3 things to discuss speedo inaccurate,Gas smell with no leaks, 3rd air shocks

[Mark Bartlett]

OK the speedo  is off by  9 miles an hour after I put on the radial tires I have a pink gear which I think is 35 teeth so it looks like I need a 38 tooth blue?    Another issue I have is a gas smell  coming from  the rear of the car no leaks I have cked it 3 times no leaks is that tube that hangs off of the rear of the gas tank with the loop is that supposed to have piece of foam or a ck valve in it Mine has none. 3rd when I put 4 people in the car it bottoms out on speed bumps I was thinking of putting some monroe air shocks in the rear just wondering if someone has done that.     Thanks

[Jason Edge]

Mark, It might be better to split these items out in to 3 topic discussions as they are very different and following one topic thread can sometimes be a challenge, much less three!

I will take a stab at the speedometer being off. There is no indication that it has been replaced, but with being 9 mph off (I am assuming at 55mph??.. it will increase as you increase speed), I am guessing the transmission has been swapped out with one with a speedo gear for the "other rear end"?

For your deVille convertible which came with a Turbo HydraMatic the two speedometer gears per info on our Rear End Differentials: Gear Ratios, Controlled Differentials and S...  Help Page would be as 40 teeth gear for the 3.21 rear end differential, or a 37 teeth gear for the 2.94 rear end differential:

-64 Turbo Hydramatic , part # A148 1859 axle ratio 3.21, 40 teeth
-64 Turbo Hydramatic, part # B135 9271, axle ratio 2.94, 37 teeth

First confirm the transmission in the car, then confirm the gear ratio of the differential. That same help page will show you a "1" stamp on differential front carrier means 3.21 gear, and "2" means 2.94 gear.

Whatever you have it sounds like you need the other and I should have either one on hand.