire size etc now that in essence does the same as changing the speedo cable gear in older cable driven cars.

 

Hope that helps.

 

Good luck with the update I know it can be annoying figuring out how fast your going.

 

But think of this, on the LOW side if your cruising down the road at or just slightly above the speed limit can't get pinched for speeding

[Chris]

Depends on the rear axle ratio and whether overall tire diameter has changed from factory tire height.

 

Take a look at this chart to give you an idea.

 

http://www.teufert.net/speedo/69-gtogp-400.htm

[geartech]

An accurate way to check it is, to have wristwatch with a second hand and hold the car at 60 on the speedo as best you can on the interstate, in a section that isn't very busy. When you hit a mile marker, note your watch and where the second hand is. When you hit the next mile marker and you've been holding it at 60, it should be 60 seconds later. (60 mph = 1 mile per minute)

Each change of the speedo gear teeth, one tooth difference results in approx. 4 mph change on the speedo (at 60mph) in my experience.

[LostnFound]

Speedo? Rule of thumb....Don't wear one!

[dimitri]

thanks everyone this did help me

and lostnfound you definately put a smile to my day

[LostnFound]

[RickWillson]

EEEEWWWW a speedo?