Topic: Speedometer Gears

[Jeffy]

I'm just using the gut feeling calculator since I've done this a few times.    About 4mph off with either pinion gear at 65mph is about right.  So that should mean 39T would get me within 1mph or better.  I don't really feel like wiring another box under the hood when I can just swap a gear in 1-2 minutes.  My stock tires were 205.75R15's not the 215.75R15's.  205's were the stock tires and most 2.5's had them with the option for 215's.  Over the years people probably end up with 215's eventually as that's the base tire for the 4.0L's.  The optional tire was the 225.75R15's.

Funny thing about these gears is that they are the same ones that are used in cars going back into the 60's.

[sharpxmen]

in that case

205/75R15 = 27''
at 65mph with 33'' tires and stock 4.10 gears, 41T speedo gear = 53.2mph  

at 65mph 33'' tires with 4.56 ratio
 > 41T = 59.14 mph
 > 37T = 65.54 mph  

doesn't get any better than that i would think

unless the tire size is 28'' and not 27'' (which is more on the lines with your calculations).

and just for the record
at 65mph 33'' tires and 4.88 ratio and stock speedo gear (considering the stock tires 27'') = 63.3 mph speedo reading
EDIT: if the stock tires are 28'' then the same setup would display 65.64mph on the speedo.

[Jeffy]

Nope, 37T was off by almost the same amount as the 41T I have in there now.  I ran the 37T with my 35's and was pretty much 'on'.

[sharpxmen]

Nope, 37T was off by almost the same amount as the 41T I have in there now.  I ran the 37T with my 35's and was pretty much 'on'.

for 35s vs 27 stock
true speed / tire / R&P ratio / speedo gear / indicated speed
65mph      / 27'' / 4.10:1     / 41T            / 65.0 mph
65mph      / 35'' / 4.10:1     / 41T            / 50.1 mph
65mph      / 35'' / 4.56:1     / 41T            / 55.8 mph
65mph      / 35  / 4.56:1     / 37T            / 61.8 mph

35 vs 28 stock
65mph      / 28'' / 4.10:1     / 41T            / 65.0 mph
65mph      / 35'' / 4.10:1     / 41T            / 52.0 mph
65mph      / 35'' / 4.56:1     / 41T            / 57.8 mph
65mph      / 35  / 4.56:1     / 37T            / 64.1 mph

so it's either the stock tires were not 27, the stock speedo gear is not 41 (i have no idea), the speedo calibration was off from factory (maybe they did not have different gears for the 205s and 215s), or the 35s were maybe 34 real diameter (would come to 64mph indicated speed in that case) - so not sure

[aw12345]

got to remember a lot of tires come up short on reported size. When I went shopping for speedo gears in early 97 I paid 15 bucks at the dealer. The last one I bought for my other Jeep was 30 bucks. So truthfully to get it accurate and be flexible the wizbang box is the way to go.

[Jeffy]

   I'll go with the gut feeling.  Math is good but experience says I need a 39T.  Hard to believe the Dealers are charging $90-120 for the gears now.

[VA_YJ]

39 tooth for $40

http://www.jeep4x4center.com/jeep-speedometer-gears/

[Bounty Hunter]

I run the Dakota Digital unit, but kinda had to since I installed the SS SYE with the Rubi speedo pickup, no more speedo gear.

[Jeffy]

39 tooth for $40

http://www.jeep4x4center.com/jeep-speedometer-gears/

Yeah, I've seen them on ebay for about the same.  I talked to PATC - http://www.transmissioncenter.net/speedometer_calibration_______va.htm

For completeness, here are some electronic signal modifiers/calibrators on the market.  The Dakota, while the cheapest is the most complicated since covers a wider range of vehicles.  Feel free to post links to others out there as I'm sure there are more.

Superlift Tru Speed - http://www.superlift.com/accessories/truspeed.asp
Jet Accuspeed - http://www.jetchip.com/JET_Transmissions.asp
Dakota Digital's SG-5 - http://www.dakotadigital.com/index.cfm/page/ptype=product/product_id=126/mode=prod/prd126.htm
Healtech Electonics SpeedoHealer - http://www.bluemonkeymotorsports.com/products_ht_apps.html
Yellow Box - http://www.yellr.com/
TS Performance ReSpeed - http://www.thoroughbreddiesel.com/ts-performance/respeed.htm

[Mozman68]

I run the Dakota Digital unit, but kinda had to since I installed the SS SYE with the Rubi speedo pickup, no more speedo gear.

X2

[Jeffy]

Took the Jeep on the highway again and was able to check the speedo against the GPS.  At about 40mph it's off by 2mph.  When I get to 65 it's off by 4 mph.  Each tooth is supposed to be worth 2-3 mph at 65 mph, IIRC.

Haven't decided on what I'll actually do just yet.

[sharpxmen]

Took the Jeep on the highway again and was able to check the speedo against the GPS.  At about 40mph it's off by 2mph.  When I get to 65 it's off by 4 mph.  Each tooth is supposed to be worth 2-3 mph at 65 mph, IIRC.

Haven't decided on what I'll actually do just yet.

just leave it as it is, not worth it - mine was off by about 10mph at 65 so you're in much better shape than i was with only 3mph (i don't think it's worth the effort to be honest).

[Jeffy]

just leave it as it is, not worth it - mine was off by about 10mph at 65 so you're in much better shape than i was with only 3mph (i don't think it's worth the effort to be honest).

I kinda got used to having an accurate speedo after a decade of it being off.

[sharpxmen]

I kinda got used to having an accurate speedo after a decade of it being off.

well, if you're at 65 and your speedo shows exactly 61 and you currently have 41 tooth gear in order to bring it to precisely 65 you need 38.47 teeth so obviously you'll need to take your pick between 39 or 38 (not sure if the latter is available). this is unrelated to the tire size since you already have the values and you'll be using the same tires but mind you for 33s will vary a bit based on the tire pressure as well, so if you have the pressure lower you'll see more speed indicated.

[Jeffy]

well, if you're at 65 and your speedo shows exactly 61 and you currently have 41 tooth gear in order to bring it to precisely 65 you need 38.47 teeth so obviously you'll need to take your pick between 39 or 38 (not sure if the latter is available). this is unrelated to the tire size since you already have the values and you'll be using the same tires but mind you for 33s will vary a bit based on the tire pressure as well, so if you have the pressure lower you'll see more speed indicated.

Choosing the correct gear is never exact.  There are too many variations with tire size, pressure, wheel size, vehicle weight, etc...  I found many of the charts out there to be inaccurate.  Some are closer then others though but it's really a gut feeling sort of thing.  I've done enough of then to know what I need though.

If I had known there would be a rise in the price I would have bought a set of them.  Or at least 35-41 as that's what I'm usually swapping to for one reason or another.  Although, if someone was going to do this for the first time I'd probably recommend one of the electronic pulse modifiers.

On another note supposedly, 1993 was a change from the 4.5" speedometer gear to the 2.5" gear.  I don't think there is any difference in the transfer case.  It looks like you can take the VSS a part and swap the upper for one that will take a shorter gear.  The only problem with that would be having to swap the plug on the transfer case harness.  Seems doable though.  Probably won't even have to cut the plug off but just take the plug apart and swap the pins.

And if the 4.5" gear supply dries up you can take a heat gun and remove the gear off a 2.5" shaft.