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PittsburghCeli
12-12-2009, 12:14 AM
Does anyone know how to make a speedometer read correctly after putting on a bigger overall diameter wheel?

I am thinking... tear apart a tranny and putting something different on the timing or put a special conversion gear in between the speedometer cables.
Thanks

Shadowlife25
12-12-2009, 12:31 AM
Lower profile tire.

1" rim increase along with a 50 series tire usually results in a 3mph UNDER reading.
So, if your speedo reads 30mph, you are actually doing 33mph.

Luni
12-12-2009, 02:01 AM
The equation isnt linear.

Its logarithmic.

If youre off 3mph at 30, youll be off more by 60. Even more at 80.

It isnt a strict percentage equation. And if youre mechanically driven, you cant change it. If youre electronically driven, they make boxes that will perform the conversion and redo the speed signal for the correct size.

jsooke
12-12-2009, 02:34 AM
GPS unit may be the cheapest way ....

rizin
12-12-2009, 02:54 AM
If it is cable driven I know that you could get gear drives that would help with bigger tires it goes between the trans and gadge. Old school stuff that I have seen it on.

lalojamesliz
12-12-2009, 03:20 AM
i always liked having this problem because the odometer also reads less :) just get a good GPS and compare the speeds you usually drive to avoid speeding tickets

Luni
12-12-2009, 06:47 AM
Ok, good luck finding something that will fit in the tranny and convert a mechanical signal. If you do, post it up

PittsburghCeli
12-12-2009, 03:55 PM
Would need some kind of electronic speed sensor extension to connect to the existing computer speed sensor connection, on top of the tranny near the cable. Then run it to a new digital speedometer.
Maybe get another stock speed sensor (http://www.autozone.com/autozone/catalog/parts/partsProduct.jsp?skuDescription=Duralast+/+Vehicle+Speed+Sensor&categoryDisplayName=Engine+Management&fromString=search&itemIdentifier=910204_0_0_3498&productId=910204&sortType=&parentId=59-0&filterByKeyWord=electronic+speed+sensor&isSearchByPartNumber=false&navValue=15900430&categoryNValue=15999999&fromWhere=&itemId=430-0&displayName=Vehicle+Speed+Sensor&store=5296&searchText=electronic+speed+sensor&brandName=Duralast) and piggyback it onto the existing one, because that would not interfere with the computer signal, opposed to splicing into the wires and trying to run a speedometer off that.

Murgatroy
12-12-2009, 04:34 PM
If it is cable driven I know that you could get gear drives that would help with bigger tires it goes between the trans and gadge. Old school stuff that I have seen it on.
In the oldschool world (read GM TH350/400/Powerglide and other assorted) you can get gears tailored to match your final drive ratio (which also changes it) and your tire ratio.

I have never seen something for an import transaxle to correct it. The best way (if it is a concern) is to either get wheels of the exact same diameter or a GPS.

For the record, a smaller diameter wheel will register you going faster than you are, as there are more rotations per mile; where a larger diameter wheel will show you going slower, as there are less rotations per mile.

Murgatroy
12-12-2009, 04:37 PM
Another gig, if you use the wheel calculator, it will show you the percentage of the size and speedo variance at a set MPH, I think it is 50MPH or 60MPH, I can't recall. As Luni pointed out, it is not set, it is more of a percentage. If it is 2% off, at 30MPH, it will show a variance of 6MPH, at 60MPH, the variance will be off by 12MPH. The faster you go, the more inaccurate it is.

And another note, factory speedos are often purposely set off. Which means your factory ride lists you at 55MPH, but you are actually traveling at 52MPH. I think it was a safety issue.

If you want to be perfectly correct, get a GPS.

Carolina91GT-S
12-12-2009, 07:44 PM
I would assume you can make an adjustment electrically if you used the JDM electric speedo sensor and a JDM cluster. I have one of each in my storage unit but haven't gotten around to trying it yet. I plan to soon since my cluster speedo died recently.

Anyone tried it?

by the way changing the ring gear inside your XMSN just to tweak the speedo reading seems a bit extreme to me.

klapa
12-13-2009, 01:15 AM
I had thought about this one myself - and looked into it a little bit.

For the cars I have experience with - Mopar - there was always the capability to change only the speedometer pinion gear in the transmission - be it automatic or manual.

This is a worm gear driven by the transmission output shaft in most cases that drives the speedo cable. On the cars I'm familiar with, this gear was easy to replace.

I looked in he BGB on this site when I was considering going from 14" to 15" wheels on m '92 STX, but didn't find much. What is needed is a parts book. The key would be to look at various combination of Toyota cars with the same transmission as you have - but different final drive ratios and/or wheel sizes. Each should have a different part number for the speedo pinion - or "vehicle speed sensor gear" - as is listed in my Corolla Haynes manual.

It would stand to reason that such different speedo pinions would be available - as the same transmissions are used in not only Celica's, but also Corolla's in many cases - all having different gear ratios, final drive ratios, and wheel sizes. Yet looking at various part dealers around - most only offer transaxles - I've not found any with individual transmission parts.

For myself - I didn't pursue this very far - as after reading around here I decided it is better to find a GT as a project car than to try and make an ST to be a GT.

METDeath
12-13-2009, 04:55 AM
Or find a place that has a gov't certified dyno for speedo accuracy, they can usually make you a correct reading speedo with the parts. Won't be cheap though...

Murgatroy
12-13-2009, 05:07 AM
I had thought about this one myself - and looked into it a little bit.

For the cars I have experience with - Mopar - there was always the capability to change only the speedometer pinion gear in the transmission - be it automatic or manual.

This is a worm gear driven by the transmission output shaft in most cases that drives the speedo cable. On the cars I'm familiar with, this gear was easy to replace.

I looked in he BGB on this site when I was considering going from 14" to 15" wheels on m '92 STX, but didn't find much. What is needed is a parts book. The key would be to look at various combination of Toyota cars with the same transmission as you have - but different final drive ratios and/or wheel sizes. Each should have a different part number for the speedo pinion - or "vehicle speed sensor gear" - as is listed in my Corolla Haynes manual.

It would stand to reason that such different speedo pinions would be available - as the same transmissions are used in not only Celica's, but also Corolla's in many cases - all having different gear ratios, final drive ratios, and wheel sizes. Yet looking at various part dealers around - most only offer transaxles - I've not found any with individual transmission parts.

For myself - I didn't pursue this very far - as after reading around here I decided it is better to find a GT as a project car than to try and make an ST to be a GT.

As long as you stay with the same diameter wheel tire combo, you won't throw it too far off.

I run a 17" rim on my ST.

And for the record, the ST is the best performance platform other than the ST185.

85gtsblackman
12-13-2009, 05:34 AM
my gts is better :wiggle:

my st is lighter, and has more power than the 5th gen st :wiggle:

:D