Speedo accuracy
#1
Passenger
Posts: n/a
Speedo accuracy
When I did the auto ('84) to manual ('85) swap I just plugged the speedometer cable back in. now I am doubting it’s accuracy.
1st redline = 30mph
forget what second is
3rd redline = 80mph
5th is 20mph per thousand rpm (and it's linear)
this is with 205/60’s.
rx-7.org tells me:
1st = 34
2nd = 56
3rd = 87
and calculated 5th with 205/60's should be around 22mph per 1000 rpm.
how accurate is the tach in these cars?
what is your speedo showing?
1st redline = 30mph
forget what second is
3rd redline = 80mph
5th is 20mph per thousand rpm (and it's linear)
this is with 205/60’s.
rx-7.org tells me:
1st = 34
2nd = 56
3rd = 87
and calculated 5th with 205/60's should be around 22mph per 1000 rpm.
how accurate is the tach in these cars?
what is your speedo showing?
#2
Old [Sch|F]ool
![](/images/misc/20_year_icon.png)
I have stock sized tires, stock transmission, etc. I have "calibrated" the speedo with those "Your Speed Is" boxes on the side of the road, and I'm apparently reading abot 10% too high at 50mph (reads 55). That works out to - yup - 20mph per 1k in 5th. (Assuming the TACH is accurate!) But the speedo isn't linerarly off, I've noticed... at 70mph (3500rpm) the speedo reads 80.
Basically I just go by the tach. 10mph per 500rpm in 5th, 8mph per 500rpm in 4th. So if I'm cruisin' at 3k in 4th I know that I'm rolling along at 48mph or so. 3rd gear doesn't work out to a round number, and 1st and 2nd gears aren't cruised at important speeds, so I just guesstimate. Really, I don't look at the speedo much anyway, just go off of the tach. Besides, on the highway, the speedo is just pegged anyway when I'm cruising at 3800 or so.![Big Grin](https://www.rx7club.com/images/smilies/biggrin.gif)
Note that 5th gear changed ratios in '84 and again in '85, and depends on 12A or GSL-SE, but 4th is ALWAYS the same.
Basically I just go by the tach. 10mph per 500rpm in 5th, 8mph per 500rpm in 4th. So if I'm cruisin' at 3k in 4th I know that I'm rolling along at 48mph or so. 3rd gear doesn't work out to a round number, and 1st and 2nd gears aren't cruised at important speeds, so I just guesstimate. Really, I don't look at the speedo much anyway, just go off of the tach. Besides, on the highway, the speedo is just pegged anyway when I'm cruising at 3800 or so.
![Big Grin](https://www.rx7club.com/images/smilies/biggrin.gif)
Note that 5th gear changed ratios in '84 and again in '85, and depends on 12A or GSL-SE, but 4th is ALWAYS the same.
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