Could someone have put larger tires (maybe 265/75r16s) on it and corrected the speedometer and went back down to 245s and left it calibrated for the larger tires? That might explain it, but if he went down to 245/70r16s, it would read maybe 4-5 mph faster.

 

If they're put on the front only, it wouldn't affect the speedo. Some Rams came stock with 265/75's though also.

 

I found the following website that calculates speed differences: http://www.tacomaworld.com/forum/tirecalc.php

Comparing a 225/75R16 (Speedo Reading column) to a 245/75R16 (Actual Speed column) I got the following:

Speedo Reading ---- Actual Speed
20 mph (32.19 km/h) 20.81 mph (33.48 km/h)
25 mph (40.23 km/h) 26.01 mph (41.86 km/h)
30 mph (48.28 km/h) 31.21 mph (50.23 km/h)
35 mph (56.33 km/h) 36.41 mph (58.6 km/h)
40 mph (64.37 km/h) 41.61 mph (66.97 km/h)
45 mph (72.42 km/h) 46.81 mph (75.34 km/h)
50 mph (80.47 km/h) 52.02 mph (83.71 km/h)
55 mph (88.51 km/h) 57.22 mph (92.08 km/h)
60 mph (96.56 km/h) 62.42 mph (100.45 km/h)
65 mph (104.61 km/h) 67.62 mph (108.83 km/h)

Hopefully the formatting on my post works and it does not display as one long sentence.

I do not know the math or physics behind the above calculations, but if it's on the Internet it has to be true, right?

The only thing I know for sure is that when I drive past one of those "Your Speed Is" radar signs, my detected speed is slower than the speed indicated on my speedometer.

 

See, if it had 225s and 245s were put on, your actual speed would be higher than the speed your speedometer says. But you said the opposite is true. I would suggest you obtain a build sheet from Dodge and see what the tire size it really came with, as the sticker on the driver's side door opening has a tendency to fib.

Heres the link for requesting a build sheet: http://www.dodge.com/wccsapp/univers...ull&category=U

 

The sticker on the door says 225/75R16.

Thanks for that build sheet link. I have requested one and will post back when I receive the original tire size.

 

Sticker on the door is often inaccurate just wait and see what the build sheet says.

 

Here is the build sheet...

Vehicle Description DODGE RAM 1500 P/U CLUB CAB Model Year 1996 Body Model BR 1 L 31 Color PGFA- Emerald Green Pearl Coat
Engine ELF- 5.2L V8 MPI Engine
Transmission DGTS- 4-Spd. Automatic 46RE Transmission

Sales Code Description
*P1 Premium Cloth 40/20/40 Bench Seat
-D5
ADAP Light Group
AJKP Convenience Group
AJPP Power Convenience Group
APAS Monotone Paint
ASPP SLT Decor Group
BAUP 117 Amp Alternator
BCES 600 Amp Maintenance Free Battery
BGJS Pwr Disc/Drum Rr Anti-Lock Brakes
BKAS 11x2 Rear Drum Brakes
CBEP 40/20/40 Split Bench Seat
CGTS Driver Air Bag Only
CHEP Deluxe Headliner
CKEP Front Seat Area Carpet
CLAP Front Floor Mats
CSRP Passenger Assist Handles
CUDP Mini Overhead Console
DGB All 4-Speed Automatic Transmissions DGTS 4-Spd. Automatic 46RE Transmission DHAS Lock-Up Torque Converter DMDS 3.55 Rear Axle Ratio DRBS Corporate 9.25 LD Rear Axle EAAC All Engines ELF 5.2L V8 MPI Engine GACS Tinted Glass Windows GBBS Tinted Windshield Glass GCBS Front Door Tinted Glass GFD Rear Sliding Window GNAS Rear View Day/Night Mirror GNMP Passenger Side Sun Visor w/Mirror GPP Power 6 x 9 Bright Mirrors GVBC All Vehicles W/Power Mirrors HAAP Air Conditioning HGAP Hood Insulation HGBS Dash Liner Insulation JAYP Instrument Cluster w/Tach JCBS 120 MPH Primary Speedometer JHAS Var Intermittent Windshield Wipers JJAS Cigar Lighter JJBP Dual Note Electric Horns JJJP 12V Auxiliary Power Outlet JPAP Power Windows JPBP Power Locks K17P Bodyside Molding LAAP Ignition Time Delay Lamp LBAP Map/Courtesy Lamp LBCP Glove Box Lamp LDAP Underhood Lamp LMAS Halogen Headlamps LPEP Cargo Lamp MBFP Bright Rear Bumper MCTP Bright Front Bumper MDAP Front License Plate Bracket MFDS Bright Grille MHAS Black Windshield Moldings MPBP Black Tailgate Applique MWDS Removable Tailgate w/Caliper Latches MXBS Front Air Dam MXCP Front Bumper Sight Shields NAA Federal Emissions NBKS EVAP Control System NFWS 26 Gallon Fuel Tank NHMP Speed Control PGFA Emerald Green Pearl Coat QGFS Emerald Green Pearl Coat RAAC All Radio Equipped Vehicles RASP AM/FM Cassette Radio RCDP 4 Speakers RDDP Fixed Long Mast Antenna SBAS Variable Assist Power Steering SCVP Leather Wrapped Steering Wheel SFBS Front Heavy Duty Shock Absorbers SGBS Rear Heavy Duty Shock Absorbers SHAS Front Stabilizer Bar SUAP Tilt Steering Column TBBS Full Size Spare Tire TBMS Tire Carrier Winch TYFP P245/75R16C BSW All Season Tires TZAP Goodyear Brand Tires WLBS Steel Spare Wheel WNDP 16X7.0 Cast Aluminum Wheels XBSS Pickup Box XCMS Vendor Painted Cargo Box YAAS Build To U.S. Mkt. Specifications

 

My truck (2wd) had 245 when I got it, door sticker says 225 I think...

 

My truck has 225 on the door placard, 265 on the build sheet, and the stealership (at my request) changed them from 265 Goodyear to 265 BFG's.

As a small FYI, the R&D for the best tyre/suspension package on the '78 had the BFG 265/75/16's selected, but between the bean counters and a long standing Jeep/Goodyear alliance (war time), Goodyear are factory fit.

 

door sticker on mine says 225/75/16 and is true, because i still have intact the factory spare