Skybolt

Does anyone know what gear is needed for a 140mph speedometer?

I am going to change the face plate on an 85mph speedometer, but am unsure about what gear to order?

Thanks.

SwampeastMike

The gear that I believe you are talking about compensates for different rear axle ratios. Merely putting a new face plate on the speedometer is not enough--the speedometer itself has to be recalibrated.

Skybolt

Really, bummer. So I f I get a new speedo the goes to 140 mph, I don't need to change the trany manual hook up gear?

SwampeastMike

Don't change the gear in the transmission unless you change the rear end ratio or substantially change tire size.

The speedometer recalibration is done magnetically, not mechanically, and is not a DIY thing.

A bit of trivia: Nikola Tesla, the man who almost single-handedly invented production, transmission and utilization of the alternating current system we still used today also invented the automotive speedometer. It was one of those little things that helped fund his experiments.

mrvette

I read long ago that 1001 rpm is supposed to indicate 60 mph.....

why 100 ONE rpm is anybody's guess......

and I have confirmed on my '72 vette via GPS that the speedo is dead nutz on up to 80 mph which is really only 78 mph......at 100 indicated, it's really only 94mph.....I ran outta room, test over......

nuff said.....

Skybolt

I knew the first part, but did not know the speedometer part. Thank you.

So I take it to change out my 85mph speedo, I just need to put in an new 140mph speedo and I'm done. Man I missed the boat on that one, I read a few articles and must have confused the gear thing. It makes sense about the trany gear and the rear end gears. Thanks for explaining.

Gale Banks 80'

When You have a Spedo rebuilt with a 140 or 255mph face they send an in line multiplier that makes it close to the right speed. Depending on the ratio, You might be able to do the same thing by just switching the Trans Gear as in the 1st post. You would have to do some math to find out if its possible. Mine is pretty close, and I'm just using a Trans Gear.

SwampeastMike

Doesn't that throw off your odometer? Unless my logic is backwards I believe your odometer will be ticking off the miles much more slowly than it should. Isn't that illegal?

Street Rat

I swapped out the 85 mph speedo to a 140 mph in my '81. I haven't had any accuracy problems. When I changed the rear end ratio I ended up changing the driven gear at the transmission end of the lower speedometer cable. This corrected the readings on the speedometer. A speedometer app on my cell phone helped me zero in on accurate speedometer readings with GPS.

Gale Banks 80'

The Spedo has been re calibrated internally so that the odometer and the speedometer somewhat match. If Your going to just change the face of the spedo then You can either chose a trans gear that makes the od right or the speed right. Not both.

Skybolt

Yep, so it looks like I am just going to buy a new re-calibrated 140 mph speedometer. Much easier that way.

Thanks for all of the input, I have a much better understanding of what was a gray area to me.

SwampeastMike

Your original speedometer can be recalibrated most likely at less cost than a new one.

Here's a place that can do it.

0Willcox Corvette

SwampeastMike is correct! The calibration won't be corrected by changing a gear..

What you should do is purchase the face and then have the gauge calibrated to the face... This is the only correct way to do this.

Unless you change the rear end or/or the tires, the driven gear is not a factor... The input is decided by the calibration of the head. Which I (or other shops) can calibrate inhouse..

Willcox

Skybolt

Care to PM me a link to your site on this or should I just call?

0Willcox Corvette

Calling probably won't get it.. The sales staff just knows parts. If you have the 140 face then box up the speedo and send it in to us. I can calibrate the head to the 140 face, that's not an issue. If you don't have the face then box up the speedo and send it in and I'll get you the face, calibrate it and get you going..

Willcox

lvmyvt76

ther you go, Willcox saves the day again!!!!