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Speedo and RPM


drkyle96

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1994 Del Sol Si; 5 speed

 

Quick question: My car reads 66 mph at 3000 RPM in 5th gear.

 

I know the speed is off by about 10%, so my actual speed is really 60 mph. Next time

you're cruising, would someone check their RPM at 60 and let me know? I am wondering

if the RPM is off also.

 

Thanks

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how do you 'think' the rpm is off?

 

Based on your response, I am assuming my ignorance in some of the finer car details has been revealed...

 

I didn't know whether the two were linked somehow. I just wanted to know that the RPM was reading true.

My other vehicle is a Dodge Truck and in 5th gear at 60 mph it's only 2000 RPM. I know about gear ratios, etc,

but it just seemed high to me.

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Yes, you are comparing apples to bass boats.

 

Considering there are light years of differences between your truck and your del sol you cannot use either one for any comparisons.

 

I don't have an SI del sol, which has a different engine and a different transmission then my car, but 3000 rpms at 65 is a typical number for a civic product of any variety.

 

Honda engines are much higher revving and higher geared transmissions then most other vehicles. It is normal for the rpm range to be considerably higher. For example: your truck cruises at 2000 at 65 and possibly hits redline at 4500, maybe 5000. Your del sol does not even engage the vtec solenoid and hit peak horsepower production until it passes 5000 rpm. i think engagement on your car is 5400 with a redline of something near 7000 rpm.

Edited by Xeryon
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those numbers sound about right

 

i actually found out the place i got my tranny from gave me a d15 tranny and i think there is only a slight difference.. but ~70mph mine is about ~3k or a little over

 

i'm not saying a honda tach is perfect but it will not be off that much unless something is wrong

 

 

 

my firebird had quite a bit longer gears and at 60 was about 2000 rpms but also didnt rev till ~7200 rpms from the factory

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Yes, you are comparing apples to bass boats.

 

Considering there are light years of differences between your truck and your del sol you cannot use either one for any comparisons.

 

I don't have an SI del sol, which has a different engine and a different transmission then my car, but 3000 rpms at 65 is a typical number for a civic product of any variety.

 

Honda engines are much higher revving and higher geared transmissions then most other vehicles. It is normal for the rpm range to be considerably higher. For example: your truck cruises at 2000 at 65 and possibly hits redline at 4500, maybe 5000. Your del sol does not even engage the vtec solenoid and hit peak horsepower production until it passes 5000 rpm. i think engagement on your car is 5400 with a redline of something near 7000 rpm.

 

 

Actually my redline on the truck is 6000 RPM - but your point is well taken, and yes the Si redlines at 7200. I was surprised that the speedo was off so much. As I stated in an earlier post, this was purchased used and I am finding a lot of gremlins. I had been cruising on roads at speeds in the del sol similar to those driven in the truck and people were flying by. I finally used a Garmin today to record my speed - and before you ask - I also had calibrated it in the truck. The speed is definitely off by 10%.

 

 

 

Is there a way to re-calibrate a honda's speedo?

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the speedometer issues were part of a huge class action lawsuit once upon a time. all honda's from the early 90's were incorrect, generally reading high and thus causing more miles to calculate then they actually had traveled. hence the source of the lawsuit - deflation of a vehicles value do to reading higher then actual mileage.

 

Nothing to be easily done aside from a slightly larger tire to offset the calibration issue.

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'91 hatch Si all w/stock circumference tires runs 3000 rpm @ 60 mph and 4000 rpm nets 80 mph in 5th gear. Speedo and tach track close. Tach is almost dead on and speedo reads about 2mph high across the range.

Edited by Dual-500
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As I recall, the core of the lawsuit was that customers could loose some % of their mileage based warranty, forcing them to pay for repairs that should have been under warranty. For example, if your odometer/speedometer was off (optimistic) 10% too high, then you'd loose about 3600 miles of your 36,000 mile warranty. Of course that is assuming the dealer and district service managers were assigning warranty repairs strictly by the numbers.

 

I know of MANY cases where extra time/miles were allowed depending on what type of issue it was. The up side is that it could protect you from getting a speeding ticket by making you think you were going faster than you were.

 

I also corrected my 6% error on my Sol by running a slightly larger diameter tire on my replacement wheels. Now it's dead on.

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As I recall, the core of the lawsuit was that customers could loose some % of their mileage based warranty, forcing them to pay for repairs that should have been under warranty. For example, if your odometer/speedometer was off (optimistic) 10% too high, then you'd loose about 3600 miles of your 36,000 mile warranty. Of course that is assuming the dealer and district service managers were assigning warranty repairs strictly by the numbers.

 

I know of MANY cases where extra time/miles were allowed depending on what type of issue it was. The up side is that it could protect you from getting a speeding ticket by making you think you were going faster than you were.

 

I also corrected my 6% error on my Sol by running a slightly larger diameter tire on my replacement wheels. Now it's dead on.

That's a good idea - never occurred to me and I'm a former 4x4 guy that's way aware of final gearing, tire circumference, etc.

 

Think I'll try a set of 205/50's next tire replacement instead of the 195's - should bring the speedo dead on.

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Is this something that would no longer be an issue if you did a motor swap? I'm guessing being the the motor and tranny would be off a newer car that it wouldn't be an issue any more?

That wouldn't really matter. In these instances, it's an issue if it's an issue and not if it isn't. For a swap, assume you mean motor and tranny. Final gearing -vs- tire circumference is what matters. Did it change? Yes, then speedo is off. No, then it's as accurate as it's accurate.

 

From one car to the next things can vary. If it is an issue, and you have a capability fo adjust tire size to correct it, that's a good way to go.

 

Most of the newer cars I'm familiar with don't use a gear in the tranny that turns a cable connected to the speedometer. They use magnetis sensors in the tranny that pick up rotation and that feeds the computer that is calibrated for the tire size.

 

When tire size changes on the new stuff, the computer needs to be recalibrated. That can be accomplished in various ways depending upon the manufacturer. Not sure what Honda does on the newer models.

 

But on the relics we restore/rebuild they are for the most part old style gear/cable setups. For a motor swap as you mention, I'm guessing it will be old school setup to old school setup.

 

Interesting thought, what gearing is used from one model to the next for the speedo cable from the tranny. Haven't really seen anyhing about that. i.e. tire rotation to speedo cable rotation. Maybe somebody else has experience with this and swaps - I don't.

Edited by Dual-500
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Here is a great reference to tire diameter specs from Tirerack too.

 

Each tire will show a lot of good info under "specs" like diameter, cross section, tread depth, etc...: http://www.tirerack.com/tires/tires.jsp?tireMake=Dunlop&tireModel=Direzza+Sport+Z1+Star+Spec&partnum=96HR4Z1SS&vehicleSearch=true&fromCompare1=yes&autoMake=Honda&autoYear=1997&autoModel=DelSolVTEC&autoModClar

 

and some more http://www.tirerack....ge.jsp?techid=7

 

It should help most figure out what they could use as an alternative. Just remember a 26"tire, compared to a 25", diameter tire will raise the car about 1/2" due to half that diameter being transfered to height nad the other half being above the hub.

Edited by MChester
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