fast speedo question?

[FL.HBODY]

so i just got my truck on the road and the speedo is saying i am going way faster than i am. maybe 10 to15 over, it is a mechanical cable drive one and i know for a fact my 91 HB has a new style trans. now do i need to change the drive gear for speedo? or can i even do that? please any advise getting it normal will help. o yea and i have 15" 2000 frontier rims would that change speed? thanks...
Modified by FL.HBODY at 5:32 PM 8/17/2009

[RT22]

I had my needle break and had to replace. To get it back accurate I pulled the needle and replaced it and it was off. My 91 king cab pulls like 2750 rpm at 7-mph in top gear. Mine was off by ten but I knew the rpm and mph in top gear. get a buddy in a known accurate car to pace you at say 55mph see how far off and pull needle and adjust. remember when you pull it the dial rotates backwards. To adjust for that pull tip of needle back enough to clear the post where it sits at zero and let needle fall to full rest position. remopve needle and adjust accordingly. may take a few time. Just get the rpm in top gear at 55 from a pace car and set needle until it hits 55 at that rpm. Then get pace car and do 40 50 60 70 etc and make sure it is accurate. leave the clear cover off speedometer until done so it can be done fast and easy.

[RT22]

Oh and yes tire size and transmission tossed it off. But needle adjust may get it close enough through the range to be good to drive. Remember check 30 40 50 60 70 with pace car when done and make sure it is close enough you do not get a ticket

[MinisterofDOOM]

Speedometer inaccuracy is exponential, so simply moving the needle does not solve the problem. The faster you go, the greater the inaccuracy, so if you're reading 15mph high at 65mph (for example), just moving the needle 15mph down will throw off your readings BELOW freeway speed inversely (you'd be reading ~5mph LOW at 40mph and TEN miles per hour low at 25mph...again, just examples).

You need to correct the input signal, or find out the exact amount (percentage) of inaccuracy so you can calculate the new "correct" readings for your speedo. One way of doing that is to change the speedo gear, as you mentioned, but that's only really possible if there's a speedo gear out there that fits your exact discrepancy.

LARGER tires should not make the speedometer read HIGH, they should make it read low. Smaller than stock tires would make the speedo read high. So you much have another issue somewhere.

[RT22]

True if it is off by different speeds at different speeds. ???? it may be off by 10 or 15 all the way through the range. need to pace it with car with accurate speedo and find out. if not then you are correct my fix will not work. But if it is constant my fix will work. The fact he has larger tires and different transmission thows in a few variables. You are on the mark he first needs to find out just how much it is off and where.

[FL.HBODY]

yea it is off exactly 10 miles an hour every where. going 40 says 50 and going 20 says 30. you can see it when you going very slow off a start rises very rapidly to at an acurate speed.... ten over.

[FL.HBODY]

i am going to reasearch the speed sensor gear. i know the needle thing will work but my O/D will register me going 50 miles when in fact i went 40. what i think happened is the different transmission in car came with the new style electric sensor and the guy just put the old mechanical one in and that threw it off. saw in a previous thread two speed sensors next to each other and the mechanical gear was exactly 1/4 inch smaller than the electric one thinking that might be the problem but dont really know. makes sense i think. will let you guys know, might just need to switch gears if possible. thank you so much for info keep you posted... you can look at the thread its zer...ility
Modified by FL.HBODY at 4:22 PM 8/22/2009