Bucket List Trip about to start

[DuckRyder]

Glad you made it home safe Jim, glad there are still some good shops out there that’ll take care of a old truck on a trip!...

Agree keep us updated on the truck!

[Jim Bathgate]

Thanks for all the nice comments and support.

I have a few questions. I used a windshield mounted GPS unit to help me navigate on the trip, and generally found it helpful, especially knowing how many more miles I had to travel to reach the goal destination for the evening. About a week ago, instead of inputting the destination, I just had the unit on without any destination plugged in. In that mode it was tracking my route on the map in addition to the speed I was traveling. I noticed that the speed was reading about 5mph lower than what my speedometer was showing. In other words, the speedometer read 60 mph but the GPS said I was only going about 55 mph. So a few questions for you.

1. For a 1968 F350 Camper Special with 159 inch wheel base and dual rear wheels, the specs call out for 8.0 x 16.5 OR 7.5 x 16 tires. I don't see any aspect ratio listed so I don't know the original tire diameter. Does anybody know? I see some currently available 8.0 x 16.5 tires listed at diameter 28.6" (89.8" circumference) and some 7.5 x 16 tires listed at 32" diameter. I have brand new (at the beginning of the trip) Kumho Crugen HT 51 all season tires which were highly rated by Tire Rack. They are size LT235/85R16 and have a diameter of 31.7" (99.6" circumference). If my thinking is correct that a larger diameter tire results in a slower speedometer reading, then I would need a tire circumference of 107.6" or 34". Seems kind of big.

2. If in fact the GPS reading of MPH is correct, then my speedometer is reading about 8% fast. Short of replacing the tires for a different size, is there a way to get the speedometer to read more accurately? I suppose that putting more air in the tires might help. The max pressure is 80 PSI and I've been running the rears at 68 PSI. Would that make that much difference?

3. Again assuming the GPS reading is correct, should I also reduce my calculated MPG reading? Over the last 4600 miles on the trip, I calculated 13.3 MPG. Should I reduce that by 8% down to 12.2 MPG?

4. Am I stupid for not noticing this sooner? I drove 7000 miles around the country at what I thought was 60MPH and it turned out I was traveling at 55 MPH. This calculates out to an extra 16 hours on the road over what it would have been. No wonder I hardly ever passed anybody.

[Ranchero50]

Sounds about right. From the pictures on the sales brochures I think the OEM rolling stock was smaller than what we normally run now. Lots of room in the wheel wells.

Ford was also notorious for miss calibrating the speedo's to avoid the liability from folks from speeding.

You can also do the math and order the correct speedo cable gear to get closer. 55 to 60 mph shows the speedo running 9% fast. You'd need to pull the speedo cable out of the transmission and count the teeth (usually they are marked on the end or are color coded too), then order one with an extra tooth. Pretty simple to do and swap out. They were pretty cheap too last time I ordered a couple.

[Jim Bathgate]

Ranchero, that sounds like the perfect solution. I didn't know you could get different gearing for the speedometer but that makes perfect sense. I've disconnected the speedometer cable at the back of the dash a number of times to do troubleshooting on the gauges. Getting to the transmission end I haven't done yet, but I'll add this to my list of things to fix on the truck over the next year. Thanks much.

[tsherry]

I bought a whole selection of speedo gears on ebay for under $20. It's a five minute job to change one.

[DuckRyder]

When you pull the gear and adapter out, shine a light in and see what color the gear on the output shaft is.

Been a while since I looked into this, but I seem to recall some combinations do not work right.

[popeyes71]

I'm a little late to the game but welcome home! I enjoyed taking the trip along with you through your posts. Maybe we'll pass each other in our bumps driving around here in SJ.

[Jim Bathgate]

Thanks for the information on speedometer gearing. I will be generating a shopping list for parts needed to fix the various issues with the truck, and this info is very helpful. Popeye, hope to see you around town.

I'm now trying to catch up on a month's worth of undone chores (Christmas lights, shopping, cleaning, etc).

Regards to all and hope you have a nice Christmas/Hanukkah season.

Jim Bathgate

[basketcase0302]

Jim,

DC used to have a kit that included an assortment but i don't see it now.
I have an assortment, (used) for sale but found there is a difference between the manual, (I remembered you told of using your granny gear' on your trip) and the ones I have are for the "slushboxes" (automatic).
Here's the DC page of them:
-mg5.mail.yahoo.com/neo/b/launch?.rand=4e1v1jbouon9h

[Jim Bathgate]

Yes, Basketcase, I have the NP435 4 speed manual transmission, so I don't think yours would work. thanks for the info.

[Jim Bathgate]

By the way, I did get a 16 and a 17 tooth speedometer gear to try to fix my speedometer over-reading situation. I changed out the 15 tooth gear on the cable to a 16 tooth and the result came out just right. Now when the speedo say 60mph, I'm really going 60mph.

[basketcase0302]

="Jim Bathgate
By the way, I did get a 16 and a 17 tooth speedometer gear to try to fix my speedometer over-reading situation. I changed out the 15 tooth gear on the cable to a 16 tooth and the result came out just right. Now when the speedo say 60mph, I'm really going 60mph.

I would hate to see how Chebby mucked this simple mechanical change up LOL?!
I'd saved many speedo plastic gears over the years< (any Ford that went to the boneyard at least gave up this simple part to me) and it paid off.