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 Help me undestand tires and speed

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Nick in PA



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Join date : 2010-12-14

PostSubject: Help me undestand tires and speed   Sun Nov 11, 2018 9:23 am

Hello Everyone,
General tire question here. I have two caprice wagons, one requires 225/70/15 tires and the other 225/75/15.
If I understand correctly, both are 225 mm wide so it is the side wall ratio is different making one tire taller than the other.
Do they calibrate the speedometers at the factory for the different tire size options or is that 70-75 difference so small that it doesnt really affect the speedometer. It seems to me that one wheel would have to turn more than the other to travel the same distance.
I was just wondering if they are interchangeable between cars. I know they will fit but just wondering about the speedometer. It made me wonder if they are calibrated in some way at the factory.
Thanks,
Nick in PA
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silverfox103
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Location : Littleton, NH & St. Simons, GA

PostSubject: Re: Help me undestand tires and speed   Sun Nov 11, 2018 11:23 am

Nick

The label on the door says 225 75 15 ?

Tom
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RedandBlack

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Join date : 2016-01-19

PostSubject: Re: Help me undestand tires and speed   Sun Nov 11, 2018 6:58 pm

Sure they'll interchange but your speedometer won't be accurate. My wagon also originally called for a 225/75R15.

Your difference would be negligible. And on these cars the tire size can be adjusted in the PCM tune, so it's likely that differences exist. The Fleetwoods, for example, call for a 235/70R15 (which is damn near impossible to find in a whitewall... and about 0.2" shorter than a 225/75).
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Nick in PA



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Join date : 2010-12-14

PostSubject: Re: Help me undestand tires and speed   Sun Nov 11, 2018 7:15 pm

Tom, yes sir, the 91 says 225 70 r15, the 92 is 225 75 r15. Both right from the door labels.
Makes sense that the computer needs adjusting like Redand Black stated. Both cars have the correct tires on them. I was just wondering how that worked if I changed them around.
HAve a good day,
NIck

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Fred Kiehl

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Join date : 2009-11-13
Age : 70
Location : Largo, FL 33774

PostSubject: Re: Help me undestand tires and speed   Sun Nov 11, 2018 11:43 pm

The 91, 92, and 93 can not be adjusted with the computer. You must change the driven gear, and possibly the VSS, depending on the number of teeth the driven gear has. If you pull the VSS, you will get about 2 quarts of trans fluid out of the hole, so be prepared.

Usually the speedo reads fast to begin with, so putting a larger tire on the car will probably make it more accurate. My 92 OCC read fast by about 6 MPH, and putting 235/75/15s on it vs. 225/70/15s, the speedo is almost dead accurate.
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Nick in PA



Posts : 59
Join date : 2010-12-14

PostSubject: Re: Help me undestand tires and speed   Mon Nov 12, 2018 9:29 pm

Fred, You know these cars inside and out. Is it just a love for the model or a GM mechanic?
Either way, thanks for the info. Very interesting.
I hope your wagon has completed recovered from that dump truck incident.
HAve a great night,
Nick
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dmg4

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Location : Geneva, New York

PostSubject: Re: Help me undestand tires and speed   Mon Nov 12, 2018 10:45 pm

You can look up the revolutions per mile under "specifications" on most tire sales sites (e.g., tire rack). If you are just changing from 225/75/15 to 235/70/15, then I'd not worry about recalibration of the speedometer or PCM. The difference in revolutions per mile in those tire sizes tires is negligible (736 vs 742, respectively....about 0.8% difference). That's less than the inherent error in your speedometer.
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Fred Kiehl

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PostSubject: Re: Help me undestand tires and speed   Tue Nov 13, 2018 8:24 am

I have two of the OCCs. I have had many experiences (some not so fun) with them, and if I can share helpful hints and kinks, I am happy to do so. I just recently found out that the valve covers are unique to the 91-93 LO3/5 engines in the B bodies.

The wagon is still in the body shop. It has been 7 months. I sure hope I can drive it again before I die.
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RedandBlack

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PostSubject: Re: Help me undestand tires and speed   Tue Nov 13, 2018 9:58 am

True - I assumed he had a 94-96 car. No info in there. So we're both correct, depending on what wagon you have.

Interesting so my wagon would have had a green driven gear because it was originally 225/75R15 with 3.23 gears (a 1992).
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dmg4

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PostSubject: Re: Help me undestand tires and speed   Tue Nov 13, 2018 10:22 am

The two tire sizes mentioned by the OP will not introduce a significant error in the speedometer, no matter which car they are running on. As Fred pointed out, unless you switch to a tire with a significantly different number of revs per mile, it's mostly a matter of matching the speedometer gear to the differential gears.
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Nick in PA



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Join date : 2010-12-14

PostSubject: Re: Help me undestand tires and speed   Tue Nov 20, 2018 7:45 pm

Fred, I hope you get it back before you die too... HA, unless you are in serious illness in which case I take back the HA and feel rather bad.. Father time is however catching uo to me as well.
Thanks everyone for all the feeback, The conversation sparked another question,
I have, and have had several 91-92 Caprice wagons and OCCs. They get great gas milege for what they are, avg 17MPG on a rodtrip with a full load. Crappy by todays standards but better than my old 73 Buick wagon by 10 MPG. Here in PA, they all struggle to get up the mountains and heven forbid you are towing something up the slightest hill. I know they have 170 to 180 HP but why are they so slow up the mountains. On some of the steeper grades the darn things cant maintain 65 MPH.
Happy Thanksgiving Everyone,
Nick in PA
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Fred Kiehl

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PostSubject: Re: Help me undestand tires and speed   Tue Nov 20, 2018 9:03 pm

Check the gear ratios. If it is a GU5 you have a 3.23, and it should do pretty good on hills. If it has GU2 it is a 2.73, and will be a bit of a dog everywhere. The LO3 is a dog no matter what. The LO5 has 50 more lb ft of torque than the LO3. If you get a chip for the computer, and put an L99 or LT1 cam in them, you get even more. The 350 from a 93 9C1 is listed at 205 HP, and the cam, and computer is the only difference. If you put the electric fans from a 93 Fleetwood on it you get another 5 HP.
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