Abstract: A tractor-trailer calculator includes several tractor-trailer indicia portions slidably supported on an elongated support structure at positions to simulate a tractor-trailer vehicle. The indicia portions include a tractor cab portion, a tractor tandem axle portion, a tractor fifth wheel and trailer king pin portion, front and rear trailer portions and a trailer tandem axle portion. Each portion is slidably supported on the support structure in overlapping relation to at least one adjacent portion whereby the overall length of the simulated vehicle and the relative positions of the overlapping portions may be varied to conform to predetermined length requirements for a tractor-trailer vehicle. Measurement devices are associated with the pairs of overlapping indicia portions to indicate their relative positions. Another measurement device is adjustably and slidably supported on the support structure adjacent the indicia portions for measuring various lengths of the simulated tractor-trailer vehicle.