The effects of tall tires

[greg g]

Based on several measured miles and about 4 of those speed limit radar signs, here is what I can report regarding the 225 75 15 tires. Odo was just about dead on against the measured mile signs in Pa, Va and NC. And the following was from those signs. In one area, my speedo was at 45 and the sign was flashing 43/44. The fact that the limit was 35 is immeterial to the data collected. On two occasions, approaching work zones, the speedo was indicating 60 and one sign said 57, the other indicated 59, this seems well within the acceptable margin of error as my Jetta puts 28 on the sign with 30 on the dial and 58 at 60. So it looks like things are honky dorey with the instruments. With us loaded up and with a full tank of gas, the engine and clutch they were pulling against a taller ratio, and with the absence of accelerator pumps, startup on hills was dicey. With the trunk unloaded and half a tank of gas, it was fine.

For those interested in fuel economy, we got 17.8 banging around the NC woods like the revenuers was chasing us, and 19.5, and 19.2 cruising the interstates on the way home at 65 on the clock. On the way down with the stop and go on the state higways the average was 18.8 for 684 miles. With 300 #'s of junk outa the trunk, skys the limit.

[Brendan D25]

Greg, which rear end ratio do you have? I have been thinking about trying taller tires for the rear. I now have P205-15s with a 3.90-1 ratio. Brendan.

[55 Fargo]

Brendan, don't want to answer for Greg, but he has a 4.11 diff I believe, with his taller tires P225 75 15, it dropped his rpm down a bit at higher speeds. You see with a flattie in good shape, even with lower gears you can still cruise at 60 mph, were just not used to an engine hummin at 3000 rpm these days. At 65 mph in my go to work truck, which is 1994 Ford Ranger, it is cruisin at 65 mph with 2100 rpm, as it is a 4 spd auto trans...........Fred

[Brad Lustig]

I've found that my Dodge's speedo is more accurate than either of my modern cars when compared to gps. About 2mph off at 60mph, too.

[De Soto Frank]

I check my speedos on the interstate, by the mile-markers and watch. Once I get the vehicle speed to 60 MPH "actual", I keep that speed for several miles (traffic conditions permitting), just to compensate for any errors in mile-marker placement.

My '41 De Soto was out on I-84 for about 10 miles last Sunday; I clocked 60 MPH "actual" at about 55-57 MPH "indicated", held consistent for nine miles.

Didn't think to check the Odo... car is running 6.50 x 16 bias tires, factory size was 6.25 x 16; De Soto has a 4.1 rear.

Greg's coupe has a 4.1 rear too - I happened to ask him during his visit on Monday.

De Soto Frank

[55 Fargo]

Did a speedo test with my 47 Chrysler, equipped with 6 cyl flattie, 3 spd standard trans, dry clutch, either a 4.30 or 4.10 diff, and very short P205 70R 15 tires. The speedo was registering 65 mph while my chase vehicle indicated I was doing only 50 to 52 MPH.

Wow is this speedo out, the fact is the stock speedo, had a 3.54 diff driving the wheels. Now with a 4.30 and tires that only have a diameter of 25.5 to 26 inches, this is really throwing everything out.

The stock Chrysler engine with 114 hp, the fluid drive and a 3.54 diff, pulls away very slow from a dead stop, I know as I have driven one very recently,and in my opinion are quite slow off the line. But once rolling cruise very nice at 60 to 65 mph.

I was thinking, my engine is only a 100 hp, with a dry clutch, there is no fluid coupler as a fluid drive provides, so going with taller gears may make my get up and go performance with a dry clutch and 3 spd trans suffer. This car weighs probably 700 pounds more than a P15 Club Coupe, so it will need lower gearing for off the line performance.

I was noticing it was a little slow after shifting into 3rd gear until I hit about 35 mph, then it would pick up speed a lot better.

So maybe , with the present engine, dry clutch, I should keep a lower rear end, for now. Does this make sense, or am I off on this one.

Having these small tires, gets the rpm really wound up at 50 mph, plan to get P225 75 15 tires, this would give it about 2 to 3 inches taller tires, which would give me a higher speed at a lower RPM.

[Young Ed]

I have a 3.73 in my p15 clup coupe. Everything else is standard. I have I think 215 75r 15s on my car. Maybe 205s. My car pulls away from stops just fine. You should easily be able to go to a 3.90 or 3.73. 3.54 might be pushing it a little too much. Also once you settle on a rearend you can change the speedo gear to get that back in line. I changed mine after the rear end swap and got the speedo back to pretty accurate.

[55 Fargo]

Thanx Ed, was thinking the same, with the present engine, weight of car etc, a 3.54 may be a lil too tall, but a 3.737 or a 3.90 maybe just right. Actually an R10 trans would be nice, none knows of any around here at this point in time...........Fred

[Young Ed]

I know where there is an extra around here. I keep thinking I should pick it up for my p15 but I think OD with a 3.73 might be alittle too much.

[55 Fargo]

Is it a 3.73 or 3.90. If availble, can you PM me with price details etc...Fred

[Young Ed]

Fred its actually an R10 OD. I could have OD in both my cars that way. Of course no OD available for trucks.

[Brendan D25]

Fred, you could look in the salvage yard in Altamont. From the pictures I have seen, there are a couple of Dodge D25s and maybe a few P15s. You could check for a 3.90 ratio. That is what is in mine. Brendan.

[55 Fargo]

Phone the yard owner this morning, he doesn't know if there are any OD trans, or what diffs are on the cars. I did ask him about the grille, he is going to check and call me are you still interested.

I have another guy nearby, he has some diffs at his place, will check him out first............Fred

[Brendan D25]

Yes I would Fred, thanks. Hopefully he has one there that is not pitted. Brendan.