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 What is the correct MPH correction for switching to 31inch tires?
#475243
07/12/04 05:09 PM
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Joined: Nov 2003
Posts: 167
OP
Wheeler
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I threw some 31x10.50 BFG A/Ts on my monty a few weeks ago...
Is the speedo going to be off since these tires are bigger (even if not much) than the stockers?
1997 Montero LS 3.5 V6 31x10.5 BFG A/T's T-bar lift Optima Yellow Top Cheap Pilot Fog lights 
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Re: What is the correct MPH correction for switching to 31inch tires?
[Re: TeaQue]
#475244
07/12/04 05:17 PM
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Joined: Jan 2000
Posts: 8,160
Web Wheeler
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Hard to say, that is the stock size for Gen IIs, SRs. I would guess if it is off, you could procure a SR speedo gear to correct.
1996 SR From the Area 51 Skunk Works. 37"BFG Tires, 5" lift, Rock Track 4:1 Case, Tom Woods Drive Shafts, Oasis Air, Front ARB, lifted gas tank, T-max winch and lots of stuff that cost too much.
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Re: What is the correct MPH correction for switching to 31inch tires?
[Re: TeaQue]
#475245
07/12/04 06:29 PM
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Joined: Apr 2000
Posts: 1,664
Body Damage is Cool
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It is really a matter of what do you dim as off. Car manufactures usually make speedometers that show speed higher than the actual one (sometimes up to 10%, one of the reasons why you are not getting stoped going 5-10 mph above speed limit, is in fact you might be actually going just above or below one.). That means now with larger tires speedometer may show a correct speed. For example after going from 31 to 33" according to GPS my speed is only 2-3 miles above shown on the speedometer at 70-80 mph.
I believe that in your case by going from 235s to 31s difference in measurements of speedometer will be about 5% which translates into only 4 mph of difference at 80 mph (I hope you usually don't go much faster than that). How it translates into actual speed only comparison with something like GPS can tell, (or a cop with radar, for the sake of it). Anyway I wouldn't worry much about changing your speedometer gear just adjust you habits slightly.
92 SR, 285/75R16 Yoko MT ARB locker GAST compressor AirLift 1000 MM10500 Snorkel custom rear bumper
02 Montero LS died protecting the master
08 PowerWagon (stock!)
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Re: What is the correct MPH correction for switching to 31inch tires?
[Re: Alex Kogan]
#475246
07/12/04 06:38 PM
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Joined: Nov 2003
Posts: 167
OP
Wheeler
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It is really a matter of what do you dim as off. Car manufactures usually make speedometers that show speed higher than the actual one (sometimes up to 10%, one of the reasons why you are not getting stoped going 5-10 mph above speed limit, is in fact you might be actually going just above or below one.). That means now with larger tires speedometer may show a correct speed. For example after going from 31 to 33" according to GPS my speed is only 2-3 miles above shown on the speedometer at 70-80 mph.
I believe that in your case by going from 235s to 31s difference in measurements of speedometer will be about 5% which translates into only 4 mph of difference at 80 mph (I hope you usually don't go much faster than that). How it translates into actual speed only comparison with something like GPS can tell, (or a cop with radar, for the sake of it). Anyway I wouldn't worry much about changing your speedometer gear just adjust you habits slightly. I never drive above 80mph in this thing <img src="/forums/images/graemlins/lol.gif" alt="" /> I usually stay below 75 as a matter of fact. off-topic: I just noticed you're located in KC! I actually live in Blue Springs (20 minutes east of KC) I've been wondering if there are places around here to do some wheeling (even if it's mild wheeling in my case <img src="/forums/images/graemlins/smile.gif" alt="" /> ) maybe we can meet up sometime. Can you give me any info on this "KC4WD" club I saw on your picture page?
1997 Montero LS 3.5 V6 31x10.5 BFG A/T's T-bar lift Optima Yellow Top Cheap Pilot Fog lights
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Re: What is the correct MPH correction for switching to 31inch tires?
[Re: TeaQue]
#475247
07/12/04 06:54 PM
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Joined: Feb 2004
Posts: 10,238
Web Wheeler
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What was your OE stock tire size? And was the speedo accurate with stock tires?
Frank
'89 [color:"white"]G-Raider[color:"white"] [color:"black"]Supercharged 3.0L, MegaSquirt 2, lockup A/T, 2.5" exhaust, 172k, Cibie H4s/Oscar SCs, Hella Micro DE fogs, Cobra CB, Superwinch hubs, LSD rear/Aussie Locker front, Bilsteins, Lifeline AGM, Rust-Oleum
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Re: What is the correct MPH correction for switching to 31inch tires?
[Re: TeaQue]
#475248
07/12/04 07:27 PM
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Joined: Dec 2001
Posts: 11,727
Web Wheeler
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borrow a GPS with WAAS correction <img src="/forums/images/graemlins/smile.gif" alt="" />
98 Montero with cold weather package
96 Toyota Land Cruiser, fully locked Mall Machine :-)
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Re: What is the correct MPH correction for switching to 31inch tires?
[Re: LRJ4x4]
#475249
07/13/04 06:59 AM
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Anonymous
Unregistered
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Actually in my Gen I I found out that my speedo was off with the stock 235's and was only accurate once I had 31's (radar gun tested). Well, much closer to accurate. Kinda interesting.
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Re: What is the correct MPH correction for switching to 31inch tires?
[Re: TeaQue]
#475250
07/13/04 09:54 AM
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Joined: Jan 2003
Posts: 384
Mudrunner
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Yeh, the speedo will be off. Compare the revolutions per mile/kilometre for your old tyres, with the revolutions per mile/kilometre for your new tyres (these can be downloaded from the manufacturers website). Work out the percentage difference, and add that to the speedo's reading (carry a calculator until you the hang of it). IE: 60mph + 5% =63mph. after a while you remember the how much to add per 10mph's. <img src="/forums/images/graemlins/cheers.gif" alt="" />
It's time for my pill now!
93, 3.0L LWB, Pajero. 200K klm,s.
91, Mazda Bravo
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Re: What is the correct MPH correction for switching to 31inch tires?
[Re: Skinnybloke]
#475251
07/13/04 12:45 PM
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Joined: Sep 2001
Posts: 8,557
Forum Moderator
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Just find on of those trailer things the police depts uses to scare people into slowing down, and camp it until you can make passes un inhibited, and test it out. 20-30-40-50-60, and make a % difference estimate from that.
You know what I am talking about. The radar trailer....Your speed is (and in huge block letters) 56
87 Raider 4D56td v5MT1
31's..Basically Stock
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