Speedometer

[Christian Doerk]

My speedometer shows about 70 mls per hour when I drive 60. I checked it several times with my GPS. My Fleetwood does have a perfect working cruise control. Could it be the issue?
Is it possible to adjust the speedometer?

[David Thomas]

Jason, very true, particularly if you have the standard get up, meaning tire size. In my case I had a lower overall ratio because I have smaller overall diameter tires from stock. What I did was essentially go back to stock gearing, overall. It is basically the same as it came from the factory but I'm running radials. If I ever went back to the bias ply tire, I would probably want to put the original pumpkin back in.

And you're right, when I went from a lower than factory gearing back closer to factory gearing, I did notice a slight decrease in off the line performance, not much, but a little.

[Jason Edge]

Changing out the rear end differential to a higher gear (lower #) is definitely one way to offset the speedometer inacurrancy due to shorter tires which effectively lowers the drive train to distance traveled ratio. That of course also alters other dynamics with a higher gear being better for highway cruising.... but not so good for stop and go city driving.

[David Thomas]

My 63 with a/c had the 3.21 rear gears, I'm running 225 radials on it. My speedometer was about 10% fast. At 60 mph it registered 66 and it gained a tenth of a mile for every mile traveled. I drove it like that for years adjusting my mind for it to indicate 5 mph faster than I wanted to go. About a year or so back I found a 2.94 pumpkin and put that in. I didn't change the transmission speedometer gear. Now the speedometer indicates very close to the correct speed and mileage on the odometer.

[Jason Edge]

Just to add, an excellent way to determine exactly how far off your speedometer is without having to take into a shop, is the next time you are on a trip in on the Interstates, note the mileage on your car at a particular mile post, and on the mile post 100 miles further on your trim note the exact reading on your odometer.   If your odometer reads say 102.6 miles you will know you are exactly 2.6% off.  

This was something I heard at the seminar on instrument gauges at the 2012 Grand National last year, and tried it out coming home up I-95 and found that my 305/45/22 tires put mile speedometer of by about 1.6% over from the shorter 305/40/22's had had been running.  In this case since I have taller tires than stock I'm traveling just a bit faster than the speedometer shows.

[Jason Edge]

The speedo head unit can definitely be off, especially if it has been bumped or handled.  I remember I got a speeding ticket WAY over the speed limit in my 1985 IROC Z and took the speedometer out, increased the gap between the magnet and outer metal speedo cup and it thirew that thing way off@ I took to local garage that would test and certify speedometer and it was off enough that they allowed my 105 in a 55 to be dropped to an 85 in a 55, and was able to get off with a PJC instead of losing my license.  If you are in college putting yourself through, you did whatever it took! LOL  Those were the days. Crazy thing thinking about it, is I just took the whole assembly out of that 85 Camaro sight unseen..no shop manual...just figured it out and did it!

[Kurt]

As a matter of fact I swapped out the gear from my parts car just to see if that would help my speedo inaccuracy. The parts car was non-ac & my CDV is an ac car. I was then getting about a +15 to 20 MPH difference depending on speed. If I remember correctly the 2.94 gear (non ac) has 11 teeth and the 3.21 gear (ac) has 13 teeth. Mine is a hydramatic so probably different than a TH.

Christian, I would be really surprised if that transmission shop swapped out this gear. Now if they swapped out the entire transmission, that's a different story.

Also to add, my truck which is a stock 2000 Silverado with stock tires is about 3 MPH off in the 70MPH range compared to the GPS. So, 10MPH could be inherent inaccuracy plus smaller tires in my opinion.

I know many of us are running smaller radial tires. Can anyone else chime in on how accurate their speedometer is with P235 75R15's? I assumed my problem was tire size but maybe the speedometer is just way off.

[Christian Doerk]

Ok, nice. Thank you. I will study the shop manual tomorrow ;)

[Jason Edge]

I will have the 3.21 AC gear for the Turbo Hydramatic transmission. 

It makes sense that if you use a speedometer gear for a 2.94 rear end but are using a 3.21 rear end, the speedometer would show about a 9% inaccuracy reading in the plus. 

As we all know a 2.94 rear end turns the wheels once with 2.94 revolutions of the driveshaft and the 3.21 turns the wheels once with 3.21 revolutions of the driveshaft. 

If you have a 2.94 speedo gear in transmission, but a 3.21 rear end, the 2.94 gear thinks the drivetrains is turning slower to go the same miles per hr, but if you actually have a 3.21 rear end (as your AC Fleetwood should), the drivetrain is spinning 9% faster to go the same speed...resulting in a 9% too fast reading on the speedometer.

Hope that is clear as mud! LOL

Jason.

[Christian Doerk]

I have to check my size of the tires too. But this huge difference?

[Christian Doerk]

The first owner serviced the transmission in the 90's as I know. Perhaps the service shop put in the speedometer gear for a non-ac car. Sounds logical!
I currently don't have the shop manual here. Is it described in the shop manual how to change the gear? Is it easy to change or do I have put apart the whole transmission? Do you have a correct gear for a ac-car, Jason?