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Discussion Starter #1
Does the size of the tyres effect your speedometer, say for example I've got 175/13's on a car, and I change to 205/14's, will my speedo reading still be accurate?
 

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If it was correct before the change, no. You will be going faster at same speedo reading with new tires.
 

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What swamper said.
Speedo reads the revolutions at the end of the tranny if I'm right, so with bigger tires you travel further in one rotation than you did before.

Cheers!
Matthijs
 

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72CAT, assuming your speedo was accurate before the size change. You can change rim sizes without effecting accurance, but the total diameter of the tire must be the same as what was on the smaller rim size. A larger diameter will result in the speedo reading slower than actual speed. and vice versa with a smaller diameter.
 

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Discussion Starter #5
You can change rim sizes without effecting accurance, but the total diameter of the tire must be the same as what was on the smaller rim size. A larger diameter will result in the speedo reading slower than actual speed. and vice versa with a smaller diameter.
Okay, thats what I needed to know, so if the original rims and tyres on the cougar are 195/14's, and they are now 235/15's what does that mean....quick lesson in tyre diameter please,
 

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Do an odometer check. We have mile markers here on roads. Over a 5 or 10 mile check divide your odometer distance reading by the actual. This will give you rate of error to multiply against speedometer reading to correct for actual speed. At slower speed the # miles per hour error is less than at higher speeds.
 

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You're tire size is indicated as: 235/50/15 for example.
- That means the width of the contacting surface is 235 mm wide
- The height of the walls (from the edge of the rim to the contacting surface) is 50 % of the tire width (235 mm). That is: 117.5 mm
- Rim size is 15"

the total diameter of the tire is the rim diameter (15") + 2 times the height of the walls (2x117.5mm).
That makes:
(15*25.4) + (2*117.5) = 616 mm

Hope this helps.

Cheers!
Matthijs
 

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Discussion Starter #8
Help......they're making my brain work, okay swamper, I reread the post twice, sprayed some doom on the cobwebs and I got it, would be easier to just go thru a radar scan and see what the cop thinks I'm doing to what I think I'm doing, bit more expensive I guess. Our roads are'nt marked off but if I go out of town I can go from one km sign to another, [distance to travel till you hit the next town are normally marked in 10 km distances] but thats km's and my car is in miles..........!

Matthijs, am saving this post so I can sit down in peace and work out what I've got on the 72 and 70. You see every now and again you do actually make an intelligent post....:D

Thanks guys, appreciate it.
 

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72cat, OK, I'll Try. Metric Tire sizes included 3 sets of number. i.e. 205/70 R 14 the first set of numbers are the aspect ratio. Than is to mean the width of the tire tread to the width of the sidewall. The last number (14) is the rim size. Tire Width will also determine what rim width you will need. Here are a few examples for comparision. (1)a BFGoodrich P195/60R-14, needs a suggested rim size of a 14x5.5 to 14x7.0. The outside Diameter of the tire is 23.2 while the Tread width is 6.3 (2) a BFGoodrich P215/70R-14, needs the same rim size but the OD is 25.9 and the tread width is 6.4 (3) a BFGoodrich P245/60R-14 needs a rim size of 14X7.0 to 14-8.5 has an OD of 25.6 and a tread width of 8.0. Have I totally confused you now???? Again assuming your speedo is correct, Get the size of you current tire and talk with you tire dealer. IF possible to do he will be able to refer you to a tire and rim size in 14, 15, or 16 inches to be able to maintain the same OD. If this is for your Cat, I would think it possible, however, keep in mind that with some of the smaller 13" rims the tires are so small that you just get a bigger rim size without increasing the OD. In that case, you can change your speedo's gears, which is relativly cheap to do. I would also consider going with a 15" rim as -at least in the US- they are more popular and there are more sizes to choose from.
 

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You can also take the above error # and multiply by the number of teath on speedometer gear. round to nearest whole # and replace gear with that # of teath.

The only differance then is to convert to mph from metric. I think you have it.
 

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Thanks Dayle, that's real kind!
I know there's lots of info in my head, but it is such a mess I cannot find it at the appropriate times... ;)

Cheers!
Matthijs
 

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Discussion Starter #15 (Edited)
Thanks again guys, swamper are you seriously suggesting I get under the car, and count the teeth on the speedo gear, cos if you are...may I suggest you seriously [email protected]#?/.... joking, guess I should know what it is without having to look.
69Kat, thanks for the link and the info, I just used the 13 and 14's as an example, altho we have 12's on our bakkie [small pickup] [very small pickup]. So if the front rim on my 70 is 5.5 to 7. then I get 215's for them, I really want to get rid of the Silverstone [left side] and Bridgestone [right side], don't ask, it was'nt me, PO's doing. I'm going to save that site as well, for later reading, when kids are all asleep and I can think straight. Would also prefer 15's but not financially viable just before Christmas. :(
Matthijs, would'nt it be nice if our brains were like computers, then we could just drag the info into the right place, click on find and there you have it. It bugs me when I say that I read something somewhere and then can't remember where, or when.
Sign of age I guess.
 

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:1poke: I can understand the not being able to think straight....LOL Let's see if I can add one more curve to it. If this is for you cat. Changing the tire size on just the front will not effect your speedo.... as the speedo is driven from the transmission. It will effect the cars look and stance though. However, some of the olders cars where built, where the speedo was driven from the wheels. These cars are easy to spot however, because when you would drive in reverse the Odometer would run backwards.
 

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72Cat,

Sure crawl under remove bolt, pull cable end & gear then count teath. It's not hard. It is usually a task that results in some dirt and grease on you though. But then again I like to get dirty.
 

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Discussion Starter #18
because when you would drive in reverse the Odometer would run backwards
Thats interesting, going to drive all the cars in reverse and see what happens:D
Swamper I don't mind getting dirty, hell I've been crawling under cars for years, but i must have been a bit slimmer in those days, these days parts of me hook on everything. She is going up on ramps Monday, actually she will already be on ramps Monday, so I may as well do it then, or, of course, get Stephen to do it. ....
 

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72cat, the last time my wife crawled under a car to change oil we lost a whole tank of gas on the groung (20+ years ago). Now she thinks all mechanical work is no problem as long as I'm doing it.
 

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Discussion Starter #20
Swamper, round about the time I got interested enough in cars to actually get underneath them my husband was trying to get the exhaust pipe out from where it fitted under the engine. Can't remember the exact details but he gave me a 10pound hammer and told me to hit it hard when he said to hit it, well he said hit it and I did, missed it unfortunately, but did'nt miss him, a visit to emergency and a couple of stiches later both he and I agreed that my help should be limited to being about 10 yards away from the car, of course that only lasted till he needed my help again.
 
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