Speedo good but Odo way off....

[Aardvaark]

Thinking my gas mileage sucked bigtime and something on my FI was wrong,
I checked my speedometer and Odometer on the trip back from the Springfield meet and got a big surprise on my '75 Manta.
With 175/80-13 tires instead of the OEM 165/80-13's, my speedo was only
about 1mph off at 70mph, reading slightly high at 71mph using the cruise control.
But the odo was way off. 15.4% error measured over 100 miles on the interstate using the mile posts. 100 miles of posts showed only 84.6 miles on the odo.
Sooo, what I thought was about 17.2 mpg at 70mph turned out to be 22.6 mpg.
Also at other average speeds, 25.9mpg @ 65mph and 19.7 mpg @ 75 mph.
Not great, but about what I remmember with my '75 Ascona on the highway.
Guess the engine is okay. I'll check the start injector for leaks and the mains for pattern just to be sure.
Anybody else check their speedo/odo for errors?
-Mark

[tomking]

How can a speedo be on yet the odo be off??
I am sure there can be some jokes made of that!!

[heimue]

My speedo and odo on the Manta are both 2.5% fast, meaning when the speedo shows (theoretically) 100 mph I'm "only" going 97.5 mph, and when the odometer shows I travelled 100 miles it really was only 97.5 miles. This is with 205/50/15 tires.

Dieter

[Paul]

Mark,
All of the car magazines of the time showed the speedo read about 3 mph higher than true speed @ 60mph, but the odometer was pretty accurate. Your 175/80 tires are slightly taller than stock which would correct the speedometer error, but would make the odometer indicate fewer miles less than actual...

As far a gas milage goes, my 75 Manta always returned good fuel milage but tuning was critical. I always got 26-27 @ 70 mph when tuned well 24 -26 when not. When I was running an air dam on the front, Ansa free flow exhaust, fresh tune up, tires aired up and cruising at 63 mph (in the days of 55 speed limit) I could get 28.5 -30 mpg on flat interstate depending on which way the wind was blowing.

The best fuel injector cleaner I've found for on the car use is made by RedLine... Good clean injectors always made a hugh difference in the ability to keep the tuning spot on.

[Aardvaark]

This Manta is the first vehicle I've owned that had more the 2 or 3% speedo/Odo error. That's why I'm suspecting slippage of the Odo pinion
driven gear on it'd shaft, the plastic one that often times splits on the VDO speedos, since it's only a press fit in the Odo shaft. Guess it's time to yank the unit for inspection.
Thanks for the RedLine injector cleaner tip. I've never use injector cleaner on any vehicle. I stick with Top Tier Gas but I don't know what the PO used.
Time for a parts store run.
-Mark

[sawdust]

Both my 71 GT Manual 4 speed and my 72 GT Auto read about 4 mph high compare to my GPS. The 72 has 13" wheels and the 71 has 15" wheels.
FWIW.

.

[Paul]

Mark,

I also highly recommend, if you haven't already, switch over to synthetic fluids in the tranny, differential and front wheel bearings. I received a noticeable bump in economy from this. I currently use Red Line: CV-2 in the front hubs, 75W-90 in the rear end and MT-90 in the tranny...

The Red line stuff really helps improve the shift quality and reduces noise. As long as the shifter shaft seals don't leak on the tranny, its a excellent upgrade.... or else the leak just gets expensive.

Also, a BIG impact to fuel milage with the fuel injected models... is having the proper thermostat!!!!! OGTS has the proper one. You can't just use the single valved thermostat that's avialable for the carburated models. The temp sending unit impacts the ability of the FI to reduce the injector pulse time and properly lean out the mixture as the engine heats up. The water must heat up to 190F at the sending unit or else the mixture will stay too rich and fuel milage will suffer..