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Topic: Speed/Odometer Correction (Read 1635 times) previous topic - next topic

Speed/Odometer Correction

All,
I know that with all the wheel/tire size options that these cars had, there was a nominal tire diameter on which the speed and distance was calculated.  Most seemed to fall around, say, 25.6" give or take maybe 1/2".  Was there any compensation made for these different sizes in the speedo cable gears, or would the same gear be used no matter what tire was being run?  Meaning, were the indicated speeds of some cars more accurate than others?  On cars with the TRX option, that tire was only around 24.9" diameter, while a car with 215/70R14's was almost 25.9" diameter.  If I put a 24.9" tire (225/50R16) on a car set up for 25.9", am I just going to have to live with the variation, or can it be corrected?  Thanks.

Speed/Odometer Correction

Reply #1 –
I don't think they really cared over 1/2" tire sizes do to logistics of using gear ratios (you'd basically need 'half sizes' for fine tuning), but obviously they did have different speedo gears based on rear end ratios. You can use the link to see what size you would need for different configurations, and you'll notice a gear will fall into the 'close enough' area of a few different sizes.

also, if you have a stock AOD, you'll want the 8 tooth drive gear option.

https://lmr.com/products/Mustang-Speedometer-Gear-Calculator

Keith
1988 Thunderbird sport
2004 Ford F150 Lariat
2008  Chevrolet Cobalt Sport
2007 Suzuki DR-Z400S dual sport/Supermoto
1988 Thunderbird LX - sold
1988 Mercury Cougar XR-7 with GST kit - gone

Speed/Odometer Correction

Reply #2 –
There were also a bunch of different gear ratios offered. The Speedo gear usually only corrects by about 3 or 4 mph, but again that depends on tire and rear end ratio.

Now if you really want your head to spin, you have to now factor in that the tire size isn't exact. I've seen as much as a 1" difference in tires of the same size, but different brands.

I upgraded to 10 holes. They are a 15" rim, and were optional on not only our cars, but mustangs as well. I run 215/65r15's. The Speedo is off a bit, but tires are cheap and plentiful. I think that my Speedo reads 59 at 60mph, close enough for me, and the $300 I saved over stock sized 14" rims.

Find a good tire calculator. I like the Tacoma calculator. You can compare stock sized tires to other sizes. Find something close to 25.5-26" tall. Then do price shopping. Read reveiws, make an educated decision, then throw on the tires you picked.

Good luck.
Quote from: jcassity
I honestly dont think you could exceed the cost of a new car buy installing new *stock* parts everywhere in your coug our tbird. Its just plain impossible. You could revamp the entire drivetrain/engine/suspenstion and still come out ahead.
Hooligans! 
1988 Crown Vic wagon. 120K California car. Wifes grocery getter. (junked)
1987 Ford Thunderbird LX. 5.0. s.o., sn-95 t-5 and an f-150 clutch. Driven daily and going strong.
1986 cougar.
lilsammywasapunkrocker@yahoo.com

Speed/Odometer Correction

Reply #3 –
Thanks, guys.  I've always kinda been a math nerd, especially when it comes to tire sizes, gear ratios, etc.  I actually like finding alternate tire sizes and things like that.  I've had a formula I've used for years for calculating tire diameters.  What I was meaning was, given a fixed rear end ratio (for example, 2.73 as many of these cars had), how are various tire diameters managed within that fixed rear end gearing?  I'm doubting that speedo gears are made to compensate for these small variations.  Does the factory just allow for maybe a 3%-5% variation when it offers various tire sizes on a car (with other factors held constant)?

I actually chose the 225/50R16 size because I have a set in pretty good condition on a car I'm about to get rid of, and the size was hopefully close enough for the Cougar I'm looking at when I have them mounted on a set of Turbo Coupe 16" snowflakes, which I already have.  I may be putting way too much thought or concern into this, but I'm just one of those people who gets hung up on things being "just right".  I know they are going to be slightly small in diameter, but I already have them and I'm on a pretty tight budget anyway. 

Keep hitting me with feedback, and thanks again for your comments.

Speed/Odometer Correction

Reply #4 –
The manufacturer's specs are probably not up to your standards.
From the 84 Shop Manual:

Speed/Odometer Correction

Reply #5 –
softtouch,
That's a cool table.  You're right.  I guess I thought the standards were much tighter than that.  I guess my bubble is busted.  So what I'm proposing is still well within specs.  Thanks for the info.