speedometer/tire size ?

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with the 235/85's on my 94 the speedometer only reads 9/10's for every mile(checked it using mile markers on the interstate)how can i calculate this to figure out how far my speedometer is off? with the taller tires my gauge should be reading less than i am actually going ( correct?? ) how do i know which gear for the transmission to buy ? i think it is reading around 5-8 mph less than actual. the stock tires were 245/75's

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94 2500 4x4 SLT,auto,182,000 miles,3:54,brilliant blue/silver,tst 250/635 set full forward,star wheel wide open,air box ported,pump turned up 20%,k&n,gauges,cat gutted,
2001. 5 3500 4x4 q-cab slt,auto,6000 miles,3:55lsd,brite silver,stock (for now)both have 12k hitches&15k's in the beds,30ft fourwinds 5er,(MY RAMS) http://albums.photopoint.com/j/AlbumIndex?u=1360183&a=10142301

 

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Iceman, I may be wrong but I think if you are going 9 tenths of a mile for every mile then you are off one tenth so I would say you are going 54 on your speed ometer instead instead of 60. 9 tenths of 60 = 54. (I wouldn't bet on this but it sounds logical to me. )

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2001 quadcab slt 2500 HO 6spd. LWB,2wd,dk garnett red, trailer tow package,camper special, anti spin 3. 54 axle, speed liner,oversized stainless steel chicken slide,66 gallon in bed aux. tank,K&N air filter,Reese 20k hitch, Terry 2000 EX 30ft. double slide 5er, 2000 20ft gooseneck for haulin jeep, firewood and huntin stuff. 5X12 tagalong for haulin the 4 wheeler. Okie Newton

 

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You have about a 4% difference between the 245's and the 235's. This is figured by revs per mile of both, 654 for the 235's and 680 for the 245's. This is current info from BFG's tire catalog. This will be very close to whatever brand of tire that you happen to have. Simply multiply your speedo reading by 1. 04 to get your actual speed according to the speedo. For example 60 registered is actually 62. 4. This does NOT mean that your speedo was right to begin with. I would suggest you use a GPS on a nice straight stretch of road (as long as you can find), set the cruise at a speed, and see what that tells you. The GPS has some margin of error but it will be alot less than a speedo and if you don't have one you should be able to borrow one for a day or two. Otherwise you will need to run at least 10 miles of mile markers to get a more accurate idea of the difference. Hope this helps.

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99 2500 QC 4X4 AUTO SB 24V LARAMIE SLT 3. 5 LSD 285-75-16'S Everything but leather NRA Member

[This message has been edited by LSMITH (edited 01-18-2001). ]

 

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http://www.tirerack.com/tires/tiretech/truksize.htm
http://pw1.netcom.com/~sgalaba/tiresize.htm

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Money Sink - 96, 5 speed, HX40-16, TST #11/280 slid 3/4 way, 300 HP Injectors, Scourge BD 4200rpm Governor kit, McLeod, 4", CATless, straight out back, chrome turn down, ISSPRO Pillar, Delvac 1 & Amsiol, K&N, Mag-Hytec, 3:54, Michelin 235s on Alcoas, Rancho 9000's, Valentine One.
#11 plate days are numbered, custom grind in hand.
Dutchmen 27' 5th Wheel.
GoldWing.

 

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Yes, but does Iceman get a speedometer gear with more or less teeth than his present speedometer gear?

 

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He will need to buy a gear with more teeth than the one he has, but without knowing how much his error is it will be a shot in the dark.

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99 2500 QC 4X4 AUTO SB 24V LARAMIE SLT 3. 5 LSD 285-75-16'S Everything but leather NRA Member

 

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Call around and find a speedometer calibration shop. It will only cost about $45. 00 for them to run it. It looks like a dyno wheel, but has a speedo on the wall. They will give you % err at various speeds. I put 33" tires on my 94' (a 6% increase over stock) and guess what; My speedo was right on at 60 mph. I was 6% fast from DC. I would recommend checking all trucks because of this fact. That's 6,000 miles at warranty time.

 

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here's the math to do it for free... BTW i think that you don't have an old-fashioned speedometer gear (i could be wrong) i don't know when they began, but an early 98 has a magnetic sensor built right into the ring gear that sends pulses to the computer. .


First... park the truck on a flat, level surface.
Pop off the Right Rear hubcap
With a framing square, measure the pavement-to axle-center height as precisely as you can. This will eliminate the uncertainty of the flat-spot effect on the bottom of the tire, rather than assuming that a "35" is really 17. 5x2


circumference of tire is "pi"x2xradius

If you know your "stock" tire's original height (and assume/confirm speedo accuracy) the percent difference larger for your new tire will be the same as your road-speed


eg... when i put on my Ricksons, I went from 14. 5" pavement to center to 16even... a difference of 1. 5" ---at this point, I could have gone thru a long involved math problem with 3. 1417 (pi) x 2 x 16 and 5280 ft/mile and 5280x12 inches per mile, and figured out how many rolls of the tire per mile, but the only thing that is different is the radius... 14. 5 is almost exactly 90% of 16... look at it one way and you get 9%, the other, there's an 11% difference... . Our speedometers are reputed to be wrong by more than 2% anyway... . when my speedo says XX i figure XX x 1. 1

DISPLAY REAL
10 11
20 22
32 35
40 44
50 55
59 65
72 80
81 89-90
90 99-100


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mildly BOMBed 98. 4 1B7KF23D4WJxxxxxx-- BD's #5 plate and 210HP injectors--BD Brake and the associated transmission bomb goodies (except for TC,currently), 4" open exhaust (short),Psychotty, camper shell, Warn front receiver, G159Unisteels on Rickson 19. 5's. . CD player is bone stock, so are the seats. PrimeLoc, also remote spin-on filter for the auto trans... Pyro,Boost and Trans temp from Isspro,
Westin nerf rails(black),
bent welding rod to hang celphone on dashboard...
240HP on Dynojet from 75-105mph drumspeed.

 

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using the calculators in the site forrest posted it says im only about 2 mph off. i will try your method rich but first i gotta go plow the 6" of snow that fell last night. i am just trying to build a case for court on the ticket i got,,he says 79 in a 65,,speedometer was saying 72-74,,proving it is off will cut my fine and points,,good thing he wasn't around after i put the new plate in and seeing what the egt's were at 110mph!!! first time my CB radio ever left me down

 

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Here is the easiest way to find out how fast you are going and also find out how far off your speedo is. Get to the interstate or where ever there are mile markers and set the cruise at any speed above a crawl. Take a good stop watch and clock how many seconds it takes between milemarkers. Divide that amout of seconds into 3600(which is the number of seconds in an hour). The reason you do this is the speed is in mile per hour and your stopwatch results are in seconds. Hence if the elapsed time between the mile markers is 52 seconds then the answer is 69. 23 miles per hour. The best results are achieved using your cruise control and on a flat surface.

 

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Rich,

That flat spot business has nothing to do with the circumference of the tire. I've seen this argument before and it didn't make any sense so I checked with my son-in-law who is a math prof at BYU. (He's handy for things like this. ) After he got the grin off his face he said that I was right. The circumference of the tire is the distance around it. If the tire is flat on one side that does NOT change the distance around the tire nor does it change the effective gear ration. The tread does not shrink or stretch a whole lot in response to air pressure especially on these belted tires. The math does not really apply exactly since it's not a circle once it has a flat spot anyway. You can check this by parking on a flat piece of pavement and marking the tire and the road. Then roll ahead a number of turns and mark the pavement again. Measure the distance traveled. Let a lot of air out of the tire and do it again, same result.

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Joe George
Eureka, CA

'95 2500 CC auto 4X4,3. 54,Combo EGT/boost guage,custom switch panel,PacBrake,TST #5,BD valve body,Automatic motorhome steps on both sides,Foldacover hard bed cover,Cummins chrome kit,Black steel grill guard,Front hitch receiver

 

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Joe g. ---difference between theoretical radius (measured from axle center to a round piece of tire-edge) and effective radius (axle-center to driving surface) should be very inconsequential. Operative phrase "should be" is dependant on degree of "squishment" on the bottom of the tire. Aside from that, which is "all academic", you're simply going to get a more accurate "raw" radius by my measurement method than by assuming that the nominal size number is truly accurate ( a "35" may be 34 1/4, or any other number that's neither 34 nor 36) There's also a big potential for error using a yardstick across the center. View angle can introduce a bit of parallax error +/-. Failure to be across the dead center can cause you to use too small of a number for diameter. ... If you know that there's going to be some unavoidable "slop" (the speedometer itself, for example) somewhere in the calculations, your final result number will be least damaged by starting with the most accurate controllable factors. For this purpose, I like the framing square and axle-center... . to beat a traffic ticket,however, I'm not above the thought of using any statistical manipulation that works in the "alleged" leadfoot's favor, so long as the judge/prosecutor are not well-accomplished geometric "mathe-magicians" themselves. #ad



I use the percentage comparison from my "stock" tire and my Ricksons to keep a (literal) running estimate of roadspeed compared to speedometer display (too cheap to get the recalibrating computer from Abbot)
Measured by the milemarker/odometer method, it's held up... . 10 milemarkers=9 odometer miles, measured on flat ground on I-15, so I mentally add 10% to my speedometer while I drive and know within a couple of % what the radar will "blink" me at... also been confirmed by pace car and by one of those roadside radar "your speed is" deals that appears near some construction zones

 

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That flat spot business has nothing to do with the circumference of the tire...
You can check this by parking on a flat piece of pavement and marking the tire and the road. Then roll ahead a number of turns and mark the pavement again. Measure the distance traveled. Let a lot of air out of the tire and do it again, same result.

Someone just recently did exactly this experiment, except that they didn't let a lot of air out of the tire - they just let out a little bit out. (Search on "dog-tracking 3500")
Even a slight adjustment in air pressure DID make a measurable difference in how far the truck travelled for each tire revolution.
I guess we need to repeat the experiment and see if we all get the same results.

 

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If the flat spot made a difference then you would see a difference speed according to tire presure. I've never even heard of that sort of thing. Race cars would be able to adjust stagger by varying tire pressure. They use tire pressure to vary spring rate and stickiness, not stagger. I notice that when they measure a tire its not on the car and has no weight on it. Tire pressure is not going to make any difference in the length of the tire tread around the circumference of the tire and that's how far the tire travels in one revolution. It may have some affect with a bias ply tire, but not a steel belted radial. The frameing square measurement would certainly be ok if there was plenty of pressure in the tire. It would be a lot easier to do then using a yard stick. I've experimented with the rolling marked tire business and got the same answer regardless of tire pressure. The time I did it was one day when I had just come off the beach and the tires had 20lbs of pressure. Same measurement after putting 60lbs in them with my on-board compressor.

 

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Relative to the original question, I think we should all hand George the prize for best answer. Dividing 3600 by the number of seconds per mile will tell you exactly what your true average speed was over that mile.

Relative to our new side-issue of whether or not pressure and the "flat spot" on the tire has any impact on distance traveled per revolution, I almost didn’t respond, because I feel this is one of those little side issues that tends to pull us apart instead of bringing us together. Too much of that on TDR lately, so, please take this continued discussion in the spirit of a fun debate among friends, not an argument! #ad


I based my original argument on a second-hand report from another TDR member who said he measured a difference. Using a bicycle tire, I got similar results - any squash changed the roll-out. But now Joe's got me thinking - the steel belted radial probably eliminates the squirm, and acts more like a tank tread than a bicycle tire. Tires that are almost completely flat don't count because they are self-destructing and really just running on the rim. Guess I'd better shut up until I repeat my measurements with a radial, and quit typing when I'm sleepy! #ad



[This message has been edited by HC (edited 01-25-2001). ]

 

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When I did it I had just got off the beach with only 20 PSI in my tires so they were squashed pretty good. I dunno why I thought of this thing, but I did. The way I did it was not too scientific, but not too bad either. I smuged a line on a tire with the point of a stick and layed the stick on the ground even with the mark. Then I drove a little way and stopped. I dunno how many revolutions, maybe 10. It was in a parking lot near the beach. I pumped up the tires to 60 PSI and then backed up until I got back to the stick. The mark was right where it was to start with.

 

[Archive]

i did the 3600 second test,,,set the cruise at 70mph and averaged 50 seconds for each mile which gives a mph of 72. i am not sure whether the 94 has a gear or a sensor. it,s not really off that much to worry about it. my shop manual doesn't say either way,but i am just curious which it does have. thanks

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94 2500 4x4 SLT,auto,182,000 miles,3:54,brilliant blue/silver,tst 250/635 set full forward,star wheel wide open,air box ported,pump turned up 20%,k&n,gauges,cat gutted,
2001. 5 3500 4x4 q-cab slt,auto,6000 miles,3:55lsd,brite silver,stock (for now)both have 12k hitches&15k's in the beds,30ft fourwinds 5er,(MY RAMS) http://albums.photopoint.com/j/AlbumIndex?u=1360183&a=10142301

 

[Archive]

Iceman, I am glad you found out what your mph error is, but I hate to tell you that 2mph at 70 is well within the tolerance for factory error. They don't have to make them perfect and they don't even get them close sometimes. As for a gear or a sensor I don't know, I wouldn't worry about it if mine was as close as yours.

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99 2500 QC 4X4 AUTO SB 24V LARAMIE SLT 3. 5 LSD 285-75-16'S Everything but leather NRA Member

 

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Here is another calculator for everyone. At the bottom it states how far off the speedo is with new tires. Keep in mid it assumes that the with the original tire size entered the speedo was correct. http://www.miata.net/garage/tirecalc.html

Another thing is, with this % error info you can determine how many teeth you need to add to your speedo gear (if equipped). Counth the teeth on the gear you have now and then multiply that number by (1+(the percent generated).



[This message has been edited by Cooker (edited 01-28-2001). ]

 

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I have had 3 sizes of tire on my 97 stock 245/75R16, then 235/85R16, and now 255/85R16. I never checked the speedo with the stock tire but with the 235s it was right on the money. With the 255s I pulled the speed gear counted the teeth got a new gear from Dodge with one less tooth. I am now witgh in one percent of perfect.