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How far off will speedo be?

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Old 09-14-2009, 05:57 PM   #1
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Default How far off will speedo be?

I just put a new rear end in the car and the old one had 2.73 gears and the new one has 3.42 gears. Can anyone tell me how far my speedo will be off till i can get it tuned?

Thanks
Andy
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Old 09-14-2009, 06:00 PM   #2
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It's not a set number, more like a percentage. It's going to vary depending on your speed.
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Old 09-14-2009, 06:01 PM   #3
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its a percentage for sure! get a tune and fix your speedo
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Old 09-14-2009, 08:04 PM   #4
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im pretty sure for that gear change every 5mph it says you are going its really around 4. so when it says 75 your going 60...pretty sure thats close to exact.
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Old 09-14-2009, 08:07 PM   #5
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if your wondering how i got that, devide 342 into 273...you get 125....that means when it would say 125 your real speed is 100. its all relative to what speed you are going. so your speedometer is always going to read 125% of your actual speed.
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Old 09-14-2009, 09:15 PM   #6
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if your wondering how i got that, devide 342 into 273...you get 125....that means when it would say 125 your real speed is 100. its all relative to what speed you are going. so your speedometer is always going to read 125% of your actual speed.

Makes very good sense. Thanks


Well i would get a tune but im planning some more stuff for it soon and dont want to pay for several tunes as i go along. Plus there is no place close to me. Im thinking maybe buy a hp tuners or efi live and do it myself?

Andy
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Old 09-14-2009, 09:48 PM   #7
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yea most the handheld tuners will re-calibrate for you, around 300 bucks maybe, me personally, i would just leave the speedometer they way it is and keep up with the flow of traffic, saves money, if you get a tuner tho with most of them u will always be able to read codes, so i guess it just depends on weather or not thats worth it, hope that answeres your question.
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Old 09-15-2009, 02:26 AM   #8
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Have you got to test out the new 3.42s? Thinking of doing the same swap myself.
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Old 09-15-2009, 05:54 AM   #9
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Have you got to test out the new 3.42s? Thinking of doing the same swap myself.

Yes had it out last night and its a major improvement over the 2.73. Had i not of run onto such a deal on the complete rear end i bought with the 3.42 in it i probably would of went with 3.73 but i am very happy with what i did. The "fun factor" went up alot for sure.

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Old 09-15-2009, 09:16 AM   #10
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Cool, its the same deal with me ran into a complete rear minus backing plates for 200
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Old 09-15-2009, 10:17 AM   #11
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if your wondering how i got that, devide 342 into 273...you get 125....that means when it would say 125 your real speed is 100. its all relative to what speed you are going. so your speedometer is always going to read 125% of your actual speed.
100% correct.
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Old 09-15-2009, 10:41 AM   #12
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way off get it tuned
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Old 09-15-2009, 01:13 PM   #13
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Default shift points wrong as well

you will want to get it tuned. Your shift points will be off as well, so it will basically short shift all the time and you will be in high gear at a lower actual speed.
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Old 09-15-2009, 01:32 PM   #14
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you will want to get it tuned. Your shift points will be off as well, so it will basically short shift all the time and you will be in high gear at a lower actual speed.
x2...i went from 2.73s to 3.73s and at WOT it doesnt shift into 3rd unless i let off the gas
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Old 09-16-2009, 12:55 AM   #15
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if im not mistaken it wouldnt short shift it will not want to shift...if u gun it ull prolly hit the rev limiter, not possitive on that one maybe someone can correct me or agree
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Old 09-16-2009, 06:50 AM   #16
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you will want to get it tuned. Your shift points will be off as well, so it will basically short shift all the time and you will be in high gear at a lower actual speed.
Yes i have noticed this being true that the shift points are off now. I dont live closer to a tuner at all. Can a local GM dealer do this maybe?
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Old 09-17-2009, 03:09 PM   #17
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If you don't want to buy an HPtuners box (which is somewhat expensive for just tuning shift points) maybe you could do a mail order tune. I don't recall who does that here because I have never needed one, but I think there are some guys around here who could do it for you and probably only have you out of your car for a week or two. You would just need to tell them what gears you went from and to and it is an easy change in the software. I did it in hptuners and it is really easy, there is basically a built in algorithm that takes care of changing an necessary parameters.

Personally, i would say you should grab the HPtuners package if you plan on doing other stuff to your car, and I don't think that a GM dealer can change anything to a non-stock ratio (3.42 is stock for a manual trans but I don't know how GM's programming boxes work).

Good Luck.
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Old 09-17-2009, 05:42 PM   #18
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another thing you could do is jump on the regional section and see if anyone with hptuners in your area can do a quick adjustment. it will cost them 2 credits ($100) to tune your car + whatever their time is worth to do it for you.
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Old 09-17-2009, 11:07 PM   #19
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where are you in Illinois? if you are in the northern suburbs, we could maybe work something out since I get down to Kenosha relatively often. Also, what year and model is your car?

D
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Old 09-17-2009, 11:24 PM   #20
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where are you in Illinois? if you are in the northern suburbs, we could maybe work something out since I get down to Kenosha relatively often. Also, what year and model is your car?

D
I have been looking at hp tuners and thinking hard about buying one cause i will be doing more tuning in the future. Just not sure which one to get? I need to do some searches for those answeres on here i guess.

The car is a 98 z28.

I live near Macomb IL, about 250 miles south and west of chicago.
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