diff ratio, tire size or speedo error?

[Kyp J]

I recently saw some discussion of high ratio diff.s and relsulting speed vs. tach readings. I think it was on this forum since Tom had some speed/tach comparisons. I know there are several treads on Mnet on this topic. I couldn't find it here when I looked for it.

I have changed the open diff to a Torsen of unknown ratio (from a 2002 SE I think, if such an animal exists. I think they told me that at the scrap yard.)

I have 205/50/15 tires on stock wheels. They are spec'd at 23.1" diam. and stock 195/50/15 are mostly 22.7" This is only a 1.25" difference in circumference.

5 speed trans.

I recently checked my speed with an old GPS unit and the result was about 75 when speedo said 70. I think the Tach said really close to 4000 at this speed. I didn't check any significant different speeds. Just 55, 60 etc to see where I wanted to be when too close to a Police car.

My question, if anyone has some ideas, is this: Would this fairly large error be from tire size, (I don't think so but my math skills are low) speedo error or differential ratio mismatching from the stock one?

If anyone is up to it, what diff ratio might that result in considering the above specs unless the accuracy of GPS's isn't good enough to determine that?

[maggie]

I have found discrepencies with my speedo also with stock tires and wheels. The speedo is reading about 8% high. I found this out when I was bringing the car back from CA. (A Texas State Trooper clocked me at 9 over. Good thing he didn't ask me how fast I thought I was going. The speedo read 96). I just switched to 17" wheels measuring 3/4" larger and drove up to the Gap last weekend. I did many measured mile checks (time in seconds divided by 3600 to get MPH) and am still running about 8% high on the speedo. 4000rpm is about 80 mph., stock 6 speed. Here's the interesting thing...the odometer is regestering milage to within <1%. I'm thinking the odometer is receiving an accurate digital pulse but the speedo may need calibrating. For now I do the mental math to compensate.

[Tom @ Fast Forward]

Changing the rim size doesn't help. You need to change the tire width and/or aspect ratio. I went to 225/50-15 and the speedo is dead accurate. I do the speed measurements with a GPS. Only real accurate way to measure.

Just for the record, I was not heartbroken to have to go to 225/50-15 to get the speedo right. :)

[99mx5]

Tom, what is your suspension camber, caster and toe settings? I know you set up your JICs to default ride height.

[Tom @ Fast Forward]

They are basically stock. I spend a lot of miles on the road and eat tires fast enough as is. ;)

I doubt any of those would effect the speedo.

[99mx5]

Sweet! BTW, up for a trackday? Theres plans for one Aug 11th at ASR. Im dying to go to Firebird... Got lots of vacation time to burn :)

[Kyp J]

Thanks everyone, that answers all my questions.

[Tom @ Fast Forward]

Maybe. If I get the 99 running again. First I had a problem when trying to start of it just going click-click-click. Changed the starter and that problem went away but it started simply growling. Turn the key a couple times and then it would start. So I took it back to the local shop here in Coolidge and they found a bad alternator. Changed that and said the bad alternator killed the battery so they changed that. Well, it is still acting like a dead battery. Maybe I have something draining it? Anyhow they have it back. In the meantime, I got in to take it to the local shop for them to look at the battery, etc and discovered that my clutch has a LOT of freeplay and is engaging about 1" off the floor. I hope that is simply a bad slave cylinder. In any case, if it back and running and starting (I hate getting stuck in the middle of nowhere with a dead battery) I might just meet you there.

The guys at the shop said it was the coldside kit and especially the piggyback engine management that makes the clutch work funny.


Sweet! BTW, up for a trackday? Theres plans for one Aug 11th at ASR. Im dying to go to Firebird... Got lots of vacation time to burn :)

[99mx5]

Yeah, next time tell them to check the fluid in the headlights while they are at it... :stuart:

[FormerDatsun510Man]

The numbers makes sense. I am running completely stock in gear ratio and tire size. At an indicated 70mph my car is doing around 67mph. If you had a 5-speed in your model year Miata you originally had a 4.3 diff ratio. The 2002 SE (a 6-speed) has a 3.909 diff ratio. So at an indicated 70mph the calculations come out:

actual speed at indicated 70mph = 67mph * 4.3/3.909 * 23.07/22.68 = 74.97mph ~ 75mph

If you want to make your speedometer dead accurate you could change the speedo gear to one with 75/70 times the number of teeth on the current one.

[Kyp J]

The numbers makes sense. I am running completely stock in gear ratio and tire size. At an indicated 70mph my car is doing around 67mph. If you had a 5-speed in your model year Miata you originally had a 4.3 diff ratio. The 2002 SE (a 6-speed) has a 3.909 diff ratio. So at an indicated 70mph the calculations come out:

actual speed at indicated 70mph = 67mph * 4.3/3.909 * 23.07/22.68 = 74.97mph ~ 75mph

If you want to make your speedometer dead accurate you could change the speedo gear to one with 75/70 times the number of teeth on the current one.


Bill

Thank you for answering the questions I asked. Now if I can understand it. I used to have a 4.3:1 and now have a 3.9:1? If so, that is good. I guess this statement is all I really needed for the question: "The 2002 SE (a 6-speed) has a 3.909 diff ratio."

A different gear for the transmission to make the speedo right would necessitate counting the teeth I currently have and getting one with 1.0714257 times the teeth on it? I said my math skills are low. Could you elaborate a bit on that aspect? Are there a lot of teeth on these gears? I wonder if getting the next toothier gear would put the speedo error into the original "optimistic" error condition. That would be better in terms of speeding tickets.

Looking at the tire difference: My attempt at mathorizing this (and rounding up) says the tire inluence is 1.8% and the speedo error is 6.7% (assuming the chosen numbers which are really approximate anyway) and since the tires would increase the ratio induced error, by subtracting it would be what stock tires would look like on the speedo?

IOW 6.7 - 1.8 = .049 X 70 (MPH) = 3.43 + 70 = 73.43 instead of 75?

[maggie]

Kyp,
I am passing off the speedo differences for now as I have switched wheel sizes from stock 16" to 17" and the O.D. tire size increased +3/4". I had a speedo inaccuracy at stock and of course with the new wheel tire combo too but cannot understand why the odometer is still accurate to <1%. this was over a measured 250 mile trip. Currently, 23.5" tire O.D., 6 Speed 3.9, 80mph indicated, 74mph true.

[Kyp J]

Kyp,
I am passing off the speedo differences for now as I have switched wheel sizes from stock 16" to 17" and the O.D. tire size increased +3/4". I had a speedo inaccuracy at stock and of course with the new wheel tire combo too but cannot understand why the odometer is still accurate to <1%. this was over a measured 250 mile trip. Currently, 23.5" tire O.D., 6 Speed 3.9, 80mph indicated, 74mph true.

I hope you didn't expect me to figure out anything technical.

I can make up speculative scenarios however. Was the odometer checked for accuracy on the stock wheel and tire combo? Could it have been reading high before? With the slightly bigger tires you would theoretically be partially correcting a speedo that read high (I think) and may have put it into a more accurate operation than stock. This would assume the odometer and speedo gauge face reflect different indications relative to accuracy. (Speedo markings painted wrong while odometer is right in the current set up. Before they were both wrong but the speedo was wronger.)

Buck:

"Wronger" is not a real word.