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99XJ on 33's - Speedo Question...

G

Germfam

NAXJA Forum User
Location
Morton, PA, near Philly!!
My step daughter just got nailed doing 81 MPH in a 65 MPH zone in good old New Jersey... Only $102.00, cheap compared to a ticket in PA...
Question, she swears the speedo said 65 MPH, is it possible for my speedo to be off by 16 MPH??
99 XJ, 4.5 inches, 33X10.50R15 BFG AT's... Original sneakers were the pweny P215/75R15's...
Thoughts appreciated... And girls crack me up when they get a ticket... She was hysterical crying and sobbing... :cry:

My 94 YJ came stock with P205/75R15's and now sits on 36X13.50-15 IROK's, and is only off by 8 MPH on the speedo...
 
Germfam said:
My step daughter just got nailed doing 81 MPH in a 65 MPH zone in good old New Jersey... Only $102.00, cheap compared to a ticket in PA...
Question, she swears the speedo said 65 MPH, is it possible for my speedo to be off by 16 MPH??
99 XJ, 4.5 inches, 33X10.50R15 BFG AT's... Original sneakers were the pweny P215/75R15's...
Thoughts appreciated... And girls crack me up when they get a ticket... She was hysterical crying and sobbing... :cry:

My 94 YJ came stock with P205/75R15's and now sits on 36X13.50-15 IROK's, and is only off by 8 MPH on the speedo...
Click to expand...

Speedos are not off by a specific amount of speed. They are off by a percentage of the speed. So at 25mph they might be off only 1 or 2 mph but at 65 they might be off significantly more. Best thing to do is pull your speedo gears (count the teeth) and get one that is suited for you new tires and gears. about $15 for a new speedo gear from the dealership.

If you can not tell the difference between 65 and 81 in a suv with 33's you shouldn't be driving (I do not care what the speedo says)!!!! (just my opinion)

Michael
 
www.4lo.com for gear calculators, but yes it is possible to be off by 16mph @ 65mph indicated...

What gears are in the Jeep? That will help make a difference but I agree that you should be able to know whether you're doing closer to 65 or 81mph.
 
It was my 17 year old step daughter, and I didn't think to tell her to stay under the speedlimit... Guess you were never a kid, it's called experience which she doesn't have... It is my fualt, but I never really checked it the speedo offset. Gears are stock, the 8.25 comes with 3.55's I think... Will have to check with the stealership...


2xtreme said:
Speedos are not off by a specific amount of speed. They are off by a percentage of the speed. So at 25mph they might be off only 1 or 2 mph but at 65 they might be off significantly more. Best thing to do is pull your speedo gears (count the teeth) and get one that is suited for you new tires and gears. about $15 for a new speedo gear from the dealership.

If you can not tell the difference between 65 and 81 in a suv with 33's you shouldn't be driving (I do not care what the speedo says)!!!! (just my opinion)

Michael
Click to expand...
 
Stock 8.25 with an auto means 3.55 and I'm not going to beat that dead horse about the gearing; let's just leave it at saying they need to be regeared.
 
Re-gear the diff, or swap the speedo gear? Seems to run and handle good. I re-geared my YJ to 4:56's for the 36's... But can I just swap the speedo gear to avoid this again? Gets decent gas mileage...



Yucca-Man said:
Stock 8.25 with an auto means 3.55 and I'm not going to beat that dead horse about the gearing; let's just leave it at saying they need to be regeared.
Click to expand...
 
Regear both, actually. 4.56 or 4.88 to match the engine to the tires, but with 33s expect to use a 39-tooth speedo gear with 4.56 or a 41-tooth with 4.88s to bring the spedometer back into normal.
 
Yes, it could be off by that much. I only switched from stock 225/70/15s to 235/75/15s and was clocked by laser (and ticketed) for doing 73 in a 60(speedo said 64/65). I even used several different calculators found online, but they really don't help too much. As 2xtreme wrote, when they're off, they're off by a percentage, not by a hard-and-fast number. I actually had to step down two teeth for tires just 1.5" larger.

Oh, yeah. Don't trust the parts guys at the dealerships about the tooth count of the stock gear. I went to two and neither one had any clue even when using the parts manuals. I also couldn't believe I had to explain to one guy, who wasn't new, how to figure out how the part #s coincide with tooth count.
 
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if you regear to 4.56 (you can't go any lower in the 8.25) then go with a 39 tooth speedo gear; if you don't want to regear, then use a 31 tooth speedo gear in your setup now.
 
Germfam said:
Question, she swears the speedo said 65 MPH, is it possible for my speedo to be off by 16 MPH??
Click to expand...
its possible, but not likely. more likely to be off by about 6-10MPH at those speeds with those gears and those tires. I think she's pullin your leg!
 
The difference between 215/75s and 31x10.50s is 10%. So at an indicated 65 (sppedo still calibrated for the stock tires) the road speed would be 65 + 6.5 = 71.5 MPH.

The jump from 215s to 33s is about 19%. 65 + 12.5 = 77.5

Not all 33s are the same. I used a figure of 610 revolutions per mile, but some run as few as 599 revolutions per mile, which means they are larger and the difference would be greater. Assuming she probably wasn't REALLY doing exactly 65 indicated, it's safe to assume she was probably doing at least 80 in true road speed.
 
Where are you? Also in NJ? I've got a 31-tooth speedo gear available here in CO.
 
Germfam said:
But can I just swap the speedo gear to avoid this again? Gets decent gas mileage...
Click to expand...

Are you GPS testing that mileage? B/C if the speedo is off, so is the odometer.
 
No, but I usually run a tank for 3 days, this is my DD... And my gas consumption has stayed around the same, pretty much run the same amount of miles a day in it and haven't noticed that I am filling up more often, been running the set-up for a year now.... Never really payed much mind to it...
 
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